If the turtle is small I highly doubt it will attack the larger fishes. Second if you provide it with a floating piece of bog wood or something it will probably chill on top of it most of the time. Could work have seen it in a 350 gallon pond or bigger not sure, at a feed shop with some LMB's and BG's they were all happy together minding their own business. Or you could get a soft shell they tend to do ok with fishes.