Nitrite is produced by the biological filter. Beneficial bacteria in the biological filter convert toxic ammonia into toxic Nitrite (NO2-). The biological filter then converts nitrite into less-toxic nitrate (NO3-). If not removed, nitrite will prevent fish from carrying on normal respiration.

In a new freshwater aquarium, the nitrite level may surge to 10 ppm (mg/L) or more, and then fall rapidly as the biological filter becomes established. The nitrite will be converted to nitrate. This process may take several weeks. In an established aquarium, the nitrite level should always remain at 0 ppm (mg/L). The presence of nitrite indicates possible over-feeding, too many fish, or inadequate biological filtration

The level of nitrate acceptable to fish and invertebrates is a subject of much debate. All agree that "lower is better". A nitrate level of 40 ppm (mg/L) or less is recommended for freshwater aquariums. Many marine aquarists prefer to keep nitrate levels as low as possible, especially when keeping invertebrates.

Partial water changes reduce nitrates. Other rechargeable filtration material is also available.