Nitrate is the final product from the breakdown of ammonia released by the fish.
Nitrate is not especially harmful to freshwater fish but is a potent plant fertilizer and can contribute to the growth of unsightly and unwelcome algae, such as green water or blanketweed. Ideally, the levels of nitrate in the Koi pond should be controlled to help reduce the likelihood of these unwelcome algae blooms occurring. It is recommended that a Nitrate Test Kit is used to determine the quantity present in the pond and control the concentration through water changes.
You might also be interested in the following items or information: