The type of food you give your Koi and the quantity you offer them will vary according to their size. The majority of proprietary Koi foods have a cereal base with different ingredients added either to enhance color 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, 1kg (2.21b) of fish food will increase Koi growth by 500gm or even 700gm in favorable conditions. In the winter, provide wheat germ 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. Wheat germ is also a good source of Vitamin E.