The Fundamentals of Feeding Koi and Goldfish
By Stephen M. Meyer
Carp and goldfish evolution has seen to it that the proper functioning of their digestive systems requires dietary variety over the long run. Monotonous diets are fine in aquaculture where fish are routinely culled out for market early in life, but ornamental pondkeeping aims for natural life spans, which places more subtle demands on fish-rearing techniques.
In this respect, no single food type, no matter how nutritious, represents an appropriate or healthy long-term diet for koi or goldfish. I doubt that any of the manufacturers of premium koi and goldfish food would claim that their products should be the exclusive diet of your fish. Therefore, you should make every effort to offer your koi and goldfish a varied diet (which does not mean different brands of pellets but rather different types of foods: vegetables, insects, etc.).
It is useful to think in terms of a base diet and a supplemental diet for your fish. The base diet provides the essential proteins, fats and most vitamins and minerals. The supplementary diet provides additional vitamins and minerals, but also other proteins and fats and, most importantly, variety.
After reading, I visted Wal-Mart and purchased some floating pellets and dehydrated "Baby Shrimp" specially formulated for Koi and Goldfish.