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Part 1 – Introduction to the RAS Build

This is “Part 1 – Introduction to the RAS build” of  Chapter 8 of the Urban Aquaponics Manual.

Chapter 8 is where we build the system…and its a big one…so I’ve broken it down into a series of sub-chapters:

  • Part 1 – Introduction to the RAS Build
  • Part 2 – The IBC Fish Tank
  • Part 3 – The Radial Flow Separator
  • Part 4 – The Packed Media Filter
  • Part 5 – The Moving Bed Biofilter
  • Part 6 – The Tricking Biofilter
  • Part 7 – Putting It All Together

In Part 1, I’ll walk you through the water flowpath for our proposed build…and then we’ll look at the tools that I’m going to use.  In Parts 2 to 6, I’ll show you how to build each of the major system components.  In Parts 5 and 6, I present you with a choice…between a moving bed bio-reactor (very effective but at a cost) or a trickling biofilter (still quite good but much cheaper).  In Part 7 – we hook our various components together.

It’s useful to have a clear picture of how our RAS will function so let’s begin by getting a grasp of the water flowpath.  Since the water pump is located in the sump, we’ll start there:

  • The pump starts and moves water from the sump to the IBC fish tank.  The water enters the tank tangentially and imparts a circular motion in the water in the tank.   Solid wastes are pushed outwards to the tank walls and fall to the bottom.  When they reach the bottom, they begin to move toward the centre point at the bottom.
  • The weight of the incoming water displaces water already in the fish tank and forces it up the suction end of the solids lifting outlet…drawing any solids that are within reach of the suction.  The water passes through the fish tank wall and into the radial flow separator (RFS).
  • The incoming water in the RFS is directed upwards into the water deflector which causes it to change direction – downwards.  The downward movement of the water encourages the heavier particles (sedimentary solids) to gravitate to the bottom of the RFS.  The lighter water (without the sedimentary solids) rises up to the weir where it overflows and drains into the packed media filter (PMF).
  • As it enters the PMF, the water is directed to the bottom of the filter.  As it reaches the bottom, the velocity of the water is reduced and it moves upwards.  It rises slowly up through the static media in the PMF exposing suspended solids in the water to the sticky biofilms on the media.  The ‘clean’ water overflows the weir and enters the moving bed bio-reactor (MBBR).
  • The water is directed to the bottom of the MMBR slowly rising up and exposing the dissolved solids to the nitrifying bacteria that live on the gently tumbling bio-media. Once it reaches the surface, the water overflows the weir and drains into the sump tank…and so on – ad finitum.

I should point out, at this stage, that there’s another layout option…one where the pump is located in the fish tank.  The water passes through the filters and then drains back into the fish tank.  This layout requires that the filters be positioned above the fish tank.  That means that we dig a hole in the ground large enough to accommodate the fish tank…or we put the filters on a platform high enough for them to be able to drain directly back into the fish tank.

The upside to this arrangement is that we no longer need a solids lifting outlet – or a sump tank – so the build is easier.  One downside is that integrating growing systems will be a bit more challenging.  And then there’s the digging part.  My view is that life is too short to spend any of it digging holes that aren’t absolutely necessary.  

The RAS Builder’s Toolkit

Building recirculating aquaculture systems, like our proposed unit, are like every other technical endeavour…may seem daunting to the unitiated but really it comes down to some very fundamental skills:

  • Cut plastic – specifically the plastic bladders of IBC’s.
    • Jigsaw
  • Cut steel – specificically the galvanised steel frame of IBC’s.
    • Hand grinder and ultra thin cutting disks
    • Hacksaw
  • Cut PVC pipe – in the range of 20mm to 90mm (3/4″ – 4″).
    • Mitre saw
    • PVC Hand Cutter
  • Drill holes – specifically those required for the installation of bulkhead fittings and Uniseals.
    • Holesaws
    • Drill and Drill bits

To this list, you can add the following:

  • Tape measure and marker
  • Eye and hearing protection.
  • Screwdrivers
  • Wrenches – or (more specfically) any device that will enable you to grip bulkhead fittings during installation.
  • Deburring tool

Before we start work, here are some other things I’d like you to note:

  • With the odd exception, I’ll be leaving all of the pipe and fittings unglued.  This is a basic recirculating aquaculture system and there will be things that we can do to enhance it…and, should you decide to embrace those enhancements at some later stage, doing so will all be much easier if we haven’t glued every fitting or piece of pipe.  Having said that, unglued pipework is a risky proposition, so we need to demonstrate some commonsense around how we set things up.
  • I’ll be using ball valves to enable us to isolate each major component.  This allows us to work on a single component without having to drain the entire system.
  • Each of the filters will be fitted with a dump valve…to enable us to clean and drain it.

That said, let’s build a fish tank.


I’ve had to call a halt on this rollout of the Urban Aquaponics Manual.  You’ll find an explanation…in this article on my blog.  I’d like to say that I’ll continue with the work but that depends on how I go with some other priorities.  In the meantime, I’m reasonable satisfied with what I’ve published here so, if Aquaponics is for you, then I invite you to make ongoing use of the work.SaveSave

















Part 2 – IBC Fish Tank

This is Part 1 of Chapter 8 of the Urban Aquaponics Manual.

“An intermediate bulk container, IBC tote, or pallet tank, is a reusable industrial container designed for the transport and storage of bulk liquid and granulated substances, such as chemicals, food ingredients, solvents, pharmaceuticals, etc.”

So sayeth Wikipedia.

Notwithstanding the uncomplimentary things that I had to say about IBC’s in earlier chapters, I do acknowledge that, for many people, they are the most cost-effective means by which to acquire a fish tank.  For that reason, we’re going to use one for this build.

My biggest issue with them is that their shape and structure can be problematic when it comes to concentrating and removing solid wastes.  Most of them are not actually square; they’re slightly rectangular.  The bottom of an IBC is not flat; it has structural moulding that discriminates against herding all of the solids into its centre.

Suffice to say, if we can make this work, you’ll be able to take what you learn and make any round or square tank work even better.

Our first task is to remove the steel retaining bars to give us free access to the plastic bladder.

