Long term, tropical fish won't stand a chance. Sorry, but you asking a predator to ignore it's prey. Might happen in the short term with a small turtle and larger fish, but once the turtle matures it will eat any fish it can catch.
At the Big Al's store up here, there are red-ear turtles in the gold fish ponds. I'm not talking about baby gold fish, more like the 3 inchers... Anyway, it's very common to walk by the ponds and see many many halfs of gold fish, because teh turtle just took a good sized chomp here and there. Pretty gross, but reality.... :\
These turtles are Awesome. They're truly Aquatic. I so quite a lot of juveniles lately in a very dear price. It's not the money but not unless you can provide them a pond when they reach adulthood this is my Turtle!
I'm new to aquaria central and to fish in general (I'm just starting my first tank). However, I am not new to reptiles and turtles. Although I haven't ever kept them , I have care for the ones at the local science museum, and I have friends that do. There are a few species of aquatic turtles that are herbivores, but they are generally fairly expensive ($30+), and do produce a lot of waste. As for red eared sliders, they can get pretty large. I've mostly cared for yellow pond sliders which get to be a similar size, and they were each 8 inches or bigger. The only fish they were kept with were bluegill and catfish in a LARGE outdoor pond, and the pond still had to be cleaned often due to waste.
If you want some good info on aquatic turtles, go to Kingsnake.com and go to the aquatic tutle forum. They might have an idea on what turtles can happily live with fish. Hope it was a bit of help!
I'm doing this right now with a musk turtle. The fish is in a 110 with sunfish and a Texas cichlid pair.
My turtle doesn't bother the larger fish, but will snap at minnows or shiners which I put in every eight weeks or so. He also loves insects and will be the first to the top of the tank if I drop in a cricket or beetle. He does very well on the pellets which I feed to the fish.
I had this turtle in a pond type tank for 18 months and never saw him leave the water, so I don't believe that he needs to. The way my tank is set up, he could easily get out of the water and onto the glass above the tank is he feels the need, but again, he never has.
A quick edit: I forgot to mention that these turtles max out at three to five inches long.
I know of a girl on another board who has 2 turtles and 2 oscars in her 125(there may be a pleco or catfish in there too, don't remember). As far as I know, she's had the set-up for awhile and it works just fine. Of course, her oscars are of a decent size.