I feed my fish Kokney Koi superior mix, you can get it in several different sizes depending on your fish. As the weather gets colder the fish eat less and in really cold spells they won't need feeding at all, I tend not to feed my fish at all from about October to May unless we get an unusually warm spell. If as you say your pond is established you don't actually need to feed them at all and as winter comes on you won't see them much as their metabolism slows down and they hide in the deepest part of the pond