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Mythconception #10 – Water Replacement

Arguably, the biggest benefit afforded by aquaponics is its capacity to produce two crops – plants and fish – for the same amount of water that it took previously just to grow the plants.

In their quest to persuade others of the water-wise qualities of aquaponics, some AP evangelists get a little carried away – to the point of claiming that you never need to change water in an aquaponics system.

One “internationally-recognized aquaponics expert” states in her book….

“In aquaponics, you never replace your water; you only top it up as it evaporates and transpires (evaporates from the leaves of the plants).”

….and a bit further on….

“Aquaponics, on the other hand, is a closed recirculating system.  The only water that leaves the system is the small amounts taken up by the plants (some of which transpires through the leaves) or that evaporates from the top of the tank.”

…..and then….

“Because the fish are constantly excreting waste, and that waste is constantly being digested and converted by the bacteria and worms, and because there are no salts to build up, there is no need to ever dump and replace the nutrient solution.”

The Case for Water Replacement

My view is that some water (aside from that required to top up water lost to evaporation and transpiration) should be changed out in the interests of the health and well-being of the fish.

In a recirculating aquaculture system, water is changed out for several reasons including:

  • Management of nitrate levels.
  • Management of pH
  • Removal of micro-solids.
  • Removal of dead bacteria.
  • Salt build up

Replacement of water is a common way to manage water quality……and, depending on the situation, this may amount to replacement of up to 10% of the system volume per day.

In an aquaponics system, plants take up nitrates – so they are less of an issue…..but the micro-solids, dead bacteria and salts remain…..and there’s another issue that’s specific to aquaponics – that of nutrient imbalances.

Different plants take up different nutrients. Different nutrients are available (or not) depending on the pH level of the water in the system. Too much of some nutrients locks out other nutrients.

So, the case for water replacement is valid – if only to avoid nutrient imbalance.

OK… the question then becomes…..”How much water should be replaced?”

As little as 2 – 3% per day is probably adequate to keep things fresh.

Now, you could measure all of that stuff carefully……or you could adopt a much simpler and less expensive approach…..and just use some of the water in your aquaponics system for watering your soil-based gardens, trees, pot plants – or for any other purpose for which you would otherwise use fresh water.

Using my earlier suggestion of 2 – 3% per day…..and taking a 1000 litre system as an example….would mean that we would use 20 – 30 litres per day……or about 150 – 200 litres per week. A very modest garden would absorb that amount without wasting a drop……and your fish will be better off for it, too.

To summarise……The whole water replacement issue is a bit like the solids removal argument.  You can build and operate an aquaponics system without dedicated filtration……and yes, you can run an aquaponics system for a period of time without changing water – but the simple fact of the matter is that replacing water (like using dedicated filtration) enables you to achieve greater productivity, resilience and versatility out of your system – than if you don’t.


15 Good Reasons to Have a Biological Filter

Biological filters, used in conjunction with effective mechanical filters, are the means by which people like you and I are able to grow our own fish.  In short……no bio-filtration; no aquaponics!

Biofilters are simply good places for nitrifying bacteria to live and play.  They provide the optimum environment for the bacteria that convert the potentially toxic wastes produced by the fish into organic plant food.

Some aquaponicists rely on the grow bed media…or the wet surfaces in their raft systems…to provide biofiltration but I highly recommend dedicated units.

Dedicated biofilters will make any aquaponics system more resilient and more productive. There are good sound reasons for having them…including:

1.   They provide for much more nitrification than a grow bed of a similar size – simply because they have a continuous flow of water through them as distinct from the intermittent flow of a flood and drain grow bed.

2.  They weigh much less than a grow bed or grow tank.

3.  They enable the staged development of an aquaponics system.  The fish tank can be set up, cycled and the fish added……and then the growing systems can be added as time and resources permit.

4.  They are much cheaper to build than another grow bed – good for those with a limited budget.

5.  They have a much smaller footprint than a grow bed – good for those with limited space.

Trickling bio-filters come all shapes and sizes

Biofilters come in all shapes and sizes

6.  They will accommodate a wider choice of media.

7.  They offer greater portability – good for people who rent their homes or who are anticipating a move.

8.  They permit greater control over overnight temperature variations in an aquaponics system.  Some people shut their pumps down overnight to limit heat loss out of their system.  While I understand why they do this, it’s a risky practice. The insertion of a dedicated biofilter into their water column will allow these people to disconnect their growing systems without prejudicing nitrification and oxygenation during the period of the day that it’s most needed.

A wide variety of media can be used.

A wide variety of media can be used.

 9.  They facilitate over-wintering more readily.  Fish can be moved indoors to avoid extreme weather….cold or hot.

10.  They are much easier to maintain than grow beds.

11.  They enable the separation of the fish tank from the growing systems in the event of disease or infestation in either fish or plants – which means that one can be treated without doing harm to the other.  

12.  They provide for more effective aeration – largely due to the continuous water flow.

13.  They are more convenient for ancillary small systems like fingerling system purge tanks or quarantine systems.

Trickling bio-filters can be used indoor or outside.

Biofilters can be used indoors or outside.

14.  They enable higher stocking densities – particularly when coupled with solids removal devices.

15.  They enable the integration of a wider range of growing systems – aquaponic or soil-based.

Biofilters come in various forms but my personal favourites are the humble (inexpensive) trickling biofilter – or the more sophisticated (and more effective) moving bed bioreactor.

Given the advantages that they offer (and in the absence of any good reason for not having them), I strongly recommend that they be a feature of any aquaponics system.



Integrated Aquaculture

Imagine being able to produce your own freshwater fish and salad – simultaneously – in your own backyard.

