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How to Lower Nitrates and Alkalinity in a Koi Pond

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Alkalinity, generally measured in ppm (parts per million), refers to the concentration of dissolved minerals, such as calcium and magnesium, in the water. The higher the alkalinity, the more difficult it is to maintain the proper pH of pond water.

Nitrates are produced from fish waste. Too high of nitrate levels can become toxic, making fish sick and/or encourage the growth of blue-green algae. For the health of the koi, it’s ideal to keep the alkalinity levels at 50 ppm to 200 ppm reports the Koi Club of San Diego and nitrates under 100 mg/L reports veterinarian Erik L. Johnson of the Lone Star Koi Club.

To lower both the nitrates and alkalinity in a koi pond at the same time you will need to do a water change.

Koi Pond Water TestingKoi Pond Water Testing

Test the pond water for alkalinity and nitrates. You can get testing kits at any aquarium or pond retailer. Follow the instructions on the package as different kits may have different instructions. If either of the levels are high according to the test results sheet on or including in the kit, then continue with a water change.

You can also buy the test kits you need at Amazon. The PondCare Master Liquid Test Kit tests for ammonia, nitrates, and pH. You can use the pool test kit by Aquacheck as an accurate measurement of alkalinity.

pond vacuumRegular Water Changes Will Reduce Both Nitrates and Alkalinity

Perform a partial water change in your pond. This involves removing about 10 to 20 percent of the total water in the pond and replacing it with fresh water. Treat the fresh water with a de-chlorinater before adding it to your koi pond as you would have when first filling your pond. You can use buckets or a pond vacuum to remove the water. 

Test the water again for alkalinity and nitrates after performing the water change. If the levels are in the proper range, then test again in a week to determine if another water change is needed at that time. If the levels are still too high, complete another 10 to 20 percent water change in 24 hours. Continue daily testing and partial water changes until the alkalinity and nitrate levels are within optimal range.

Test the pond water weekly and perform partial water changes as needed based on the test results. How often you need to complete a partial water change will depend on a variety of conditions unique to your pond. Some may need weekly water changes, while others need only monthly water changes.

References: How to Change Pond Water

Lone Star Koi Club: Nitrates

Koi Club of San Diego: Alkalinity

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