Then, we mark up the top in readiness for cutting.  Removal of the top allows access to all internal surfaces of the IBC – to give it a thorough cleaning – and for ongoing management.  

This particular unit contained glycerine in its former life – non-toxic, water-soluble and easy to remove.

An electric jigsaw is my weapon of choice when it comes to cutting IBCs and other plastic containers.

The dump valve enables the IBC to be emptied and the space immediately behind the valve is a trap for solid wastes.  To prevent your toddler (or your sister’s toddler) from operating the valve, we’re going to zip tie it in the shut position.  And then we’ll plug up that space behind the valve to prevent solid wastes from accumulating there.

I’d like to be able to drain this tank directly through its bottom but, the pallet arrangement doesn’t easily lend itself to that, so I’ll install a solids lifting outlet (SLO).  This is a fancy name for a simple device that uses the weight of incoming water to displace water already in the tank…forcing it up a pipe and out of the tank.

Clear as mud…right?  Well, hopefully, this simple diagram will clarify things for you.

We’ll be setting this IBC up so that the solid wastes are directed to the centre of its bottom…so it’s logical that we’ll place the suction end of the SLO over that point.

Before we get too concerned about the SLO, however, it’s time to modify our IBC for its new role as a fish tank.

Step 1 – Remove the retaining bars at the top of the tank.


Step 2 – Mark out a square section to be removed to provide our tank opening.


Step 3 – Cut and remove the plastic top to create the opening.


 Step 4 – Mark out the exit point for the SLO – and drill a hole of the appropriate size.

There are two way that I’d propose for the

Step 5 –


Building the SLO is a simple matter of assembling some PVC fittings and a couple of short sections of pipe.


In the meantime, I invite you to comment…to express any concerns that you may have…and to provide ideas or suggestions that you feel will improve the book – or add value to it. 

Selecting the Components

This is Chapter 7 of the Urban Aquaponics Manual.

In the last chapter, we developed a design specification for a small recirculating aquaculture system (RAS).  Now. it’s time to select the components that we’ll need for the build.

Before we go too much further, I need to say that I don’t propose to provide a set of plans or a materials list for the design that we’ve produced.  Just accommodating the measurement system differences from one country to another makes such an undertaking a nightmare.  

Ultimately, you’ll make component choices based on…

  • the amount of fish that you want/need to produce
  • what’s available to you in the way of materials and equipment
  • the amount of money that you have to spend
  • your abilities and skills
  • your personal preferences

…so, I believe that it’s more important that I provide you with options that can accommodate your specific circumstances.  

We’ve all heard the old adage…”If you give a man a fish, you feed him for a day.  If you teach a man to fish, you feed him for a lifetime.”  

My goal, by the time you ingest the contents of this Manual, is that you should be able to take the principles that I describe and apply them to the broad question of system design rather than just be able to build one specific system.

Our goal is to produce clean, fresh food…so the materials from which our components are made – and any prior use that they may have had…must be safe for humans and fish.  Reject any tank or vessel where you can’t be certain of what it contained before it became available to you.  

At this stage, we’re just looking at components.  While, for the purposes of illustration, I’ve approached the design process in a linear fashion, I recommend that you read the entire manual before you start to get too set on your design.   Our basic design is just that…basic!  There are plenty of things that we can do to enhance the basic design and I want you to have the opportunity to consider those tweaks and bells ‘n’ whistles before you finalise your plan.

That said, let’s commence our component search.


The key imperative of a fish tank is its ability to facilitate the removal of solid wastes.

Concentrating solids within reach of the drain is the consequence of tank shape and design…and managing water movement within the fish tank.

The ideal fish tank is robust, round in shape and will have a slightly sloping bottom with a centre drain at its lowest point.

Water returning to a round fish tank is directed tangentially at the surface.  This creates a ‘hydrocyclone effect’ – setting up a slow circular movement in the water in the tank.  A weak centrifugal force causes heavy matter in the water (solid wastes) to move outward to the wall of the tank and to slowly spiral down to its bottom eventually moving across the tank bottom toward the drain.  

You can replicate this effect – on a tiny scale – by swirling the last mouthful in a tea cup while observing the dregs gathering in the centre of the bottom of the cup.

Most smaller purpose-built aquaculture tanks function like this. 

They are also usually quite expensive.  Secondhand units may become available but, you’ll need to act quickly since they are usually in high demand from aspiring small fish farmers.

Other off-the-shelf options (in order of preference) include:

  • round plastic or fibreglass tanks with flat bottoms…like re-purposed rainwater tanks or large round livestock watering troughs.
  • square plastic or fibreglass tanks…(preferably with rounded corners) like produce bins.

The desirable circular movement to which I referred earlier can be created in round tanks with flat bottoms – and even square tanks – and we’ll cover that in more detail as we get into the construction of our system.

Rectangular tanks are the most difficult to accommodate as fish tanks so I’d recommend that you avoid them if at all possible.

While I prefer those made of food-grade, high-density polyethylene or fibreglass, people have managed to repurpose all manner of containers for use as fish tanks.

DIY Fish Tanks

So long as we bear in mind the need to concentrate solid wastes so that they can be removed from the fish tank, the scope for viable do-it-yourself fish tank options is limited only by your imagination.  

This situation is made possible by the existence of the food-grade low density polyethylene liner.  LDPE liners are tough but flexible and can be used to line fish tanka that are built inground, on the ground and above ground.

They can be used in conjuction with existing concrete, steel/aluminium or brick/masonry structures – and you can build very serviceable fish tanks from timber and/or plywood and line them to make them waterproof.

Intermediate Bulk Containers

IBC’s are plastic vessels (generally with a capacity of 1000 litres or 250 US gallons) contained within a galvanised steel frame with a pallet base.

Notwithstanding that they are probably the most widely used off-the-shelf fish tank option in the world, I’m not a fan of IBC fish tanks – for the following reasons:

  • Encouraging a circular flow in an IBC can be difficult and that can negatively impact solids removal.
  • They are not fully UV-stabilised and will begin to fall apart over time.
  • It’s hard to know what has been stored in them. They are often used as mixing tanks for herbicides and pesticides.
  • They will always look like IBC’s.

