I would advise against any turtle unless you are willing to loose fish.The current inhabitants are to small to house w/ turtles & not be eaten if the opportunity arrived.The size of the tank will give the fish a chance to outmanuever the predator & snails are easier prey so you might not have to many loses. Just have a 10 gallon ready to put the turtle in.If you do get a mud or musk stinkpots(musk) are most common & cheap. Mississippi & Three stripe muds also aren't that much. You will need to make sure there are enough area the turtle can get to the surface as they aren't open water swimmers & need to rest rather than continue swimming like sliders do. If the fish are to have a chance let the center be open where they can see the turtle coming.