Nitrite
Nitrite is formed by the conversion of ammonia by nitrifying bacteria. Nitrite is toxic to fish because it binds with the hemoglobin in fish's blood to form methemoglobin. Hemoglobin carries oxygen through the body while methemoglobin does not, so fish in high nitrite waters may suffocate even if sufficient oxygen is present. If enough methemoglobin is present in a fish's blood, it will cause the blood to appear brown instead of red. Consequently, nitrite toxicity is often called "brown blood disease."
If nitrite is present, a water change should be performed. If the fish species in the tank will tolerate increased salinity, salt can be added to the aquarium to increase the level of chloride in the water. Chloride levels should be at least six times greater than nitrite levels. Chloride prevents the uptake of nitrite by the fish's blood thus eliminating the effects of brown blood disease. However, the addition of salt/chloride does not remove nitrite from the system, and the source of the nitrite must be controlled.
After the cycling period, nitrite should be tested once per week. Once established, a well-managed aquarium should have no detectable levels of nitrite. If nitrite is present, management of the aquarium should be evaluated to ensure:
- The biofilter is large enough.
- The biofilter does not need cleaning.
- The aquarist is not overfeeding (based on what the biofilter can handle, not what the fish will consume).
- The tank is not overstocked.
- The tank is not excessively dirty.
- The biofilter is not too clean (vigorous cleaning of the biofilter will remove nitrifying bacteria).
- The alkalinity or pH has not dropped and negatively affected the biofilter.
- No chemicals have been used that have killed the bacteria in the biofilter.