KH is imperative to a pond's pH balance, as it is the “usable part of your pH”. It doesn’t matter too much to the fish when pH is going UP. But they will leave home when it suddenly drops DOWN! When your pond runs low on KH, which should be 100-300 ppm, the pH becomes unstable and can crash. If it goes down to 80 ppm, nitrification stops and pH becomes unstable.