I have some pretty big feeder goldfish in my outdoor pond along with a ton of mosquitofish. The turtles don't expend any energy chasing the fish, but if a fish is injured or not swimming quickly it gets eaten. (pretty darned big Female RES, pretty darned big female Phrynops hillari, very darned big 2.1 Florida soft shells and several pond turtles.)
I have goldfish that are now quite large and a bunch of what is sold as rosy feeders,even though some are gray in color, They all survive under the ice if and when the pond freezes over.The feeders multiply but have never been over crowded, somebody eats the small ones, either the fish or turtles. In the pond are northern red belly, eastern painted, RES, and common snapping turtles.
I have a few small predator fish that are very small now. Spotted shovelnose catfish, peacock bass, arowana just to name a few. Probably wont have the turtles in the tank one the fish get up in size.
I've had guppies and native Gambusia in my turtle tanks at various times. The mud turtles ignore them after the first hour or so but as the others have mentioned any that become old or 'slow' seem to disappear.
The Staurotypus will chase around and suck in newly introduced fish for a while but then those that outlast them get ignored--except at night when I'll occasionally hear a pow at the waters surface. That means a minnow got inhaled.
Larger fish can avoid the Staurotypus but if they wander in front of the turtles the latter will make a grab at them.