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Is soft water better for keeping koi?

This is a topic we have looked at before and we are revisiting, as some koi enthusiasts advocate keeping their fish in very soft water. Should you do the same?

We think that you shouldn’t!

There is a theory that soft water promotes koi growth and impressive colour. However, the available science indicates that the fish are more likely to thrive in water with a DH level (degrees of hardness) of no lower than 6. Hardness is most commonly expressed as milligrams of calcium carbonate equivalent per litre. Water containing calcium carbonate at concentrations below 60 mg/l is generally considered to be soft.

What is osmoregulation?

Koi find the process of osmoregulation much more difficult in softer water. Osmoregulation is the process during which the fish pass salts into their bodies and expel water and ammonia back into the pond. In soft water, the difference in salt concentrations results in the Koi having to work much harder to prevent the salts within their bodies from diffusing out through their gill membranes.

Harder water enables the fish to reduce osmoregulation and they will then experience less stress. If the koi become stressed due to struggling with osmoregulation, their immune systems will be weakened and they will be prone to disease.

Adding salt to your water

Bicarbonate ions buffer the water, reducing any PH changes, another cause of stress in Koi. Koi fare best in a carbonate hardness of 150-300 mg/litre or 9-18 degrees of hardness. In most Koi ponds the water is too soft due to the fact that there is no natural mud bottom that leaches minerals into the water. Marine salt and sodium bicarbonate increase hardness and so will also cause the pH to rise.

A permanent salt solution of 0.1% is best for Koi. Check your pH level if you add salt, and do not use table salt. The salt used to make salt water aquariums is the best choice. Salt will not evaporate out of the water and needs to be replaced only if the pond water is drained for any reason.

Changes in pH levels

Soft water facilitates dramatic swings in pH levels. Fish require a stable pH level between 6 and 9. However, acid rain storms or water changes can cause significant and immediate changes in softer water which will affect the pH level of the koi’s blood.

Locking in toxins

Hard water also helps koi by locking in toxins quickly to stop them impacting the fish. This is especially true of metals. Copper, lead and zinc dissolve quickly in soft water. Carbonate hardness also provides a source of energy for nitrifying the bacteria which eliminate ammonia and nitrite from the pond. In other words, hard water helps the biomass in the filter to better prosper.

You may experience unintentional changes to the KH and GH levels in your pond. KH is a measure of alkalinity while GH is a measure of the total dissolved minerals in the water. Changes can be caused by ceramic media as this strips calcium from the pond water. If you use ceramic media, check your KH and GH levels regularly and buffer with mineral packs when required.

There is no scientific evidence that soft water helps koi to grow faster or than it enhances colouration. However, it is accepted that soft water prevents red colouration from breaking down or developing the black areas known as shimmies.


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