Turtles are super messy. I have my tank set up where I have to drain and refill it weekly because the water gets so dirty. I would guess if you had fish in it, the rule would be don't get too attached to what fish you put in there because they will get eaten if the turtle has the chance. Red eared sliders poop, pee, and eat in their water. Your turtles will also have little respect for any decorations in the tank including live plants. They can be quite destructive, and also will eat them. You will have to make sure any plants in the tank are safe for turtles. I throw anacharis in mine every once in awhile for him to eat, and he loves them. Mixing fish and turtles can be done, but it will probably be a very high maintenance tank with probably at least twice weekly water changes.
