Hard or Soft Water – which is best for Koi?

This article was first written for and published by Koi Magazine in 2009

One of the most controversial and hotly-debated topics in the koi hobby at the moment is that of ‘hard or soft water?’ – but what is water hardness, how does it affect fish, and crucially, what’s best for koi?

 Water hardness relates to how acid (soft – low mineral content) or alkaline (hard – high mineral content) water is and can be measured in three different ways.  Firstly, by measuring the pH (potential Hydrogen) which is a measurement of the acidity of water and is measured on a scale of 1 to 14 – 1 is acid, 7 is neutral and 14 is alkaline.  Secondly, by measuring KH (Carbonate Hardness) – the measurement of alkalinity of water, or its ability to resist acidification. Thirdly, the GH (General Hardness) which is the mineral content of the water such as Calcium, Magnesium and sodium and can be measured using a TDS (Total Dissolved Solids) meter (however, this also measures other dissolved materials in the water such as fish waste, salts, metals, fish medications etc).  Keeping Koi in a filtered system is an acid making process and when you feed your fish, ammonia is excreted which is then broken down by the bacteria in your filter into nitrite and then nitrate.  During this process these bacteria produce acid and CO2, and use up both oxygen and alkaline materials. As this alkalinity or kH gets used up, it can lead to a drop in the pH and if the pH drops below 6 then the filter stops working (“crashes”)and the water in the pond  becomes  toxic to your Koi.  An ideal pH value of 7.5 to 8 will give you sufficient level of alkalinity to buffer the effects of acid produced.  In Japan the water is soft and is often at the lower limits of what is suitable for Koi.  This is something of a problem for Japanese breeders as it is not good for growing Koi fry and as a consequence they must artificially raise the pH of their fry ponds in order to provide the correct environment for their young Koi to flourish.  Because of the low pH, some breeders place oyster shells permanently in the re-circulation systems in their fish houses in order to raise the KH and many have a constant trickle of fresh water supplementing their recirculation systems, providing what is effectively a constant supply of fresh KH to replace what is used up during filtration. The Japanese however maintain that a pH between 6.8 and 7.4 enables better growth rates and hi development (although genetics play a much bigger role in this than soft water).  In conclusion, science shows us that a pH of 7.5 to 8, hard water, will provide the healthiest environment for your Koi and that, for the Japanese, producing Koi in soft water is something of a triumph over nature!