Nitrates are a natural byproduct of animal and fish waste. Bacteria convert fish waste into ammonia, which is converted into less-harmful nitrites, which are then converted into nitrates that feed algae and other microorganisms. Too much nitrate in your pond will cause it to become clouded with algae and could harm your pond fish. Managing nitrates in your pond is an ongoing maintenance task, but the work can be minimised with a variety of filtration methods.
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 build-up 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.
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 per cent 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 socket. 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.