In the wild (and John’s ecosystem pond) koi can feed off many foods: insects, algae, aquatic plants, seeds, worms, and anything they can get off of the pond bottom. So, he doesn’t really even have to feed his koi fish.
Things get interesting when he finds out what else they can eat (besides koi fish food). He can feed them human food too, even the Cheerios he’s eating.
Koi fish like fruit, finely cut vegetables, shrimp, and everything. Of course, if you feed them a poor diet they will start showing it (just like us). Moderation and a good diet are key to healthy koi fish.
John has most of these foods. Now he just needs to know how to feed koi fish.