Regardless of what I say, some of you will opt to use them anyway – so, if you’re certain about their previous use and they are really cheap, I’ll do what I can to help you to address their shortcomings later in the manual.  We may even experiment with putting a bit of ‘lipstick on the pig’ to make it look a little less aesthetically confronting.

Mechanical and Biological Filters

I’ve built filters out of all manner of off-the-shelf and recycled containers.  Some worked better than others.  The ones that I liked the most just happened to be those that were the easiest to clean.

Not surprisingly, those that were usually the easiest to clean usually worked best…largely because a clean filter works better than a dirty one.

With the exception of swirl separators, which must be round, shape doesn’t matter too much.  Having said that, I have a preference for round filter tanks mainly because they are readily available in a variety of sizes and they’re relatively inexpensive.

And, at the top of the list for cost and availability, is the ubiquitous recycled blue plastic barrel.  

These two barrels are 130 litre (30 US gallons) – perfect for our emerging RAS design.

Indeed, the only drawback of these robust vessels is the colour.  That can be addressed by buying new plastic barrels (available in a range of colours at four times the price of recycled ones) – or by cladding them in something a bit more aesthetically pleasing.

Water Pumps

Water pumps are the means by which we recirculate the water through our RAS.

For our purposes, they tend to be of two main types – submersible pond pumps or externally mounted centrifugal pumps. 

Pond pumps are cheap, very convenient to use, require minimal plumbing and are suitable for most urban aquaponics applications. The principal limitation of pond pumps is that they are best suited to low head applications. Flow rates will diminish quickly once the pumping head increases.

Two sumbersible pumps of the type commonly used by backyard fish farmers. The unit on the left is designed to be used as a submersible but also as an externally mounted pump if required.

Externally-mounted pumps generally cost more to buy but usually move more water for a given power consumption – and they are better suited to applications where the water has to be pumped up heights of greater than a metre. Their installation is also a bit more complicated.

An externally-mounted centrifugal pump – available in various sizes and usually reliable and long-lived.

A Few Pump Hints and Tips

  • Depending on your application, it may pay to consider using two small pumps rather than one larger one. The benefit of multiple pumps is that, if one pump fails, the other will keep your system going long enough for you to discover the problem. This is simple risk management.
  • Avoid the use of submersible sump pumps – they are generally not rated for continuous operation – and they can be power-hungry.
  • It may pay to buy more pumping capacity than you need initially – to cater for the likelihood that you’ll expand your system.
  • While independence from the electricity grid is a worthwhile goal, solar-powered pumps add a new layer of complexity to the establishment of an urban aquaponics system. Keep it simple to start with. 240-volt (or 110-volt for US readers) pumps will provide for relatively reliable and inexpensive recirculation during your formative stages as an urban fish farmer.

Air Pumps or Blowers

I regard air pumps as essential equipment because low dissolved oxygen levels are the principal cause of fish deaths in small aquaponics systems. In any case, fish, plants and nitrifying bacteria all benefit from high dissolved oxygen levels.

In the event of water pump failure, good supplementary aeration may be the difference between a minor nuisance and a disaster. Air pumps are cheap insurance.

Our little system is going to require aeration at several points:

  • Fish tank – continuous
  • Moving bed biofilter – continuous
  • Packed media filter – periodic…when cleaning
  • Plant growing system – continuous

We can have one larger air pump that meets all of these requirements – or we can have two (or more) air pumps to deal with specific parts of the system.  Air pumps (particularly those with diaphragms) can fail at short notice so having a couple of smaller air pumps might be a useful risk management strategy.

Pipe and Fittings

PVC pipe in the range of 25mm – 50mm (1″ – 2″) is widely used for backyard fish farming and is easy to work with.  PVC pipe and fittings in our preferred size range are of two main types…pressure and drainage.  The types are not compatible with each other although experimentation (and the judicious application of heat) will enable you to reconcile the types where circumstances demand it.

PVC fittings come in wide range of sizes and types. Their cost quickly mounts up so limiting to them those necessary is a good idea. Having said that, most backyard fish farmers have a substantial collection of PVC fittings…”just in case…”

We’ll be using 25mm (1″) PVC pressure pipe and fittings for the water supply side of our little system and 50mm (2″) for all drainage pipework.  

Some people like to use larger stormwater pipes and fittings on the drainage side but I’ve found that the lower water velocity of 90mm+ pipework often allows solids to settle out in the pipes – with the potential to create anaerobic zones.

Control valves may be required in some situations and the two most common types in use are ball valves and slide valves.  Ball valves are available from the same places that stock the PVC pipe and fittings.  Slide valves are nicer to use, more expensive to buy and are usually only available from specialty aquarium/aquaculture suppliers.

Connecting Tanks and Pipes

Secure connections of pipes to tanks are achieved through the use of bulkhead fittings (also known as tank outlets) Uniseals and flange fittings.  Each of these has their place in RAS construction and we’ll learn more about them in the next chapter as we begin the build.

One of the challenges with this chapter, was deciding what to leave out (rather than what to include).  The list of gadgets that can be included in a small-scale RAS is long.  What we’ve covered here will allow you to build a RAS that is productive, resilient and versatile.  You can always reflect on the available bells ‘n’ whistles later – as you sit down to eat your first fish and salads.

In the meantime, I propose to build our little 1,000 litre system using the following:

  • IBC – not because I like them but because (regardless of what I say) some of you are going to use them. 
  • Blue barrels – I’ll use three of them to create a radial flow filter, a packed media filter and a moving bed biofilter.
  • PVC pipe and fittings – we’ll use 25mm (1″) for the pressure pipework and 50mm (2″) for the drainage.
  • Pump – 4500 litre submersible pond pump.
  • Bulkhead fittings, Uniseals and flange fittings – to hook it all together.


In the meantime, I invite you to comment…to express any concerns that you may have…and to provide ideas or suggestions that you feel will improve the book – or add value to it. 









Designing Your RAS

This is Chapter 6 of the Urban Aquaponics Manual.

