It depends on the size of the turtle compaired to the size of the fish. My parents have a pond with koi and goldfish in it. The larger fish, ~4-12" long are left alone by the 10" painted turtle. However, the dozens of baby fish and any tadpoles they put into the pond mysteriosly seem to vanish.

Rule of thumb, if the fish can fit into the turtle's mouth, they're going to end up in the turtle's belly.