Koi fish eat just about anything, from small bugs and insects, to plants and algae at the bottom of the fish pond, to store-bought koi fish food. Koi will even eat people food. They love cereal, lettuce, shrimp, rice, peas, watermelon... pretty much anything we like, koi fish consider food.
Why does koi fish spit out food?
Softening/Breaking Up Pieces. This is probably the most common cause of fish spitting out food in the home aquarium. It indicates that the food that you are giving them is either too hard or too large for them to eat in one go.