Designing any food production system starts with the question… “How much food do you plan to produce?” 

The design of an urban aquaponics system begins with questions. too…

How Much Fish?

We’re going to keep it simple for the purposes of this design discussion so let’s assume that we’re going to grow enough for one person to eat fish once a week…so we’ll need about 50 fish.  We’ll also assume that each fish will be around 500 grams (one pound) at the time of harvest.  That’s 25kg (50lbs) of fish per year.

Once you establish how many fish you want to grow each year, you’ll want to know…

How Much Fish Tank?

 We can accommodate our 50 fish in a cubic metre…about 1000 litres (250 US gallons)…of water.

Over time, you’ll come to appreciate that everything in aquaponics starts with feeding the fish and that leads us to…

How Much Feed?

Fish in recirculating aquaculture systems are most effectively fed a percentage of their bodyweight – each day.

Fingerlings may be fed up to 8% of their bodyweight but that figure decreases over time to the point where they may only be getting 1% at the time of harvest.

Each fish will, at the time of harvest, be around 500g and will (based on a daily feed rate of 1%) be eating 5 grams of feed per day,  Fifty such fish will be eating 250 grams (0.5 pounds) per day.

Assuming a feed conversion ratio of 1:2 – one kg of fish biomass for each 2kg of feed provided – we can expect that our 50 fish (each weighing 500g) will consume a total of 50kg of feed throughout the growing period.

While it’s interesting to know how much feed we’ll use in total, the more important number, for our immediate purposes, is the maximum daily feeding rate of 250 grams…because that figure will allow us to calculate the size of the filtration system that we are going to require to deal with the metabolic wastes of our 50 fish – with a total weight of 25kg (50lbs).


How Much Filtration?

Back in Chapter 3 – Understanding Filtration, we looked at all manner of different mechanical and biological filtration devices.  For what it’s worth, the list of filtration devices that I chose to ignore is far bigger than the one that I provided.

While choice is a wonderful thing, introducing too many choices into a learning situation becomes confusing so, from this point on, I’m going to focus (based on my experience) on what I think will work best for you rather than attempting to cover every possibility.

Our filtration system will comprise three elements:

  • a radial flow separator – to capture sedimentary solids
  • a packed media filter – to capture suspended solids
  • a moving bed bio-reactor – to nitrify dissolved solids

If this is all sounding pretty complex, let me assure you that, behind each of these fancy names, is a simple blue plastic barrel.  It’s how we fit out each barrel that determines its function – and name.

We’re going to build these filters in the next chapter so, for now, all we need to do is work out how much filter media we need.  Once we know that, we’ll be able to determine the size of the barrels we’ll need.

The radial flow separator contains no media so that one’s simple enough.  The packed media filter is almost filled with media so that one is easy, too.  That leaves us with the moving bed bio-reactor.

Manufactured plastic media is very effective, is self-cleaning and will deal with a predictable solids loading so I’ll be using AnoxKaldnes K1 filter media for this design model.


AnoxKaldnes K1 manufactured plastic media – excellent bio-media.

How Much Bio-Media?

The manufacturer of K1 claims that each 50 litres of media will deal with the metabolic wastes arising from the use of 750 grams (0.75kg or 1.6 pounds) of fish feed per day.  That figure applies to industrial wastewater treatment and commercial aquaculture and it assumes that there is some heavy duty filtration equipment upstream of the moving bed biofilter.  

Our design will feature some inbuilt redundancy…so we’ll be using 50 litres of K1 to deal with the wastes from our 50 fish…based on a maximum daily feed rate of 250 grams (0.25kg or about 0.5 pound).

When sizing a moving bed biofilter, I calculate the amount of media to be 60% of the total filter volume.  So, if our filter was going to be, for example, 100 litres we’d use about 60 litres of media.  Since we’ve already decided that we need 50 litres of media (a standard shipping volume, by the way), a 100 litre plastic barrel will suit us just fine.

In fact, we’ll use three 100 litre (25 gallon) barrels to house our entire filtration module.

One More Thing…

This particular design will feature something that we haven’t spoken about previously – a sump tank.  We’ll look more closely at the sump tank, what it does and its capacity in the next chapter.

Now that our system has taken on a physical dimenstion, it’s time to address the two things without which no fish can live…water and oxygen. 

How Much Flow?

Implicit within the notion of a recirculating aquaculture system is the idea that water flows from the fish tank through the filtration modules and back into the fish tank.  That flow is created by a water pump.

When calculating the size of the water pump to be used in a RAS, I take the total system volume and double it.  What we are looking to do here is to move the entire capacity of the system through the filtration unit twice per hour.

The fish tank, filters and sump tank contain 1400 litres (around 370 US gallons).  If we double that figure, we’ll be looking at a total volume of 2800 litres (or 750 US gallons).

Pumps are rated in terms of the volume of water that they will pump – per hour – so that would suggest we need a pump that will move around 3000 litres/hour.

For reasons that I’ll clarify as we get into the construction of this system, we are going to want a bit more than than that amount. so, for now, I’ll be proposing that our pump will have a capacity of 4000 – 5000 litres (1000 – 1250 US gallons) per hour.

How Much Oxygen?

Our fish need oxygen..and so do the microbial organisms that facilitate nitrification.    

Having said that, the matter of how much oxygen we’ll need depends, to some extent, on the plants we grow – and how we grow them – since plants need oxygen, too.

Suffice to say, at this stage, oxygen is as fundamental to recirculating aquaculture as water.  Quite simply, without it, nothing of value to us will live.  We will, however, address the matter of how much oxygen we need – and how we’ll provide it – when we get into selecting our system components.

OK, let’s take a look at what we’ve got so far.

