Quote Originally Posted by Apollo

Does this mean that you're going to do water exchange's to get you nitrates below 80 ppm? My test kit goes up to 140 ppm...is water exchanges better for the fish than reducing there food supply?
What I was referring to is that the API test kit does not give an accurate nitrate color representation once you get above 40 ppm. In order to place the nitrates in the color range of 40 ppm or below you need to dilute your test water with non-nitrate water and multiply the results by the dilution ratio. As an example, if you dilute the fish tank water in the test tube by a ratio of 4:1 with distilled water (4 parts total, 3 distilled, 1 FT water) then you would multiply the measured results by 4.

Once the nitrates reach a steady state equilibrium between the amount of food you are feeding the fish (that ultimately produces the nitrates) and the amount of nitrate uptake by the plants, reducing the fish food supply we eventually reduce the amount of nitrates in the water, assuming there are no other changes, like a change in the number of plants or their size. So, adjusting the amount of fish food over a given period of time is a good way to regulate your nitrate levels in your aquaponics system, if you have enough fish. Fish have a very wide feeding range.

This places the primary role of the fish as a nitrate producer rather than a food supply for humans. By changing water instead of adjusting the feeding rate and keeping it at maximum grow-out daily amount, you are giving the fish a primary role as a food supply to be later harvested. They can be both, but they have only one primary role; which can change over time like while cycling your system.

Adjusting the fish food supply is a good way to control your water chemistry while your plants are maturing, and even after. Doing a water exchange will also accomplish the same thing but that may not be a good or healthy environmental option. You can always dump some of the water onto an outdoor garden or other non-aquaponic plants to help with the use of the discarded water, but only if your nitrates are too high.

Oliver