Hello. Interesting you say you have no nitrate in your tank water. I've been in the fish keeping business quite a few years and never heard of this. Since nitrogen is a major part of the air we breathe, there would be a trace in our water. Our water here can contain up to up to 20 parts per million In most parts of the US, the water can contain nitrate levels up to 40 ppm. Natural water still has some nitrates, around 5 ppm. I haven't tested my water in several years, I never found the need to know the chemistry end of things. I never found it important to keeping a healthy tank.
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The nitrogen in the air we breathe is nitrogen gas and this does dissolve in water as N2 gas. This is not the same as nitrate, NO3.
We measure the three nitrogen compounds - ammonia as both free ammonia NH3 and ammonium, NH4 combined; nitrite as NO2; and nitrate as NO3. Of these, we aim to keep ammonia and nitrite at zero and nitrate below 20 ppm, and as low as possible. Ammonia and nitrite harm fish quickly; nitrate is more long term.
Ammonia is excreted by fish and other micro-organisms. Where a water provider uses chloramine as disinfectant, this also provides ammonia when dechlorinators split up chloramine into ammonia and chlorine. Bacteria and aquatic plants take up ammonia so there should never be any detectable ammonia in a fish tank.
Nitrite is made from ammonia by bacteria. More micro-organisms take up nitrite and turn it into nitrate. Plants do not make nitrite, they turn ammonia into amino acids then to protein.
Nitrate in fish tanks comes from two sources - made from ammonia via nitrite by bacteria and in the source water. In a planted tank, very little nitrate is made as plants remove ammonia and don't turn it into nitrite so there's none to turn into nitrate. In a non-planted tank, all the ammonia made by the fish etc ends up as nitrate.
Many fish keepers battle nitrate as they have almost the legal limit in their tap water. Those of us lucky enough to have very low nitrate in our tap water have little trouble keeping nitrate very low; and in a planted tank that can be as low as zero as the ammonia made by the fish is not turned into nitrate.
Aquatic plants use ammonia in preference to nitrate as they have to turn nitrate back into ammonia to process it, and this takes energy. It is more energy efficient for them to use ammonia and only use nitrate when ammonia is used up. Terrestrial plants prefer their nitrogen as nitrate which is why these can be grown with their roots in a tank to lower nitrate.
The test kits we use cannot detect trace amounts of ammonia, nitrite and nitrate in tank water - or tap water. There will always be undetectable amounts in our tanks.