RAS Design Specification

Our proposed recirculating aquaculture system will:

  • raise 50 fish to a harvest weight of 450 – 500 grams (one pound) in about 30 weeks – subject to fish species.
  • utilise a fish tank with a capacity of around 1000 litres (250 US gallons).
  • require around 250 grams (0.5 pounds) of fish feed per day by harvest time.
  • feature a filtration system – comprising a radial flow separator, a packed media filter and a moving bed bio-reactor – each housed a 100 litre (25 US gallon) plastic barrel.  Fifty litres of AnoxKaldnes K1 (or similar) will be adequate to nitrify the metabolic wastes from the 250 grams of feed that we will feed our 50 fish by the time that they reach harvest.
  • utilise a 100 litre (25 US gallon) sump tank.
  • turn over the entire volume of the system – about 2800 litres (750 US gallons) – twice per hour.
  • use a water pump with a capacity of 4000 – 4500 litres (1000 – 1100 US gallons) per hour.

Standard+RASThis simple schematic representation of our RAS shows the major components and the water flow path.

While it doesn’t look like much yet, this little RAS will yield lots of clean, fresh fish.  It will also provide some other valuable outputs – about which we’ll talk more later.


OK, so what if you want/need something bigger – or smaller? 

The amount of fish to be produced can be doubled – or halved – by simply doubling or halving the specification numbers.  

Indeed, you could scale this system up to provide five times as much fish by making proportionate adjustments to those numbers.  A system of that size is, for most people, on the upper limits of a family fish production unit.

In that situation, my preference would be to have two (or more) smaller units rather than one larger 5000 litre system.  I have good reasons for feeling this way but I’d like to address larger systems in greater detail later in the manual.

In the next chapter, we’ll find ourselves something to use as a fish tank and filtration modules…and all of the other bits ‘n’ pieces that we’ll need to build our very own recirculating aquaculture system.


In the meantime, I invite you to comment…to express any concerns that you may have…and to provide ideas or suggestions that you feel will improve the book – or add value to it. 



Things to Think About

This is Chapter 5 of the Urban Aquaponics Manual.

In previous chapters, we looked at what recirculating aquaculture is – and how it works in a basic microbiological sense.  Most importantly, we should have connected with the fundamental notion that aquaponics starts with a recirculating aquaculture system.

Before we leap into the design and construction of a RAS, however, let’s take the opportunity to consider a few things that will impact your system design.  

Don’t allow these considerations to overwhelm you.  Just have them in the back of your mind as you sit down to plan your system.

Up until now, we’ve been talking about recirculating aquaculture systems.   The considerations in this chapter apply equally to the RAS – and its attached growing systems.

Health and Safety

My inclusion of Health and Safety at the top of this list is deliberate.

Every day, we hear of people who have been killed or seriously injured in so-called freak accidents.  In truth, however, there’s usually nothing accidental about health and safety incidents (as they are more appropriately called)  around aquaponics systems.  They are are almost always preventable.

The health and safety risks that apply to aquaponics systems include:

  • Drowning
  • Electrocution
  • Poisoning
  • Manual Handling 
  • Structural Collapse

A fish tank is no less dangerous than a swimming pool or a spa. How will you ensure that small children cannot climb into your fish tank? The ideal is to cover the tank but the least that should happen is that you should be able to exclude children and pets from the area.

Electricity is an essential part of any aquaponics system but it does not suffer fools lightly.   Think carefully about how you will manage prospective electrical hazards.

To prevent your family from ingesting toxic substances, or to avoid poisoning your fish, you should ensure that your system components are made from safe, inert food-grade materials.

If you are contemplating the use of recycled materials, you need to confirm that they have not previously been used to contain toxic substances.

Manual handling is another issue that requires careful consideration, too. There’s no shortage of heavy things to lift and a hernia or a dislocated disc are a high price to pay for a momentary manual indiscretion.

Manual handling injuries are not the only weight issues. A 200-litre (55 gallons) drum of water weighs around 200kg. A 1,000 litre (250 gallons) weighs a metric ton. Given the potential for injury to people (and damage to property), there’s no place for sloppy construction.

Environmental Control

Fish and plants (like everything else) grow best in a particular environment.  While that environment will include water quality, dissolved oxygen levels and pH, our main environmental concern (for design purposes) is temperature.  Our secondary concern, specifically for plants, is light.

Temperature will impact your choice of fish species and the types of plants you can grow – and when you can grow them.  The amount of natural light that is available to you will also directly impact plant production.

You can control the environment in which your fish and plants grow.   Indeed, you can keep warmwater fish species in the depths of a Montana winter.  As a general principle, however, the further away from the optimal temperature range that you get for your preferred fish species, the more money you are going to have to spend to heat their water.

Similarly, you can grow plants in a basement or warehouse that never sees sunlight but providing artificial lighting of the correct photoperiod, intensity and spectrum is going to require significant investment.

Points of Failure

A recirculating aquaculture system is a life support system.  

If it stops functioning, for whatever reason, the living organisms that it supports will die.  An aquaponics system may experience catastrophic failure for a variety of reasons including:

  • Power interruptions
  • Equipment failure
  • Serious leaks or bursts

So, when sitting down to design your system, you need to undertake a bit of ‘what if’ analysis.

What if the power supply is interrupted? What if the pump(s) seize? What if you experience unseasonal rainfall, wind or extremes of hot or cold? What if you had to leave your system unattended for a day – or a week?

Think of every piece of pipework…and every fitting…as a prospective point of failure and design your system accordingly.

System Scale

If your system is to be housed in an urban backyard it will need to be reconciled with other backyard activities including entertaining, play area or pet space.


Herbicides, pesticides and chemicals will kill your fish and have no place near an aquaponics system.  The planet is well overdue for a respite from its most troublesome organism…humans…so cut it a break and use  materials that have the lowest possible environmental impact or those that can, at least, be fully recycled.


Your choice of system components should take account of their lifespan.

Cost Effectiveness

A key question when making any investment is “How quickly do I get a return on my investment?”  Your system design should provide you with clean, fresh food without breaking the bank.

Once the system has been built, it will cost money to operate.  Your biggest variable operating expense is the energy required to run the water and air pumps – and to heat/cool the water in the water in the fish tank – and your system should be designed to minimise these costs.


The ability to empty a system and to relocate it is a distinct benefit for people who rent accommodation. The system will also retain its resale value if it can be moved relatively easily.  Consider the use of rubber slip joints and barrel unions to enable you to dismantle and re-assemble the components as needed.  Similarly, consider quick release couplings for water hoses, air lines and electrical/data connections. 

