Nitrite toxicity


Nitrite elevated >2ppm
Nitrite levels are 2ppm or above and the lower the pH the more toxic the NO2 will be.
Gills brown/red
Gills have a dark brown or dark red colouration.
pH water low <6.5
A few species of freshwater fish (Discus) can tolerate lower than 6.5 pH.  If lower than this and problems, suspect this a possible cause.
Respiration rapid
Fish is breathing very fast, much faster than is normal.
Breathing at surface
Fish stays at surface usually at an inclined angle and breathes or sucks air.
Hanging at surface
Fish stays at water surface all the time.
Listless and lethargic
Fish moves very lazily in water, much less than normal, lacks interest.


Elevated levels of nitrite. Cause is inadequate biofilter, so that the Nitrobacter bacteria which convert NO2-NO3 are not yet present in sufficient numbers. Dead Fish, excess food or overcrowding, can also raise nitrite levels.


Important to find and rectify cause same. Nitrite is more toxic at lower pH values, so in fresh water problem is likely to be more severe. Can bring on latent diseases if not tackled rapidly. Should not be a problem, in well run farms or aquaria.


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