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Is soft water better for keeping koi?

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.

Some koi keepers have alleged that soft water is better when keeping koi. They believe that very soft water encourages enhanced growth and improved colour development. But they are wrong! Freshwater fish, including koi, should be kept in stable conditions and in water with a PH between 6 and 9. The degree of hardness should be no lower than DH6. The degree of hardness is made up of two factors – KH and GH. KH is a measure of temporary or carbonate hardness and GH a measure of the total dissolved minerals in the water. GH is hard to change whereas KH can be altered relatively easily, both intentionally and accidently.

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.

The problem with soft water

Soft water inhibits the process of osmoregulation. This is the process by which the fish absorb salts into their bodies and then expel water and ammonia back into the pond. If osmoregulation is inhibited, the fish will become stressed. Increased levels of stress will impact their immune systems and potentially lead to disease. Soft water with a lower KH is much more vulnerable to sudden changes in the pH level and as this controls the pH level of the Koi’s blood, sudden changes are bad news!

Hard water protects fish from toxins

In addition, hard water holds toxins more effectively, preventing them from passing to the koi. This particularly true when it comes to metals. Copper zinc and lead dissolve much more readily in soft water. KH (carbonate hardness) also provides energy for nitrifying bacteria, in other words, the good bacteria that eliminate ammonia and nitrite. So, it won’t come as any surprise that hard water helps the biomass in the filtration system as the bacteria are better able to flourish.

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. It is important to note that ceramic media can strip calcium from the water and this can impact both KH and GH levels. If you utilise ceramic media, it is crucial to test the hardness of your water regularly and to keep a mineral pack to boost hardness 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.

What’s the science?

There is no scientific evidence that soft water promotes koi growth or that it enhances their colouration. However, it is accepted that red colouration is supported by soft water and is less likely to break down.

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.

Don’t use tap water

You should never fill or replenish your koi pond with water straight from the tap. Tap water contains numerous toxins including chlorine, ammonia, aluminium sulphate, sodium hydroxide and chloramine. These are toxic to koi in very low concentrations. The amount of toxins in tap water varies from one area to another and can fluctuate dramatically within the same area. For instance, your local water company may choose to add extra chlorine to the water in order to neutralise pollution when they are conducting infrastructure repairs.

In short, you don’t know what is in your tap water from one day to the next and so water must be purified before finding its way into our pond. The majority of issue with koi are caused by the quality of the water. Make sure yours isn’t too soft!


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