Well, you can…..using integrated aquaculture.

The three important elements in any integrated aquaculture system are plants, fish and beneficial microbiology. Put simply, you feed the fish, the microbes turn the fish wastes into plant food and the plants clean the water for the fish.

The best known manifestation of integrated aquaculture is aquaponics – the combination of intensive aquaculture and hydroponics.  Aquaponics is not, however, the only way to integrate the production of fish and plants.

While the fish and plants are the visible elements of the integration, it’s microbiology that makes it all possible.  While this microbiology is very complex, the aquatic nitrogen cycle is easy to understand and is the part that allows for  the conversion of toxic fish wastes into plant food.

Nitrogen Cycle

The waste produced by the fish breaks down to produce ammonia.

When the ammonia levels in the fish tank reach a certain level, bacteria (Nitrosomonas) begin to colonise the system. As the numbers of these bacteria build, the ammonia (NH3) is converted to nitrite (NO2). As the ammonia levels drop, the nitrite levels increase. The nitrites (like ammonia) are toxic to fish.

When the nitrite levels in the water reach a certain point, other bacteria (Nitrobacter/Nitrospira) begin to colonise the system. These bacteria convert nitrites to nitrates (NO3), which are far less harmful to the fish.

While the microbiology associated with aquaculture is complex, the equipment needed is very straightforward.

To produce freshwater fish in your backyard, you’ll need:

  1. A fish tank
  2. A pump and some fittings
  3. Mechanical and biological filtration

That’s it! These three components comprise a basic recirculating aquaculture system. You just add water and some fish……and start doing some water tests.

The first successful closed loop integration of fish and plants was called the Integrated Aqua-Vegeculture System (iAVs).  It was invented by Dr Mark R McMurtry in 1985.

In its simplest iteration, iAVs consists of a fish tank and sand bio-filters. 

Subsequent developments saw the emergence of what became known as aquaponics…the integration of recirculating aquaculture and hydroponics.  

Aquaponics comes in many forms but the dominant systems are:

  • gravel culture – flood and drain aquaponics
  • deep water culture – raft aquaponics

Both recirculating aquaculture and hydroponics create a waste stream.  In a conventional recirculating aquaculture system, nitrates are removed through water replacement where a predetermined volume of water is dumped each day. In a conventional hydroponic system, inorganic salts are used to provide nutrients for plants. Once the nutrient levels drop below a certain level, they are also dumped. In both situations, the wasteful disposal of nutrient-rich effluent creates environmental issues.

When they are integrated, however, the waste streams are reconciled – to the benefit of both fish and plants.

Integrated aquaculture is not limited to iAVs and aquaponics. There are also a number of soil-based options.

Since they all grow plants, the choice of a particular system boils down to personal preferences and the availability of resources.

Regardless of the method used, integrated aquaculture differs from conventional horticulture in a number of important ways.

The first (and most obvious) distinction is the source of nutrients – the fish.  Integrated aquaculture effectively provides two crops – one is fish and the other plants – for the same volume of water that it would otherwise take just to grow the plants.

The other very important difference is that, since chemical herbicides or pesticides are toxic to fish, they cannot be used in recirculating systems.  Clean chemical-free food is the result.


The list of Australian freshwater fish that can be produced in a backyard includes:

  1. Barramundi
  2. Murray Cod
  3. Silver Perch
  4. Jade Perch
  5. Sleepy Cod
  6. Eel-tailed Catfish

Freshwater crayfish include Yabbies (Cherax Destructor), Redclaw and Marron.

Every region throughout the world has its own freshwater fish species.

Pelletised rations, specially formulated for native freshwater fish, are available from fodder stores.


Integrated aquaculture lends itself to virtually any plant…particularly food plants.  The specific method will vary according to the type of plants being grown.

I’ve been engaged in integrated aquaculture since 2005…and I’ve written hundreds of articles on the subject.  In fact, I self-published what was arguably the first book on the subject in the world (The Urban Aquaponics Manual) back in 2007.  This material is being reviewed and will be available on this site.

This article was first written in 2009.  It was reviewed in September 2017.

The Origins of Microponics

In the beginning (about 40 years ago), there was Integrated Aquaculture.

The New Alchemists were probably the people most responsible for integrated aquaculture as we currently know it.

Aquaponics is an offshoot of Integrated Aquaculture……with a particular focus on fish and plants.

When I first set out to develop a concept of small-scale integrated food production, I called it Integrated Backyard Food Production (IBFP).  Like aquaponics, it’s a descendant of integrated aquaculture.   With its inclusion of micro-livestock, however, IBFP has retained a more direct relationship with integrated aquaculture.

Integrated Backyard Food Production became too much of a mouthful and so it became Microponics.  The name suggests its own origins…..the combination of micro-livestock, micro-farming and aquaponics.  I subsequently discovered that the name is also used to describe an obscure grafting method using in hydroponics.

Microponics differs from mainstream aquaponics (aside from its integration of micro-livestock) in a number of ways and we’ll explore those in detail in subsequent posts.

The conviction that Microponics might be of interest to other people arose from experiences that I had on the Aquaponics forum that I co-hosted.

From the outset, I encouraged discussion about “aquaponics and related food production matters”……including chickens, ducks Japanese quail, rabbits and other micro-livestock. 

Interestingly, one of the most popular threads on the forum was one on Japanese quail. Several other non-aquaponic threads (meat chickens, Black Soldier flies, etc) also attracted substantial patronage.

This satisfied me that there were other people out there who (like me) were interested in food production in a broader (and more traditional) context than just aquaponics.

Microponics is for those people.