I have 4 RES, between 2 and 4 inches in diameter. The guys at the local reptile store told us that they would eventually be fast enough to eat fish, but not until they got bigger.
I put a few small minnows in the tank, and the next morning, there was one guy left. A few weeks later, I added 5 more small fish, and they ate all of them - except the same guy who survived before.
I'm not too concerned with the fact that they eat the fish, I'm more concerned that the larger turtles eat all the fish before the runt can get any.
I've more or less given up on fish as food until all the turtles have a fighting chance. Instead, I've been feeding them tiny crickets. They go crazy over those.

