Do Koi do better in hard or soft water?
Koi do better in hard water.
A stable pH level is required to keep Koi. Hard water is full of minerals, which act as a buffer to prevent any pH fluctuations. It is easier to keep Koi in hard water than soft water. In a closed water system like a small Koi pond, the pH level will remain stable for longer periods of time with less water changes.
If you keep your Koi in soft water, you will need to do water changes more often, in order to keep your pH level stable. A "pH crash" can occur if you fail to do regular water changes. A pH crash happens when the mineral content in the pond water becomes too low to buffer and the pond water quickly becomes to acidic. A rapid change in the pH level is toxic to fish. Since soft water has a much lower mineral content than hard water, the chance of a pH crash happening is far greater.
We recommend that you use hard water for your Koi pond. It helps create a stable environment for the Koi fish.