Nitrates

Nitrate (NO3-) is produced through the conversion of ammonia and nitrite into nitrate. A high nitrate level indicates a build-up of fish waste and organic compounds, resulting in poor water quality and contributing to the likelihood of fish disease. Excessive nitrate also provides a nitrogen source that can stimulate algal blooms. 

Maintaining a low nitrate level improves the health of fish and invertebrates. A nitrate level of 40 ppm is recommended for freshwater aquariums. 

(Ref. 2), (Ref. 6)