I have a southern painted turtle in a 55 gallon with some rosy(fathead) minnows. He ate one when I first added them, but has ignored the rest, they have gotten quite large (4 inches +- ).
I try to keep red ramshorn snails in there to help with algae under the spot light, but i swear he has snail radar, the larger ones might last a couple days, but once all of the small ones are gone their time is usually up. Luckily I have a large supply of them (weather i want to or not) I try to keep a good supply of duckweed in his tank as well, but it's not so easy to find around here for some reason.
I have play sand as the substrate, with a couple pieces of driftwood and random rocks for him to scrape on. I use a whisper 40i and a CFS 500 for filtration. I also transplant a lot of the Vallisneria i pull out of my planted tank into his.
I think I need to make an above tank basking area, the uv lights grow algae like mad. and the mineral deposits from keeping the water level a little lower aren't very nice looking. (i have one of those floating basking platforms)
"A fish tank isn't a pet, it's a t.v. that you gotta feed."