Hi, I have a 40 gallon tank that houses a common musk turtle along with some guppies, rosy barbs, and a gourami. He has eaten some of the fish before, mainly guppies. I had a rainbow shark at one point but he didn't last long, however I don't know if he died on his own and the turtle simply cleaned him up. But there is no such thing as a turtle that won't eat a fish when he can. I find the mud/musk/stink pot turtles to be the best candidates for tanks with fish though. They're much less agile than many other species of turtle so might be a good option for you.