Feed me!

A Koi's metabolism functions best at temperatures above 70° F or 21c.  High protein foods are difficult to digest when the water temperature falls below  65° F or 18c

see this link for temperature conversions

Koi still need sufficient carbohydrates for energy when water temperatures are low.  This is particularly true for Koi less than 1 year old. 

Never overfeed your fish! Koi do not have a stomach.  Feed smaller quantities more frequently for better nutritional absorption.  

Feed only as much as the fish will eat in about 3-5 minutes.  Koi pellets should be fresh and used within three months to prevent oxidation of the food. Always remove uneaten food as this will foul the pond water and cause your filter to be overloaded and then you will have ammonia and/or nitrite problems or even worse..... bacterial disease.

Feeding and growth rate
The feeding pattern, and thus the growth rate, of koi depends on many factors, such as water temperature, water quality, stocking density and genetic background Koi feed most actively at temperatures in excess of 15c(59F), thus sexually immature fish can grow rapidly during the summer months when the temperature is warmer. Once koi are mature, their growth rate slows considerably; in sexually mature fish, most of the food eaten is utilized in producing eggs or sperm in preparation for breeding. However, unlike many other vertebrates, fish continue to grow throughout their lives and it is easy for pampered koi to reproduce and continue to grow because of their artificially high feeding rates. The cycle of rapid growth in summer and retarded growth in winter produces rings on the fish's scales (much like the growth rings of a tree) and a koi's age can be determined by counting these rings. Some koi-keepers, however, who heat their pools in the winter, continue to feed their koi throughout the year. If a koi has continued to feed during winter its rings will not be clearly defined and it will therefore be difficult to age.

Water quality affects the rate of growth because koi lose their appetites and may even stop eating if their environment is poor. Poor water quality can also affect the fish's metabolism, thus hindering digestion of food.

The stocking density of the pond can also have marked effects on koi growth rate. In a lightly stocked pond, koi will become sexually mature while still of a relatively small size (25-3Ocm/10-l2in) and once mature, growth is retarded. Although koi in a densely stocked pond will mature at a much larger size (50-60cm/20-24in), competition for food will slow the growth rate, food will be more scarce and the 'battle' for it can stress some koi. You will have to decide on optimum stocking levels for your own koi pond bearing this in mind. In fact, these considerations are really most important for koi farmers.

The genetic background, too, influences the size to which koi will grow; as the children of tall parents tend to be tall, and short parents produce small children, so the same is true of koi. This is obviously a simplified view of gene action, however, because of the significant role the environment plays in influencing size.

Water temperature affects fish more than any other single factor. Fish are ectothermic - their body temperature fluctuates in accordance with the temperature of their environment, usually remaining 10C higher. As the temperature drops, the ability of the koi to digest and assimilate food decreases. In the winter months, therefore, at temperatures below 10C(50F), it is a good idea to feed cereal diets that the koi can digest quickly and easily and that do not stay in the gut too long. High-protein diets linger in the gut and can cause severe problems; the bacteria found in the fish's gut, which play a role in breaking down some less readily digestible substances, such as cellulose, may become pathogenic if food is retained in the gut too long. As the water temperature rises, however, the koi need protein for growth, repair of damaged tissue and injuries, and for reproduction. In the summer, koi will benefit from a high-protein diet containing 35 to 40 percent of fishmeal-based protein.

Nutritional content of food
Food contains various elements, such as proteins, fats, carbohydrates, vitamins and minerals, which are essential for all animals to maintain healthy bodies, grow and reproduce. Here we explain what these elements are and why they are important components of food.

Proteins are made up of amino acids. Thirteen essential amino acids should be included in any fish diet, although there are about 20 found in natural proteins. An adequate diet contains sufficient quantities of both essential and nonessential amino acids to allow the koi to grow, repair damaged tissue and produce either eggs or sperm. Deficiency of protein or any of the essential amino acids causes koi to grow more slowly and, if this dietary problem continues, can result in a deformation of the spine.(Spine deformities may have a number of other causes, such as disease, however).

Fats provide a source of energy to koi; their important role is in providing fattyacids, such as triglyceride and phospholipids; vital components of membranes surrounding all cell walls. Koi can make almost all of the fatty acids they need with the exceptions of linoleic and linolenic acids, which are essential and must be provided in the food. Linolenic acids are required for growth.

All fats are made up in a similar way to proteins, but of fatty acids, rather than amino acids, bonded together with glycerol. If essential fatty acids are omitted from the diet, symptoms of fin erosion and heart and liver problems may result. Fats have a low melting point and are thus more easily digested by koi. They are found in fish, soya and corn oils and in high concentrations of wheatgerm. Fatty acids become rancid on exposure to air - a chemical process known as oxidation. In koi, the liver is the chief organ for storing fats and if stale food is fed to them it can result in disease and death. Unfortunately, rancid food has no outward appearance of having 'gone bad'. It is, therefore, worth buying smaller packs of food, rather than a large quantity which is utilized slowly, and never keep food from one year to the next.

Carbohydrates also form a scource of energy for koi but fish metabolize them lessx readily than omnivorous birds or mammals. Too much carbohydrate is very bad for koi health, resulting in either degeneration of the liver or an excessive storage of these substances as glycogen, leading ultimately to heart failure.

Vitamins are essential for the normal metabolism and growth of koi, and requirements of some are increased during spawning. Vitamins are complex-structured substances, needed in only small amounts in the diet, but deficiencies can cause clinical disorders.

