Nitrate is the final product of the biological breakdown of ammonia. The presence of nitrate in a pond is a good indicator that the filter is functioning and the bacteria are removing toxic ammonia and nitrite from the water. Sources of nitrate could also be from tap water or run off from general garden fertilizers.

Nitrate is a vital food for plants as it helps aid their growth and development. It is recommended that nitrate levels should not exceed those in the tap water supply by more than 50 mg/l, and while nitrate is not toxic to fish, it can feed problematic algae in the pond. High nitrate levels can be reduced in the pond by dilution from partial water changes (tap water in the UK contains nitrate, so care must be taken that the nitrate levels are not being increased by water changes). Nitrate is also removed by plants, so increasing the plant density will aid in its reduction.