Alot of people keep them with fish esp. larger fish with no problems. Whenever people say "All turtles will eat fish" it's theoretically because most if not all turtles in the wild would.. Doesn't mean when you add a FRT he will eat every fish but it's a chance you have to take with mixing turtles/fish
Aquatic turtles are oppotunistic feeders. Most of the vegetarian ones eat mostly plant matter but if given the chance they will take meaty foods including fish!