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Is soft water better for koi?

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.

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.

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.

Ceramic media

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.

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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