Better Science: Aquaponics and the Nitrogen Cycle

What is an appropriate nitrogen level for a freshwater fish tank? When checking the nitrogen level of our aquaponics system yesterday, we wondered the same thing. Unaware if our nitrogen level results of 20 ppm indicated anything good or bad, we traveled back in time to those days of high school biology and chemistry and did some research on the nitrogen cycle.


The nitrogen cycle, like our aquaponics setup, starts with fish poop. The fish poop decays into ammonia, an incredibly toxic substance. Bacteria living in the water (nitrosomonas) then eat the ammonia creating a byproduct of nitrites - also a very toxic substance. Then another kind of bacteria (nitrobacter) consumes nitrites in the water, creating a byproduct of nitrates. Now, this is where things get healthy.

Nitrates are good. We want nitrates, especially when working with hydroponics, because nitrates are a fertilizer. Plants and algae thrive when the nitrate levels are high—which would explain why the herbs and lettuces we're growing over our fish tank are green and growing. Healthy plants in (or above!) a fish tank are an excellent indicator that the nitrogen cycle is acting as it should.
After the plants consume most of the nitrate, the freshly and naturally filtered water recycles back into the tank and the fish don't swim in clean and clear water. Not to mention we get freshly grown herbs and salad greens out of all of this, as well...thanks, science!