Koi fish are delicate creatures that need water with a specific pH. Anywhere between 6.8 and 8.2 pH is acceptable. However, koi fish will do best in water with a 7.5 pH. Unfortunately, many things can interfere with this careful pH balance, including poor maintenance, pond plant growth, or lawn products washing into the pond. And, there is often high pH in a new pond, which, ideally, should be adjusted before the koi fish arrive. Here’s how you can lower the pH in the koi pond.

Test the pH of the Pond

Before you make any adjustments to the pond, you’ll want to make sure it truly is too alkaline, or high in pH. You’ll need a test kit from your pond supplier to test the water. Ideally, you’d be testing once a week, because it can take some time to get the pond back to normal if its pH becomes too dramatically high or low.

If the koi pond’s pH is above 10, the koi’s lives are in immediate danger, and they should be removed to a small household tank. You can then remove all the water from the tank and add in water with the correct pH.

Change Water or Add a pH Adjuster to the Pond

There are two main ways to reduce pond pH. You can remove and replace some water (as freshwater has a low pH of 7.) Or, you can use chemicals to do the same. You can choose your pH adjuster from your pond supply store. Alum and white vinegar are koi-safe options.

Make Gradual Adjustments

Changing the pH very suddenly can be a shock to the koi fish. It’s as if the temperature in your home suddenly changed by a few degrees, all at once. You may prefer that new temperature, but it’s still a surprise. And for koi fish, it could negatively impact their health.

So, if there are already koi in the pond, try only to adjust its pH by three tenths per day. If you choose to replace the water, only do ten percent of the volume of the pond at a time. If you’re using vinegar, add only a half cup to start and measure the results. You want to be careful about adding too much at once, or you can harm the fish.

Is Vinegar Safe for Koi Ponds?

White vinegar is safe to use for koi ponds, so long as you make your pH adjustment slowly, and don’t pour the vinegar directly onto a fish. That said, it is safer to simply use the water replacement method, as it is much less likely to result in a dramatic change in pH.

What if I Over-Correct?

What if, in an attempt to lower the pond’s pH, you end up lowering it too much? The solution is simply to add in some baking soda, about one teaspoon for every 10 gallons of pond water. Very low pH is actually more dangerous for koi fish than high pH, so be sure to test after every adjustment that you make.