Nitrite / nitrous acid

Nitrite NO₂ and nitrous acid NO₂ are in an equilibrium that depends strongly on the pH value and also on temperature and salinity. The lower the pH value, the more the balance is on the toxic nitrous acid side.

Nitrite is formed in the water by incomplete nitrification (= biological oxidation of ammonia to nitrate) or by incomplete denitrification (= biological reduction of nitrate to atmospheric nitrogen). Especially during the start-up phase of an aquarium large amounts of nitrite can be generated. During the start-up phase of a Deni filter, high concentrations of nitrite can occur; however, due to the very low flow rate, only small amounts enter the aquarium.

Nitrite can be removed by improving nitrification through the installation of a biofilter or the introduction of live rock and/or live sand. The use of ozone also oxidizes nitrite to harmless nitrate.

Drop tests, test strips and photometer tests measure the sum of nitrite and nitrous acid. If the pH value is not known, these tests have no meaningful value - unless nothing has been detected. For an assessment of the toxic content of nitrous acid, at least the pH value must be known. The pH value should be measured at the beginning of the illumination period. A table or online calculator can be used to assess toxicity. Nitrite and nitrous acid cannot be determined with ICP. Also, high nitrite concentrations influence the nitrate measurement.

Nitrite / nitrous acid are toxic from approx. 0.1 mg/l at low pH values (freshwater aquarium). In marine aquariums above pH 8, concentrations up to 10 mg/l can be tolerated by fish.