I've kept turtles for many years now... It really just depends on the individual turtle. Some species do better than others. For instance Maps seem to ignore fish completely while sliders usually end up eating most fish. I have found if you raise fish with them from a very young age, you have a better chance of it working out long term. I have a 125gal with a 8in female RES. I threw in a few dozen feeder guppies one day and she chased them around for about a week and then lost interest. I then got a pair of small Convicts and she chased them around for a few days as well but qickley lost interest. They have since spawned and are defending their fry. The RES is actually scared of them, as they are very aggressive parents and do not back down to the turtle. After seeing how well the Convicts worked out I decided to try and put a Salvini in with her that was causing some problems in one of my other tanks. Well, unfortunately the Sal didnt last more than an hour without being eaten... So, its pretty much hit or miss for the most part. I'd test the waters by starting with something cheap!