Your choice of plant growing systems is particularly important if you need portability.  


Having tanks and growing systems at a comfortable working height is an issue for everyone but particularly for people with disabilities.  Can you overcome space limitations (with a small system) by mounting some components on robust castors? 

Ease of Operation

Your filters will require regular cleaning.  Do you have drains at the lowest points in the system to ensure that there are no places for water and organic matter to be trapped and become anaerobic?

Are thermometers and digital displays located so that they are easy to read?


Whether you get to engage in food production may require that you satisfy your partner that you are not going to create an eyesore in your backyard.

Similarly, your neighbours may begin to take an unhealthy interest in your system if they perceive that their property values are negatively impacted by your activities.

You may argue that what you do in your own backyard is your business but local government authorities will take a different view if they start receiving complaints from disaffected neighbours.

A neat and tidy system is also easier to operate and keep clean.

Nuisance Potential

Nothing will bring the wrath of the local health inspector down on your head faster that something that stinks or attracts vermin.

Still water is a breeding haven for mosquitoes and, if it contains nutrients, it can become anaerobic and will quickly produce bad odours.

Managing your system in a healthy state is essential.

System Location

Whirring pumps and running water might be music to your ears but could well drive a neighbouring shift worker to distraction. Locating your system out of hearing range will avoid this issue.

What are the other design implications of your preferred location?  Does your proposed plant growing area have enough sun?   Or too much?  Is your fish tank going to be located inside our outside? If outside, what is the likely effect of sun, wind and rain on your fish tank?  What is your closest access point to power and water?

The system design should also integrate well with other food production units.  You may decide to extend your backyard self-sufficiency endeavours to include laying chickens, meat chickens, fruit and nut trees, quail, rabbits, worms and other possible integrations. You should design your system with this in mind.

Size Does Matter – and Small is Beautiful

This implied contradiction simply suggests that choosing the optimum tank size is a question of balance – too small and you’ll become a slave to the system – too large and you’ll chew up too many resources while trying to achieve a useful result.

For backyard purposes, I suggest that your first tank be of 800 to 2000 litres (200 to 500 US gallons). A system of this size will allow you to produce 15 – 50kg (30 – 100lbs) of fish per year without the need for you to become its constant companion as you juggle the production parameters.

For the purposes of this discussion, this is a small system…not to be confused with the micro ‘demonstration of concept’ units that people sometimes build in their homes.

You can always increase the size of your system once you satisfy yourself that aquaponics is really for you and once you’ve had the opportunity to educate yourself properly about some of the options that are available to you.

In any case, if you can’t operate a small system, you won’t be able to operate a large one.

Even if you are planning a larger system, having two or more 1000 litre tanks makes more sense (particularly in an urban aquaponics context) than having one large tank. You can keep fish of different species and ages and managing risk is easier if you have several smaller tanks.   Losing some of your fish might be annoying but losing all of them would be a tragedy.

Smaller tanks are also easier to move about and cheaper to cover and insulate.

You may be thinking, by now, that designing an aquaponics system is much more complex than you previously realised.   The truth of it, however, is that it’s much simpler than it sounds.

In the next chapter, I’ll show you the process that I use to design a small recirculating aquaculture system.


In the meantime, I invite you to comment…to express any concerns that you may have…and to provide ideas or suggestions that you feel will improve the book – or add value to it. 

The Aquaponics Fork in the Road

This is Chapter 4 of The Urban Aquaponics Manual – 4th Edition.

In a Chapter 2, we looked at how aquaponics works from a basic microbiological perspective…and I said a properly-designed aquaponics system was a recirculating aquaculture system (RAS) to which growing systems were (loosely speaking) attached.  Consistent with that direction, Chapter 3 looked at the filtration methods that are at the heart of a RAS.

Then I revealed that there was this creature called the basic flood and drain system…where media grow beds allegedly doubled as the filtration system.

Here’s where I explain what I meant when, back in Chapter 2, I referred to “informed decisions” – and here’s where you get to make what is arguably the most important choice that you will make with respect to aquaponics.

First the explanation…

The Basic Flood and Drain System

The Basic Flood and Drain System (which I also refer to as the Speraneo model) comprises a fish tank, a pump and a grow bed that contains media like gravel, expanded clay pebbles or lava rock.

The water is pumped from the fish tank up into the grow beds. Once the water reaches a predetermined level it drains back into the fish tank.


It’s simple to understand, easy to build and operate – and (within particular constraints) it can work.

It should come as no surprise, therefore, that the basic flood and drain system is the most commonly used backyard aquaponics system in the world.

Tom Speraneo inadvertently discovered that he could take a gravel grow bed (long used in hydroponics circles) and adapt it to:

  • capture and mineralise the fish solids.
  • facilitate nitrification
  • aerate the water
  • grow plants.

It all sounds very positive, so far. So, what’s the problem?

Well, there are several actually but, before we get into those, it’s appropriate that we should learn a bit more about how the Speraneo model came into being.

Aquaponics Biggest Mistake

Many people who are interested in aquaponics know that Missouri farmers Tom and Paula Speraneo popularised what is commonly termed as flood and drain aquaponics.

For the uninitiated, flood and drain aquaponics in its simplest guise comprises a fish tank and one or more media (usually gravel) grow beds.  Nutrient-rich water is pumped from the fish tank into the gravel grow beds before draining back into the fish tank.

What far fewer people know is how the Speraneos came to be involved in aquaponics and where the idea for their basic flood and drain system originated.

In the mid-1980’s, Dr Mark R McMurtry invented the Integrated Aqua-Vegeculture System (iAVs) – the first successful ‘closed loop’ production of vegetables using the metabolic wastes of fish.

iAVs comprises a fish tank and sand biofilters (in which the plants are grown).  It’s simple to understand, easy to build and operate – and it definitely works.

Following the completion of his PhD dissertation at North Carolina State University, McMurtry undertook a series of trips to showcase iAVs and its benefits for allied faculty staff, students and aquaculture industry professionals.

In December 1989, one such trip to Arkansas put McMurtry in contact with Tom and Paula Speraneo at the University of Arkansas in Little Rock.

