Koi

Hardness is one of the most misunderstood characteristics of water, because it involves some rather complicated chemistry. In a nutshell, hardness describes the levels of minerals in water, primarily minerals containing calcium (Ca) and to a lesser extent, magnesium (Mg).

Water with lots of these minerals is called ‘hard’, whereas mineral-poor water is called ‘soft’. Water hardness is important for two main reasons: firstly (and most importantly), it keeps pH levels stable and relatively high. In any pond, pH tends to get more acid over time – even over 24 hours, natural processes can cause pH levels to change.

By absorbing or releasing acid, depending on whether it’s plentiful or scarce, hardness minerals effectively keep pH constant, which is very important to the well-being of the fish. Because they prevent extreme shocks in pH, the hardness minerals are described as ‘buffers’, and the level of buffers in the water is called its ‘buffering capacity’. As the minerals are used up in the pond, the buffering capacity will decrease over time.

The second reason is that fish use minerals to make their structures, like scales and bone. Koi kept in water with high hardness levels will have brighter skin, scales and colours.

It’s easy to see why high levels of water hardness are so desirable, and this is easily achieved in ‘soft’ areas by adding limestone chips or shells (such as mussel or oyster shells), commonly in the filter compartment.

Hardness may be described in terms of degrees (°dH) of ‘general’ or ‘carbonate’ hardness – abbreviated as GH or KH – and these two types of hardness are simply a measure of different parts of the hardness minerals. Both are very easily measured with test kits. Anything over 8°dH for GH and 4°dH for KH is fine. High or low levels will probably not directly impact on a Koi’s health, but the appearance of the fish might be affected and the repercussions on other aspects of water quality should not be underestimated.

Written by Adrian Love