Despite this importance to both fish health and water quality, it is often a poorly understood subject. Indeed, many fish keepers never check water hardness. This seems particularly true of koi keepers to whom it is especially relevant because of the high levels of nitrification taking place in a well-stocked koi pond.
Water hardness has a major effect on pH and pH stability.
It will affect the toxicity of many common substances, including some fish disease treatments. It also has a major effect on fish osmoregulation , a process you will recall that is vital to fish health. Hardness is a measurement of the concentration of divalent metal ions such as calcium, magnesium, iron, zinc etc, usually acquired as rainwater percolates through rock. In most water it consist mainly of calcium and magnesium salts, with trace amounts of other metals. The sum of both types of hardness is called the general or total hardness.