A week later, he facilitated a 3-day interactive discussion/workshop at the Meadowcreek Project in Fox, Arkansas for the usual mix of faculty, staff, students and other interested parties – including the Speraneos.

The Speraneos returned home keen to construct an integrated aquaculture system based on what they’d learned from its inventor.

As it turned out, they weren’t able to afford the sand that was central to iAVs’ effectiveness, so they dug up their gravel driveway for use in their system bio-filter.

Let’s remember that the efficacy of iAVs relies on the use of sand (not gravel) so this was a significant change and one that would have serious implications for iAVs – and aquaponics.

Meanwhile, oblivious to the fact that his work was about to be usurped by a mistake, McMurtry had begun a promotional tour of sub-Saharan Africa and Middle Eastern countries.

When he returned, he became aware of the Speraneo’s substitution of gravel for the sand and he counselled them at length about their choice – but they persisted.  This aberration would subsequently be popularised as the flood and drain aquaponics system.

This “mistake” – subsequently to become wilful ignorance – was what best-selling author Malcolm Gladwell would later describe as a “tipping point” – one that would have profoundly negative implications for aquaponics.

The sand bio-filter is the heart of the iAVs “living machine.”  The substitution of gravel for sand impacted the design in several ways including:

  • a dramatic reduction in mechanical filtration capability
  • a dramatic reduction in soil microbial types and population numbers
  • reduced aeration of media bacteria and plant root zone
  • reduced nutrient utilization and system stability
  • a significant reduction in feed conversion rate and fish growth
  • increased capital costs with reduced fish and plant yields
  • increased risk profile
  • increased operating cost per unit of production

One of the key features of the iAVs design is its versatility.  A backyard farmer – or an impoverished villager – or a protected cropping greenhouse operator could use the same system design.

The first casualty of the change in media was iAVs‘ commercial potential.  The basic flood and drain system never gained commercial traction because gravel does not lend itself to the mechanisation and automation that is a feature of controlled environment agriculture.  Sand, by contrast, had been used in hydroponic greenhouse culture for decades – subject to all of the usual constraints associated with greenhouse culture.

The iAVs could be built and operated by a humble villager with some seeds and relatively little guidance.  The basic flood and drain system, by contrast, requires a connection to the grid, a pump (or two) and ongoing access to mineral supplements.   The basic flood and drain system also required greater skills and knowledge to offset the heightened risks that it poses.

As an aside, the Speraneos (who initially gave credit to McMurtry for their introduction to what was yet to become known as aquaponics), eventually used their utilisation of gravel as a point of sufficient difference (in their minds at least) to assume ownership of the concept.

This process of taking a system design and “tweaking” it (with a view to assuming ownership of the idea that underpins it), was to become a recurring theme in aquaponics.

Anyway, the Speraneos developed an information package and promoted their system through an Internet mail list (the fore-runner of the discussion forum).

Interestingly, when this information package first became available, purchasers were asked to agree (by way of a binding legal instrument) not to market their own information packages. It seems that the Speraneos were not keen to have done to them what they had done to McMurtry.

This requirement obviously lapsed at some point because, in 2005, Joel Malcolm bought the Speraneo’s information kit and “tweaked” it into an Australian context.  Australia’s ABC Gardening TV program ran a segment on Malcolm’s home-based system and the basic flood and drain system enjoyed a new surge in popularity.  Regrettably, however, the “new” flood and drain system had the same basic flaw – the media particle size.

Various other kit makers (including Murray Hallam and Sylvia Bernstein) adopted the Speraneo flood and drain system and, while they “tweaked” the model too, none of them managed to grasp the toxic tipping point – the gravel instead of sand.

To summarize, the substitution of gravel (or clay pebbles) for sand was not just a minor detail – it was the aquaponics difference between chalk and cheese.   The iAVs is a living machine whereas the basic flood and drain system is, given a convergence of common (indeed likely) events, a killing machine.

In terms of its filtration efficacy, McMurtry has characterized the use of gravel to capture solids in the biofilter as “attempting to catch BB’s with a basketball hoop.”

It’s important to understand that the difference between iAVs and the Speraneo model is much more than one being usurped by the other…or any philosophical notion.

The basic flood and drain aquaponics system was/is nothing more than a big mistake – an unfortunate mutation with nothing like the productivity, resilience and versatility of its iAVs predecessor.

Anyway, this manual is about aquaponics and, since iAVs is not aquaponics, it’s time to focus on the technical issues of the Speraneo model.

Earlier, I said that the basic flood and drain system relied on the gravel grow beds to:

  • capture and mineralise the fish solids.
  • facilitate nitrification
  • aerate the water
  • grow plants.

The simple fact is that the capture and mineralisation of fish solids in the gravel grow bed is at odds with the nitrification and aeration functions of the grow bed.

In other words, particulate matter consumes oxygen – and, in certain circumstances, inhibits the conversion of ammonia into nitrite and (subsequently) nitrate. The greater the quantity of this particulate matter, the greater the amount of oxygen that is required to deal with it.

For an understanding of how this happens, let’s hark back to what we said about ammonia when we looked at the aquatic nitrogen cycle.

“As the fish digest food, they produce solid wastes – and they include urea, uric acid and faeces. Uneaten food also contributes to the solid wastes in the system.

These solid wastes eventually yield ammonia – through a process known (not surprisingly) as ammonification.

The family of bacteria that facilitate this conversion of organic nitrogen into inorganic ammonia are called heterotrophs.”

Not bloody Heterotrophs again?

Heterotrophs are as essential to the operation of any aquaculture/aquaponics system as autotrophs – the nitrifying bacteria – however the relationship between the two types of microorganisms is not without its problems.

The first issue is the rate at which their numbers grow – relative to each other.   Autotrophs multiply relatively slowly – where heterotrophs multiply very rapidly.

This means that heterotrophs can overwhelm autotrophs – indeed eat them – to the point where nitrification is stalled.

OK, so what is likely to cause heterotrophs to multiply to the point where they might actually inhibit nitrification?

The answer is solid wastes – in the form of urea, uric acid, faecal matter and uneaten food.

