Test nitrate levels using the test kit according to directions.
Check that you have a large enough filter for the pond. Use activated carbon as the filter media and replace it regularly to reduce nitrate levels.
Fill buckets with tap or rain water. Fill with 10 percent of the volume of the water in your pond. For a 1,000-gallon pond, for example, you will need 100 gallons of water. Leave for at least 24 hours to dechlorinate if using tap water.
Remove 10 percent of the pond water with an empty bucket. You can use this water for your lawn or garden.
Add the standing bucket water to your pond. Repeat this process once a week.
Introduce more aquatic plants, which naturally remove excess nitrates from the pond.
Ensure you do not overfeed the koi. Don't give them more food than they can eat within about 5 minutes.