A JBL community member contacted us with following question: ”The KH in my pond is very high (e.g. 18 °dKH). The dilution through rainwater alone is not enough to solve this problem. Can this be harmful for koi and goldfish in the long run?"
Even though the rain with its soft and mineral-free water ought to make it softer, in rare cases your pond water can remain hard (high in minerals). While it is true that koi in Japan are mostly bred in soft water, they can cope equally well with harder water. But they need to get used to a change in water hardness comparable to how the rain would slowly soften the water. The same also applies to goldfish. You only need to avoid extremes: Hardness degrees (GH and KH) below 4 ° or above 20 ° are not ideal. It doesn’t really matter to the fish but the effects of the low hardness to the chemistry and biology of the water are dramatic. With hardness degrees below 4 ° dKH the pH value is no longer stable and can fluctuate enormously. If the pH value in the morning is, for example, 6 and then 10 in the evening the animals (bacteria and microorganisms) in your pond would need to cope with a 10,000-fold change of the acid or base components, as a result on the logarithmic scale of the pH value. If you determine hard water in your pond with GH and KH values above 20 °, you definitely need to test your starting water (tap or well water). If it is much softer, there must be factors you haven’t noticed increasing your water hardness! These can include calcareous stones or substrates.