Nitrate / Nitric acid

Nitrate NO₃ and nitric acid HNO₃ are in an equilibrium that depends strongly on the pH value and further on temperature and salt content. The lower the pH value, the more the balance is on the side of the toxic nitric acid. In everyday aquarium life, however, nitric acid plays no role, as the pH value would have to be extremely low to have a toxic effect.

Nitrate is formed exclusively by so-called nitrification, in which ammonia/ammonium is oxidised to nitrate via nitrite. There are a few ways to remove it: see under Links: Checklist Nitrate.

What do too high nitrate values indicate?
Excessively high nitrate values indicate that the nitrogen cycle is not functioning fully. Nitrate is only "unwillingly" - because it is energy-intensive - taken up by corals, algae and zooxanthellae through nitrate assimilation. Zooxanthellate corals tend to produce more zooxanthellae (symbiotic algae) at higher nitrate levels: the corals turn dark (brown). To prevent this, the nitrate concentration should be SLOWLY lowered. However, it should not disappear completely.

What do too low nitrate values indicate?
Low nitrate values first indicate that the nitrogen cycle is functioning, on condition that neither ammonium/ammonia nor nitrite can be detected. If nitrate is no longer detected, it is uncertain whether the corals are supplied with nitrogen. They are normally supplied by the uptake of ammonia/ammonium, by plankton (particulate organic nitrogen = PON) or by dissolved organic substances (DON = dissolved organic nitrogen). It is hardly possible to make a statement about the uptake of plankton in the aquarium - there is rather no plankton present (compared to natural reefs). Feeding replacement plankton and dosing organic products can ensure the nitrogen supply, but this cannot be checked on the basis of water values. Only the nutritional status of the corals is informative, but difficult to recognise, especially by beginners. Indirectly, nitrate can be used to determine whether there is a nitrogen deficiency or not: if nitrate is still detectable, there is no lack of nitrogen.

In drop tests, test strips and photometer tests, nitrate is measured almost exclusively - nitric acid plays no role. Nitrate and nitric acid cannot be determined with ICP. High nitrite concentrations also influence the nitrate measurement.

In principle, nitrate is non-toxic. Several g/l are tolerated by the animals. However, high nitrate concentrations have a fertilising effect and can lead to algae and cyanobacteria plagues.
SPS aquariums: maximum 5 mg/l nitrate (but it should be detectable).
typical reef aquarium, soft corals: 10...50 mg/l nitrate
Fish aquarium: maximum 50 mg/l nitrate