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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.