More solids = more heterotrophic activity.

The other issue is that rapidly multiplying heterotrophs consume large quantities of oxygen from the water.

So, the problems for the fish are twofold – they can run out of oxygen and/or, in the event that nitrification is stalled, they’ll be affected by ammonia toxicity.

We’ll look at the role of oxygen in aquaponics, in depth, in the section titled “Managing Water Quality.” At this stage, it’s sufficient to know is vital to the survival and wellbeing of fish, plants and beneficial bacteria.

Once dissolved oxygen in the water drops to sub-lethal levels, fish begin to die – quickly. Even if they don’t quite reach that point, low dissolved oxygen levels stress fish – and stressed fish are more prone to disease and parasitic infestation.

In fact, low dissolved oxygen levels (or stressors arising from low DO) are the leading cause of fish deaths in aquaponics systems.

OK…so what’s the solution to the solids issue?

The best way to deal with sedimentary and suspended solids is to capture and remove them from the water column.

Now, this viewpoint flies in the face of aquaponics fundamentalists who argue that the solids contribute to the overall nutrient mix in an aquaponics system. They contend that the solids will be trapped in the grow bed, mineralized by composting worms and eventually become part of the nutrient mix.

While I don’t argue with the basic mineralization proposition, here’s why I suggest that solids be removed:

  • Bio-filters (including grow beds) function more efficiently when solids are removed.  
  • Both fish and nitrifying bacteria require oxygen. Fish wastes and uneaten food consume oxygen and, in extreme situations, will drive dissolved oxygen levels down to the point where fish can no longer survive.
  • Built up fish wastes create pockets of anaerobic (without oxygen) activity resulting in de-nitrification – the opposite of what we’re trying to achieve.
  • Grow beds will require less frequent maintenance if solids are removed. Regardless of how many worms you have in a grow bed, there will still be some sediment left in the bed. Over time, this sediment will (unless removed) build up and will eventually impair the biological functionality of the bed.
  • Solids irritate the eyes and gills of the fish – and stress them. Stressed fish become more susceptible to disease.
  • Solids can harbour harmful pathogens.
  • Working with clean grow beds (and clean hands) is a more pleasant task.

OK…but, by removing the solids, aren’t we wasting nutrients that would otherwise be available to our plants?

First, we need to understand that there are three types of solid wastes – dissolved, suspended and sedimentary. The dissolved solids – and the smaller fraction of the suspended solids – remain in the water and undergo ammonification and nitrification.

Indeed, up to 75% of the wastes produced by the fish in the system – having passed across their gills – are in the dissolved form. Put another way, up to 75% of the nutrients in the water are in dissolved form.

Second, the simple fact of removing the solids ought not infer that we are wasting them – quite the contrary.

The solid wastes can/should be processed so that they deliver up any remaining nutrients. The nutrient-rich water is then decanted from the sludge and returned to the aquaponics system.

The remaining sludge contributes nothing useful to the system. In fact, it can harbour harmful pathogens and irritate the fish’ eyes and gills and the best place for it is the compost heap or the worm farm.

OK… what prompted the confusion around the removal of solids in the first place?

A Matter of Dogma

Earlier I said that the basic flood and drain aquaponics system was the most commonly used backyard layout in the world.

That begs the question…“If it’s so problematic, why are so many people using it?”

Fair question…and here’s the answer…

  • Few people knew about it’s iAVs heritage. For several reasons, Mark McMurtry wasn’t around to defend the iAVs method and, while the Speraneos knew about it, they obviously decided that it wasn’t in their interests to press the facts around iAVs.
  • Its inherent simplicity appeals to people. It’s easy to understand, build and operate…and it works (right up to the moment that it doesn’t).
  • During its rise to worldwide prominence, kit manufacturers owned three out of the four largest aquaponics discussion forums in the world, and they promoted it as the aquaponics ideal. They exploited the fact that it’s easier to sell something if you don’t confuse the purchaser with all of the things that could go wrong.
  • Most of the people who set out to build the layout didn’t understand its pitfalls – they got caught up in the hype and simply didn’t know what they didn’t know.

As far back as 2007, I argued in support of the use of dedicated mechanical and biological filtration in media-based aquaponics systems.  I met with such a barrage of criticism from aquaponics fundamentalists that I built four basic flood and drain systems side-by-side.  Over a period of nine months, I trialled three Australian native species – and a diverse range of plants – in these units.

New System - 29 Dec 08 003 (Small)

The pretty picture belies the biological unhappiness that’s happening in the fish tanks.  This is the truth of the basic flood and drain system – it’s presentation as a sustainable way to grow food is largely an illusion.

To summarise the outcome, most of the plants grew very well.  Regrettably, the fish suffered almost from the outset.

Various 003

Plant production was no issue with the basic flood and drain systems that I built but, the presence of sedimentary and suspended solids means that there’s a heightened risk of disease and death for the fish.

To summarise…the basic flood and drain system is a very bad idea…particularly for fish. It was the product of wilful ignorance and it continues to be aquaponics’ biggest mistake.

For those who are attracted to the basic approach, my advice is to learn about the Integrated Aqua-Vegeculture System (iAVs), build the real deal…and reap the benefits.

The Improved Basic Flood and Drain System

For those who want to persist with gravel (or clay pebbles, lava rock or other coarse media) grow beds, I still recommend that they be thought of as hydroponics growing units to be attached to a recirculating aquaculture system.


The inclusion of mechanical and biological filtration will make a vast difference to a basic flood and drain system. From the fish’ perspective, it’s probably the difference between life and death.

So long as you have an effective means of capturing the sedimentary and suspended solids, you can utilise the gravel grow bed as a biofilter.  There are still advantages, however, to including a dedicated biofilter into your design and we’ll explore those in detail in the next section.

So, having put what I hope is a reasonable case for using a recirculating aquaculture system as the basis for all aquaponics systems, it’s time to head over to Chapter 5 – Designing Your Recirculating Aquaculture System.


In the meantime, I invite you to comment…to express any concerns that you may have…and to provide ideas or suggestions that you feel will improve the book – or add value to it.