Hikari and sho koi food
It is sufficient to feed Koi commercially available Hikari and Sho Koi food. These have been tested and made with the right proportions of nourishment for the Koi. Commercial Koi food is designed not only to be nutritionally balanced, but also to float in order to encourage your Koi to come to the surface.
feeding your koi fish
Koi will recognize the person feeding them and gather around them at feeding time, which is fine -- as long as you do not overfeed your fish. One positive aspect of hand-feeding (other than the fun of it!) is that when the koi are eating, it's possible to check them for parasites and ulcers. In the winter, their digestive system slows nearly to a halt, and they eat very little, perhaps no more than nibbles of algae from the bottom. Their appetite will not come back until the water becomes warm in the spring. When the temperature drops below 50 degrees Fahrenheit, feeding, particularly with protein, is halted or the food can go rancid in their stomach, causing sickness.
supplemental koi food and snacks
You can get advice from your breeder, pet shop (or us!) on the type of snacks or goodies to give your Koi. The usual supplementary foods are small prawns and worms. These are usually given as treats to your Koi to encourage them to feed from your hand. This trains the fish to swim to you each time you walk by, as it may expect such treats from your hand. This type of conditioning is actually unhealthy for your Koi unless you hand-feed only occasionally. If too many snacks become a habit, you may have to do a great deal of work in maintaining the water clarity on your koi pond. Plus, a fat Koi does not look beautiful. It will move sluggishly as its body is heavy. When the fish lacks exercise, it will get sick. As with ourselves, treat your Koi with care and feed in moderation.