Determine Nitrate Level
Monitor your pond visually. If the water is cloudy or you notice excess algae growth, your nitrate levels are probably high. This is a result of the natural nitrogen cycle. In natural ponds, there are enough plants to use up excess nitrates and keep the water balanced. Many backyard ponds are heavily stocked with fish rather than plants, which can lead to a buildup of nitrates.
Purchase a pond water test kit. Be sure the kit you buy specifically tests for nitrates. Many basic pond test kits only test the pH of the water, or test pH and ammonia levels as these are the two elements most likely to be toxic to fish.
Follow the manufacturer's instructions to test your pond water for nitrates. Typical ponds have nitrate levels between 50 and 100 parts per million (ppm). Nitrate levels of up to 200 ppm are generally acceptable. Perform a water test every two to four weeks to monitor nitrate levels in your pond.
Lower Nitrate Levels
If your water test or visual inspection indicates high nitrate levels, check your pond filtration system. It may be clogged or the filter may need to be replaced. You may need a larger filtration system.
Perform a partial water change. Use a water pump for large ponds or buckets for a smaller pond. Remove 10 to 15 percent of the water from the pond. Replace it with an equal amount of distilled water at ambient temperature. This will dilute the water and disperse the nitrates.
Add aeration to your pond. Connect several pond-sized air stones to an air pump and distribute them throughout your pond. Connect the pump to an electrical outlet. You should see air bubbles floating up through the water from the air stones.
Spend a few minutes every day scooping leaves and other debris out of your pond with a long-handled net. This will help keep the nitrate levels in check and will beautify your pond.