cheerios, worms, shrimp, daphnia, mosquito larvae, green peas, oranges, watermelon, berries, duckweed, and water plant roots are some of the foods koi will eat.
They do - albeit koi can eat a wider variety of food due to a bigger mouth
What I love doing is putting a strawberry in my pond. They'll nibble for ages and then I'll just find the green bit swimming around
Dog food is the favourite in my pond. Very messy though, so I only feed it very rarely.