Fluval Test Kits accurately measure water parameters to help you maintain a balanced and healthy environment for your fish and plant life. It’s important to always test your source water as it may contain pollutants hazardous to your aquarium.Nitrite develops when beneficial nitrifying bacteria break down ammonia. This becomes extremely toxic to fish when high levels of nitrite persist in the aquarium for more than a week. Newly established aquariums are prone to higher longer lasting levels of nitrite. Densely populated aquariums with marginal filtration will exhibit persistent nitrite levels, improved filtration, proper maintenance and regular testing for nitrite is recommended. Test weekly to ensure nitrite is detected.