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.”
“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…..so 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.
* Salt build up
If salts removal is a key reason for water replacement and you transfer that to a garden, over time you are also just transferring your problem.
JohnMc….When we talk about salts, we’re not just talking about sodium. In any case, the different types of plants that grow in soil-based gardens will use some of those salts – and rainfall flushes salts (of various types) from soil-based gardens.
For five years now I haven’t done any water changes with my AP system. 300 gallon stock tank buried in the ground up to the rim, eight half barrels (4 – 55 gallon blue barrels) filled with #57 gravel, 15 lake bream, numerous tomatoes, squash, banana peppers, carrots (they look funky but they do grow in gravel), broccoli, spinach, buttercrunch lettuce, strawberries, and a crazy growing rose bush (Yeah, in the AP system!), and I’m sure there are a few things that I’ve forgotten to mention.
The point is… no water changes. If you have a properly balanced system, there shouldn’t be any need for water changes. The rain donates to the system and I’ve had to top up the water during the summer months where there wasn’t any rain.
I wonder however if the water replacement thoughts come from people who have raised fish over the years and have become occustomed to the water change requirements.
Everyone is different of course. Just grow something. Doesn’t matter about the little things. Just grow your own food. Tastes good doesn’t it?
That’s my opinion and I’m sticking to it.
Mr Bill….With respect, the fact that someone has been doing something for years without change, has never impressed me as a reason for not striving for something better. I usually stock fish at higher densities (for reasons of sustainability as much as anything else), and water quality is a real priority for me.
“Balance” is a word that is often used to resist the use of dedicated mechanical and biological filtration….and other water quality initiatives…..like water replacements.
As I never tire of pointing out…..you can run a system without dedicated filtration (or water replacement) but you’ll get greater productivity, resilience and versatility if you do.
I have been running an aquaponics system for approx one year. During this time we have harvested many crops of vegetables and currently we are harvesting our Jades, which have reached a size of 750to 1000gm. our system runs 12 growbeds located in a 17x6m greenhouse with the fish living in a 3000l s/steel dairy tank. Total water capacity is approx 4500l. I agree with most of your comments for instance I clean grow beds on change of crop (rotating crops around the different beds). I change approx 25% of the water each week, using the waste water to irrigate fruit trees and vegetables in the adjacent garden.I do not rely solely on grow bed filtration , utilising a mechanical filter housed in a 44 gallon drum.
The most difficult problem continues to be pest control in the greenhouse, particulary thrips and slugs. Currently I use small amounts of pyretheum and some yates bacterial control sprays, however these only limt the problem…any ideas?
Keith Lay….My system designs all require that the fish production be able to be separated from the plant growing units. Pest management is one of the main reasons. It’s all very well to talk about integrated pest management (an idea that I like incidentally) but, when you’re faced with the choice between reaching for the pyrethrum or getting nothing for your investment of time and other resources because insects have laid claim to it….it’s time to remember that you can’t eat principles. There are lots of organic remedies that will still kill fish so being able to isolate your fish and growing systems is a vital part of being able to hold your own with insect pests.
Keith, have you tried beer baits for slugs?