Turtles WILL eat fish. That's what they do in the wild. Fish are (one of) their natural foods. Yes, turtles are slow. But they can move with lightening speed when dinner is swimming by. They can and do catch fish in my pond - and eat them. They will especially catch slow swimmers like fantails and goggle-eyes.
I have a lot of turtles (just discovered a nest of 18 eggs -from my African sideneck, I'm guessing, and am totally delighted!). I keep the turtles in a seperate fenced hopefully-escape-proof enclosure with their own ponds. I do have fish in there, but they are cull fish and 22c PetSmart fish, totally intended to be food. However, some of them have lived for years and are quite large.
But ryunkins, fantails, telescope eyes - never. Sushi!