Nitrite is produced in the aquarium in the forms of fish waste, uneaten food, and by nitrifying bacteria in your aquarium's natural cycle as it breaks down ammonia.  Low levels of nitrite inhibit respiration and suppress the immune system, while high levels cause suffocation in fish.  The API® NITRITE TEST KIT reads nitrite levels quickly and accurately in your tank, and tests parameters from 0 to 5 ppm.