Fish waste together with uneaten food and dead fish or aquatic plants are decomposed in a process known as the Nitrogen Cycle. This process starts by the creation of ammonia and in a nut shell, bacteria convert ammonia to Nitrites and another bacterium converts Nitrites to Nitrates. For more information on the effects of ammonia on aquatic life and products that may be used for treatment of high ammonia levels, have a look at the Water Quality Category.
Nitrite (NO2) is formed during the second stage of the Nitrogen Cycle and is harmful to pond fish. Concentrations as low as 1.6 mg/liter of water can adversely affect your fish, causing them to gasp at the water surface and become very lethargic. Severe cases or long term exposure to nitrite can be fatal to pond fish. It is vital to the health of you pond fish to eliminate all nitrites. Nitrites should be tested for regularly and test kits can be found in the Other Helpful Products Category.
Nitrate (NO3) is continually produced by an established, fully functioning biological filter. In the pond nitrates are consumed by plants and algae as a source of food. If too few pond plants are present or the pond is overstocked or the fish are overfed, nitrate levels can raise to a harmful concentration. Nitrate levels greater than 50 mg/liter of water can be harmful to your fish and encourages unsightly algae growth such as free floating microscopic algae, making the pond look like pea soup or string algae that needs to harvested and/or treated. Although nitrates are far less toxic to pond fish, testing for nitrates can help diagnose other problems and help to create an established aquatic ecosystem. Nitrate test kits can also be found in the Other Helpful Products Category.
To find products to help treat ammonia buildup, check out Pond Maintenance > Water Treatments > Routine Maintenance