Why do koi fish eat goldfish?

They don't - infact once a koi can identify the fry as a cousin they won't eat them. Unless you have a Jeffrey Dahmer koi, just a joke, no such thing… Koi are schooling fish and then tend to prefer swimming with their kin rather than eat them. read more

All fish will eat fry, baby fish, but koi eating goldfish is a myth I think. I had a large pond for over a decade, and multiple koi breeds and goldfish, not intending to breed. A vendor erroniously supplied numerious female koi to me and I ended up breeding koi for years. read more

In this respect, no single food type, no matter how nutritious, represents an appropriate or healthy long-term diet for koi or goldfish. I doubt that any of the manufacturers of premium koi and goldfish food would claim that their products should be the exclusive diet of your fish. read more