Enhancing koi colour with koi food

The ornamental aquatic hobby exists because of our enchantment with the striking beauty of the many thousands of different fish species – and no more so than in the world of koi. Admired for their size, grace and beauty, it stands to reason that if we can improve an aspect of their image, our enjoyment will also be rewarded. Such is the case with colour.

There are many factors that conspire against koi (and koi keepers) when endeavouring to enhance their colour. Rather than exploring the factors that can negate our colour enhancing activities, it is more productive to concentrate on the most significant factors that will control a koi’s colour – and they are:

a). The koi’s genetic make up (which has coded for the koi’s current (and future) skin, colour and pattern characteristics b). The colour enhancers that are stored and expressed in the skin.


Let’s just take the colour red as an example. Colour in koi is produced in a similar way to photographs in newspaper where from a distance, a photograph will look detailed, well defined, and clear. Yet under closer examination, particularly through a magnifying glass, the picture consists of thousands of tiny dots of ink, each working together to produce a picture of dark and light areas. Skin pigmentation in koi is caused by dots (colour cells called chromatophores) the intensity of which is determined by how densely the dots are packed and how intensely each dot is coloured.

A koi’s genetic code will determine both factors, with the role of colour enhancement through feeding only being able to improve the colour of each colour cell (rather than increase their density in the skin). However, there is always hope as that same genetic code will also code for colour development where pattern and the appearance of other chromatophores may develop in the future.

The role of colour enhancers

Koi are only able to exhibit pigments if they receive them (or their precursors) in their diet. Feed a koi a completely colour deficient diet, and over time, colours will fade to form a very ‘unornamental’ off-white skin colour.

What causes colour to fade?

Carotenoids are the massive group of colour enhancing compounds that are stored and exhibited in koi skin. They are a group of chemicals that impart colour by the way they absorb and reflect light. Those that refract higher wavelengths of light (reds)are more desirable than those that refract the lower end of the spectrum (yellow). They are organic in nature, and are very closely related to Vitamin A, and similar in structure to vitamin E. Due to their similarity in chemical structure to these 2 vitamins, they behave in a similar way in living tissue, being very reactive and unstable, easily degraded in oxygen heat and light.

As carotenoids do degenerate over time, in the same way that a leaking bucket needs to be topped up to keep it full, koi require a constant supply of carotenoids to keep the chromatophores packed with carotenoids.

Keeping the bucket of colour topped up

Relying on us completely for their nutrition, the colouration of koi in artificial koi ponds also depends on them receiving carotenoids of the correct quality and quantity.

As mentioned earlier, carotenoids are a group of closely related organic chemicals, where through a number of subtle molecular changes, specific carotenoid pigments can be converted into others in the carotenoid family. Many different organisms (including shrimp, krill and koi) have the ability to convert certain pigments into others. Shrimp and krill are marine crustacea that are largely harvested off Norway and the Antarctic, and are renowned for their red pigmentation caused by the carotenoid astaxanthin. However, these filter feeding organisms feed on marine algae that is suspended in the water (and definitely not pink!), converting a range of pigments (carotene, lutein, and zeaxanthin) into the red pigment astaxanthin.

Koi can perform a similar trick with carotenoids, but to varying degrees depending on the tissues involved. For example, if fed pigments commonly found in algae, such as spirulina, (carotene and lutein), upon digestion and assimilation, koi choose to deposit these yellow carotenoids unchanged in their flesh.

Fortunately however, they do convert such pigments into the more desirable red pigment astaxanthin, enhancing and improving their red colouration. Salmon and trout also have an interest in astaxanthin, making their flesh look natural and appetising. But unlike carp, they are unable to convert other pigments (such as lutein and carotene) into the desirable astaxanthin which must be fed directly in their diet. Koi absorb and convert carotenes very efficiently into astaxanthin, and is the secret behind the apparent anomaly of enhancing the red skin in koi by feeding them on green algae.

Natural or artificial colour enhancers?

There are two approaches to enhancing colouration in koi – the scatter-gun approach (using natural sources of carotenoids) and the precise approach using synthetic colour enhancers.

Natural sources:

There are several recognised natural sources of carotenoids suitable for colour enhancement. Like any natural commodity, qualities and pigment content can vary from source to source, and being organic, can be liable to degradation during food manufacture. However, natural sources are also renowned for offering a superb range of carotenoids giving koi (who have the ability to convert carotenoids)excellent colour potential. For example, marigold petals have more than 20 different carotenoids, which koi can work with and manipulate. They also have a high concentration of these compounds (approximately 9000mg per kilo), whereas shrimp or krill meal will only have about 200mg per kilo, with the added issue of the exoskeletal material have an exceedingly high ash content.

Furthermore, there is a price to pay for koi using natural carotenoid sources as the process of converting them into more desirable astaxanthin requires energy.

Artificial sources

Aware of some of the limitations with natural sources of colour enhancers, much research has led to the formulation of potent, stable and effective man-made colour enhancers. These will provide koi with a precise amount of pigment in the form in which it will be deposited in the skin. So rather than feeding a natural source of unknown quantity or quality of carotenoid which koi will convert into red, they can be offered a metered dose of pigment in the final form in which it will be used.

Natural Foods that will enhance colour. Most foods that contain elevated levels of carotenoids can be used as a colour enhancer. Recognising that koi can manipulate carotenoids into the form that they require, it is preferable to provide pigments that koi will need to manipulate less.

Good natural sources of red pigmentation include shrimp, prawn and krill, with similar red pigments found in high quantities in red peppers (paprika). That will get your koi’s eyes watering!


There is always a risk of overdosing, when feeding koi with whole foods selected for their high carotenoid content. If carotenoids are fed in excessive amounts, area of white tissue can turn muddy brown or yellow. This is caused by the effects carotenoids have of light. While white areas of skin will not deposit red pigmentation because they do not possess red chromatophores, high levels of carotenoids circulating in the tissue will inevitably interact with the light and cause whites to suffer. Premium koi diets will contain ‘safe’ levels of colour enhancers, giving koi the benefits of a precise and effective formulation.

Look for foods with the following ingredients as they will enhance the colouration of your koi. Top colour enhancing compounds:

Spirulina. This has an above average carotenoid content that is easily assimilated, being found in very simple algae cells

Paprika. Red Pepper Meal. More potent than spirulina, and contains red pigments ready to be absorbed and assimilated immediately.

Marigold Flower Meal. Probably one of the most potent natural sources of carotenoids available. The pigments require manipulation by koi to convert them to reds.

Artificial Colour Enhancers. Astaxanthin and Canthaxanthin are guaranteed, potent sources of colour in the form that will be exhibited immediately in the skin.

A Yeast – Phaffia rhodozyma. This is easily digested and is a recognised source of carotene and astaxanthin.

Kill blanketweed and string algae.