Nitrites are the second stage in the nitrogen cycle. Nitrifying bacteria are readily available in the tank, and will build a colony as soon as the nutrient source (ammonia) is available.
While ammonia is converted predominantly by the species of nitrosomonas, nitrobacter is mainly responsible for converting nitrite into nitrate. When a new tank is set up, the nitrogenous compounds will rise to high levels. This enables the bacteria to form a colony and start the conversion process (nitrogen cycle).
Nitrosomonas and nitrobacter are aerobic bacteria and need a constant flow of oxygen in order to survive and perform their tasks.
Aquarium nitrite levels should be at an undetectable level at all times after the tank has fully cycled. While not as dangerous as ammonia, nitrite is still a highly toxic chemical, and causes stress for fish even at levels as low as 0.5 ppm. Levels exceeding ten to twenty ppm are lethal over a period of time. Immediate action is required if high nitrite levels persist after seven to ten days.
Nitrite interferes with the oxygen metabolism by destroying the hemoglobin (oxygen-carrying cells) of the fish and aquatic livestock.