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Nitrite (NO2) is the toxic by-product of the nitrifying bacteria (Nitrospira) in a filter or substrate consuming Ammonia.

It is only mildly less toxic than Ammonia but it still can kill aquatic animals if its levels get too high.

Like ammonia, the toxicity of nitrite is related to pH.
Two forms of nitrite are present in water: the nitrite ion (NO2-) and the more toxic nitrous acid (HNO2). The amount of each of these that will be present is pH dependent and as the pH decreases the HNO2 form prevails and is therefore more toxic. The form HNO2 can diffuse freely across gill membranes and is much more toxic than the nitrite ion.[1][2]

Nitrite poisoning is also known by aquarists as Brown Blood Disease.

Nitrite damages the nervous system, liver, spleen, and kidneys of fish and other aquatic animals. Even low concentrations of 0.5mg/l over extended periods can cause long term damage. Nitrite binds the oxygen carrying hemoglobin in blood therefore fish can suffocate even if the oxygen in the tank is sufficient.

Given time (normally 3–4 weeks) in a normal process of a new tank cycling, nitrites are converted into the much less toxic nitrates by the nitrifying bacteria. However if the levels of nitrites do not come down, then the nitrites will cause the animals to struggle for oxygen as the nitrites damage the gills of fish and will cause long term damage to their immune systems and stress them greatly.

  • Methylene blue helps nitrite stricken aquatic animals by helping them to carry oxygen in their blood and can prevent deaths. See link for more detail.

Lethal dosage[edit]

10–20 mg/l (ppm) is considered lethal. Species vary in their tolerance. If you can measure it, it's harmful as at low levels it damages internal organs.

Preventing Nitrites[edit]

  • Get a nitrite test kit so you know where you stand!
If levels are not going down, then you may need to do several things:
  • Check your filter is not blocked or has reduced flow. This will kill off your nitrifying bacteria (the brown mess) in the filter.
  • Do a large water change to dilute the nitrite levels.
  • Add some filter bacteria from either an established filter (ask a pet shop) or from a commercial bottle.
    Examples are:
  • Add some salt. See Salt article for dosage.
  • Add a chemical treatment that removes or neutralises the nitrites.
  • Reduce feeding until levels have dropped to zero.
  • Increasing aeration of the water will help a little.
  • Do not overstock your tank. Remove some animals if need be.


  1. Chen, J.C., Cheng, S.Y., 1999. Recovery of Penaeus monodon from functional anaemia after exposure to sublethal concentration of nitrite at different pH levels. Aquatic Toxicol. 50, 73-83. See ScienceDirect
  2. Seachem discusses Nitrite toxicity versus pH