Koi carp are some of the most desirable fish on the market. Their inclusive breeding has allowed fish keepers to achieve extravagant and exotic-looking fish, comparable to pedigree horses and house pets like cats and dogs. Just as with any genetically specifically inbred pedigree pets, their breeding has led to a host of immune problems for koi, making them far weaker and more prone to disease than their natural counterparts, making it vital to maintain their environment. Achieving prime water quality is the most important facet of koi care and needs to be the number one priority for all Koi enthusiasts.

It’s not as simple as just buying some Koi to live in your garden pond and their ability to live for some time is not indicative of hardiness. Their delicate immune systems mean that specific conditions are required for their survival. The following are commonly found chemicals and salts in the water supply.


Ammonia is a form of nitrogen that occurs in water, that can have a toxic effect on aquatic life. It’s produced for industries such as commercial fertilisation. It is naturally occurring, with its source being decaying organic matter. Ammonia levels in the pond water should be kept to an absolute minimum. If there are high levels of ammonia present in your pond water, your koi can have difficulty discharging it, leading to a potentially fatal toxic build-up in the blood. Ammonia levels should ideally be kept to zero.


Nitrites are naturally occurring salts that can be found in groundwater. They come from fertilisers and get into the water through runoff, natural mineral deposits and sewage. Nitrite can affect the water supply by stimulating bacteria growth. This bacterium can be toxic to aquatic life. As with ammonia, nitrite is best kept to zero.


Nitrate is a natural compound of oxygen and nitrogen and is found in water due to intensive fertilisation in agriculture. Nitrate seeps into the groundwater or runs off into local rivers and can lead to high levels of nitrate contamination. Nitrate does not have an organic effect on grown koi but can increase stress levels due to its presence in the water, leading to a higher chance of catching diseases. In young carp, nitrate can impede their growth. For healthy Koi, nitrate levels should be kept below 50 ppm.


Chlorine is a chemical that is added into the water supply to kill harmful bacteria and disinfect it ready for human consumption. When untreated before fed into the pond water, it can lead to harm to gill tissue in fish by causing corrosive injuries. Toxins such as chlorine and chloramine should be kept to an absolute zero.


Levels of metals above 0.1 m/l are likely to cause irritation, damage to the gills and bacterial infections. In addition to this, the water reacts with metals like iron to form free radicals that lead to erosion of the gill tissue. In addition, some metals such as lead can have a neurotoxic effect on the fish.  Levels of metals should be kept to zero.

Water pH.

Water pH should be kept between 7.0 and 8.0 – ideally exactly in between at 7.5. Higher than 9.0 can cause alkalosis, lower than 6.0 can cause acidosis. Alkalosis can lead chemical burning or chapping on the scales. Acidosis, particularly for younger fish, can lead to increased stress, lowering body weight. In addition, it can stop fish eggs from hatching.    


Phosphate is an essential element, a compound of phosphorous and oxygen. It is commonly found in soil and fertilisers. Runoff water from agriculture can lead to it entering the water supply. Water companies have also added additional amounts of phosphate into the water supply to prevent pipe erosion. Fish food itself contains phosphorous because of the vital part it plays in the diet of the fish. However, this does lead to phosphate being released by the fish as waste. High levels of phosphate in the pond can lead to the rapid growth of pondweed, affecting the pond ecosystem. In addition to pondweed, pond scum develops with high levels of phosphate. Phosphate has also been known to have fatal effects on younger koi.

How can you reduce levels?

Using a filter, such as the 10” 3-Stage Pond Dechlorinator, will remove particulates and sediment from your pond water supply, this includes the salts and heavy metal particles that can have toxic effects on Koi carp.

Other tips on looking after your Koi.

As well as keeping the water filtered, it is important to keep the pond oxygenated and cleared of waste regularly. Make sure to carefully monitor your care in different seasons as well. Koi can survive in a wide range of temperatures but struggle with sudden temperature changes. Watch for ice in the winter, make sure to keep large areas of water surface uncovered for gas exchange. In summer, try keep a degree of shadow cover over the pond if it is too shallow to protect fish from the sunlight.

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