Koi fish are edible. Like many fresh water fish, koi fish can be steamed, baked or breaded and fried. Recipes can be found from all over the world.
The primary reason people refrain from eating koi fish is the cost. A single koi fish can cost hundreds of dollars, so this fish makes an expensive meal. Low quality or discoloured koi fish are sometimes bred and used as bird food. A dealer of koi fish might offer these throwaway fish for a cheap price. In New Zealand, these fish are considered pests, having been imported as pets and released into the wild where they've damaged the natural ecosystem. Catching and eating the fish is encouraged in New Zealand.