Vitamins are divided into two categories: fat soluble and water soluble. Fat-soluble vitamins are found in a variety of forms, all of which are metabolized slowly and can be stored in the body fat. An excess of fat-soluble vitamins can lead to a condition known as hypervitaminosis, which, depending on the vitamin, can lead to clinical disorders. Water-soluble vitamins are easily absorbed and are not usually excreted.

All essential vitamins are supplied in more than adequate quanities in proprietary koi foods and it would be unusual for the hobbyist to meet clinical symptoms associated with defieciency.

Minerals, too, aid basic metabolic functions, as well as performing their own duties, which include building skeletal structures, osmoregulation, building of nerves, and maintaining the efficiency of gaseous exchange in the blood system. Little is known about mineral deficiencies in fish, probably because most minerals are absorbed from the surrounding water. Normally 12 percent of the diet is made up of minerals, being contained in fish food in the form of ash.

Nutritional requirement of koi
The type of food you give your koi and the quantity you offer them will vary according to their size. The majority ofproprietary koi foods have a ceral base with different ingredients added either to enhance colour or aid digestion. Choose a food size that is small enough to be eaten by the smallest fish in the pond, otherwise they may die of starvation. If you keep very small koi with larger ones, offer a mixture of large and small foodstuffs, but always make sure that young fish are adequately fed. Most koi food comes in two forms: floating and sinking. Koi are bottom-feeding fish and are, therefore, better suited to sinking food. This is made by compressing the meal through a die at high pressure. The meal is held together with fats and, again, takes a long time to be dissolved by cold water. The disadvantage with relying on this type of food is that you will be deprived of seeing your koi feeding. Fortunately, koi will take food from the surface - indeed, you can even tame your koi to feed from your hand - and special expanded (hollow) foods have been formulated for this purpose. These are steam cooked to create an outer shell, which protects the food particle from saturation for a while.(Once the pond water has dissolved the shell, the food will sink.) Another advantage with floating food is that it is easy to see when the koi have eaten enough. Take care when feeding sinking types - uneaten food can easily pass unobserved, particularly in murky water.

Provide just enough food so that after five minutes it has all been eaten. On koi farms in summer, smaller koi (i.e. below 15-20 cm/6-8in in length) are fed five percent of their body weight per day, where-as large mature koi (i.e. over 20cm/8in long) are only fed two percent. The nutritional value of koi food is calculated by weight and not by volume. Normally, lkg(2.21b) of fish food will increase koi growth by 500gm or even 700gm in favourable conditions. In the winter, provide wheatgerm based diets on a maintenance basis, in order to keep the body functions ticking over, since it is too cold for the koi to grow. Wheatgerm is also a good source of Vitamin E.

Feeding to enhance colour Koi are highly valued for their colour and certain additives can be included in the diet to maintain and enhance the fish's natural coloration. Carotene affects the red pigmentation, but if used at too high a concentration, even the white pigment on the koi will turn pink. Spirulina platensis also enhances and fixes the red pigment, but does not turn the white to pink to the same extent. It is a type of algae found and cultured in Mexico and eaten by the people, since it contains a high level of protein. Initially, it was fed to koi on account of its nutritional value and not because of its colour-enhancing characteristics. Now that these have been established, it is fed to koi for only one month each year, usually during September, but can be given at any time, even during colder periods, at temperatures of 10c, but no lower. Some koi farmers feed it for the month before the fish go to market to bring out the best colours in the koi.

Good coloration is not only enhanced by good feeding, however. Healthy fish tend to have much brighter colours than diseased ones. To bring out the colour in koi, particularly the white, you must provide good living conditions. Strong red and yellow pigments develop well in waters rich in green phytoplankton (single-celled plants). Because koi are difficult to see in green water, it helps to feed a diet that will enhance the red pigmentation. Black pigment is enriched in hard water with a pH level of 7.5-8.5. Remember, however, that changing the pH and hardness of the water can affect the toxicity of ammonia and nitrite.

Livefoods
Koi will relish a variety of livefoods, including cockles, worms and prawns. Earthworms can be fed to the fish all year round and, like prawns and cockles, are high in protein and soon become a favourite treat - a sure way to gain the affection of your koi. Tadpoles from frogs are another great treat in spring and, in summer, silkworm pupae, imported from Japan, are an excellent source of protein. Feed these pupae only as an occasional treat, however, as they have be shown to cause a diabetes-like disease in koi. Chironomid larva and mosquito larva are a popular diet for small fish, though not easily available. Daphnia ('water fleas'), plentiful in earth ponds, are one of the first foods for koi fry but tend to be too small for adult koi. Maggots are not recommended as they can carry harmful bacteria from the decaying flesh. Do not rely on livefoods to form the staple diet for koi, however, but offer them as a supplement to the regular diet. If koi are fed on these titbits alone, there is a risk that nutritional diseases will set in as the result of a lack of vitamins or amino acids.

Other foods
Koi will accept many foods thrown to them in their pond, but many of these are of little or no nutritional value and may even harm the fish. Brown bread is acceptable, but white bread contains a mild form of bleach, which does the koi no good at all. Do not offer beans, peas or corn, since koi are unable to digest the hard outer casing of these foods. Koi will take lettuce leaves and may also eat duckweed and other plants around their pond (with the exception of blanketweed, which is too coarse for them to pull off the sides).

Reprinted from "The Tetra Encyclopedia of Koi"; Tetra Press