When koi fish are in the wild, they eat plants, worms, insects, seeds, algae, etc. They regularly hunt along the pond floor and the surface. When in a home pond, you can feed koi fish almost anything humans can eat. They include shrimp, fruits, vegetables, and foods that aren't high in carbohydrates but high in protein. Bread and its kind are known to be hard on fish's stomachs. You can also feed them fish food pellets.