06. Feeding koi goldfish in a backyard pond

As koi keepers, feeding koi is one of our most enjoyable activities. It represents the time when we are able to ‘connect’ with our koi, and partake fully in an interactive relationship with our dependents. I would even be as bold to suggest that koi keeping would lose a great deal of its appeal if we were not required to feed them.

How we feed our koi and what we offer them in their diet is determined by the koi themselves. Our feeding practices must at least take into account that our koi are warm water, opportunistic omnivores that will show natural variations in their nutritional needs throughout the seasons. Over and above these basic criteria, the food we offer our koi can also achieve additional benefits for us and our koi. Koi food can be used to enhance the health and coloration of koi and different types of food will have differing effects on the water quality.

Food – The pond’s software.

Having spent hundreds (if not thousands) of pounds on our pond’s infrastructure and hardware (pond, pump, filter system etc) that will work to maintain the quality of the pond’s environment, we need a to pay equal attention to the software that we use to run the system. The food that we offer our koi (and pond system) can be regarded as our choice of operating system in the same way that the software that is installed to operate the hardware of a PC. It will determine how most aspects of your system function, particularly a dictating how your koi perform. Just as ‘garbage in – garbage out’ is true for PCs, it equally applies to koi food and what we can hope to achieve by feeding it.

Koi food. How does koi food work as our ponds ‘software’ and what results can we expect to achieve when using it?

Koi are omnivorous fish, requiring both animal and vegetable matter in their diet. Complementary ingredients are milled and blended in a food to produce a complete and balanced diet that will fulfil all of the dietary requirements of the pigs of our pond. Our koi are then able to achieve what they need to from their diet.

Energy: A fish’s priority when feeding is to meet its energy requirement. Our koi require energy to maintain their metabolism and general body functions as well as generating the physical movements of swimming, eating and breathing. Koi are able to ‘burn’ a whole range of foods to release energy, with some food sources burning leaner than others. For example, if koi use starches and similar carbohydrates as a source of energy, they excrete very clean waste products of carbon dioxide and water. However, if the same koi used proteins as a source of energy, the waste products will include nitrogen, which is released as ammonia, having a negative effect on water quality.

Growth: Only when a koi’s energy requirement has been met can a koi expect to achieve growth through its diet. Growth is defined as a permanent increase in size (usually measured as length) and can only be achieved by utilising protein in the diet. Consequently, high-protein diets provide the means for higher rates of growth, especially in warmer water. The quality and digestibility of the sources of protein in the koi’s diet are also fundamental as to the koi’s ability to utilise them. For example, fishmeal and a feather meal are both potential sources of protein in their diet and yet fishmeal is so much more effective because it is highly digestible and therefore available for use in growth.

Health: The typical koi pond is an unplanted, steep-sided deep water pond, heavily stocked with fish compared to a natural water body. Consequently, koi are completely dependent upon us for their food, and we must give them a complete and balanced diet for them to remain healthy. In practical terms, this means that when our koi have been fed on such a diet, they will not need to gain nutrients from another source. In this way, the health of our koi is maintained just as if they were feeding on a natural diet and in some ways, a koi food can offer our koi other health boosting benefits. In addition to the health gained by feeding a balanced diet, our koi’s disease-fighting abilities can be enhanced by adding specific immuno stimulants to the diet. These act by stimulating an increased production of white blood cells, thereby reducing the threat of disease in koi.

Colour: Koi colour is produced by millions of colour cells in the skin, each storing a pigment specific to the type of colour cell. The colour within the cells is produced by specific carotenoids that koi extract from their diet. One of the most effective natural sources of Carotenoids is microscopic algae (the type that are responsible for green water). Other natural sources include shrimps (that too have fed on algae) marigolds and even paprika. As our koi ponds do not provide our koi with natural sources of carotenoids, we can achieve similar colour enhancing effects of a natural pond by adding these colour enhancers to the diet.

Water quality: Besides koi food having a direct effect on koi performance it can also have an indirect effect on their health by the way it interacts with the quality of the pond environment. Just as an operating system might cause your PC to malfunction, a food can also cause your pond a few problems. We should be aware that when we feed our koi, we are also feeding our pond, relying on our hardware to deal with whatever our koi leave behind. An increase in nitrates for example is inevitable (and unavoidable) but excessive feeding, or offering a food that is inappropriate for that time of year will lead to other water quality problems. Other ways that food can impact water quality include the discoloration of the pond water (due to a build-up of dissolved organic carbon) or the occurrence of a frothy or scummy surface. Furthermore, because koi rely on their artificial diet to supply them with vitamins and minerals, some of these will inevitably leach into the water. For example, the unavoidable accumulation of phosphates will lead to an increase in blanket weed and other nuisance algae problems.


Koi are opportunistic feeders or ‘pigs are the pond’, which means that they are for ever on the lookout for a tasty snack, happy to try anything that their snout may turn up. We should work with a koi’s feeding lifestyles and offer them food on ‘a little and often basis’, as preferred by this aquatic pig. Koi do not have stomachs, and research has shown that they digest and utilise the food that is offered more efficiently if it is fed in small but regular offerings. In this way the food travels slowly and spends longer in the gut, enabling better digestion and assimilation.

Seasonal feeding.

Even though koi are a warm water fish (optimum growing temperature of 27 degrees C, the seasonal cycle still plays an important role with their physiology. Koi physiology has been shown to benefit from a period of inactivity, when the temperature falls well below 4 degrees C. As the environmental temperature drops, so too does their appetite and energy requirement. The capacity for growth of this cold-blooded animal also drops with temperature, so the food we offer at these lower temperatures must also change, for the benefit of our koi and the pond’s water quality. Koi should be fed a lower protein diet that is easy to digest at these temperatures (typically a wheatgerm-based diet). If koi are fed too high a protein diet when they cannot utilise it, the excess will be excreted, leading to potential water quality problems.

In summary, koi rely on the food we feed to provide them with everything they require for health, growth and colour. That same food (and the way we offer it) will also have a strong influence on the quality of the water in which our koi live. In this way, we should regard koi food as the operating system for our pond, and check regularly that our hardware is operating well on our specific choice of software.

The 5 Nutrient Groups – What they do for our koi

Protein. Levels of approximately 30% are typical but will be higher for a growth food and lower than this for a low temperature food. Protein is present for growth and repair and is the greatest contribution to the cost of a diet. Sources of protein include both animal and plant derivatives and meals such as fishmeal, soya, wheat and egg.

Carbohydrate. These are the complex sugars such as starch and cellulose. They are all plant in origin and are included in high quantities in koi diets as an inexpensive source of energy. They are also included as a source of fibre to aid digestion.

Oils. Oils are included as fish or vegetable oils. Usually less than 10% in the diet they are used by koi in the production of new tissues etc. If too much oil is included then pellets appear greasy and oily as in trout pellets. These tend to be too rich for koi and can lead to water quality problems.

Vitamins and Minerals. Manufactured diets will include artificial vitamin and mineral supplements to boost these vital ingredients. Koi have an advantage over us in that they are able to source minerals not only from their diet, but also from the pond water. Hence it is prudent to keep mineral levels in the pond boosted through regular additions of clay or other mineral supplements.

Kill blanketweed and string algae.