There are other cold water fish you can keep instead of goldfish and koi,
but it is not recommended to mix them. Catfish, bluegill, bass, regular
type pond fish or game fish I believe they are called, unfortunately they
don't show up very well in a green "algae"or black bottom pond and they tend
to eat goldies as they get bigger, not always, some people even on here keep
catfish in their ponds with their koi and goldies. I was very tempted to
get a small catfish until I was warned that I would end up with no goldies
left and a very big and full catfish.