Best feature for a alligator snapping turtle tank?

i have rocks, partly submerged wood, filter turned into waterfall. i also need something to keep his tank clean. i have some snails in there, but they are getting overwhelmed. there is some food that does not get eaten, and algae. i was thinking about some gold fish? .. it is a 55 gallon tank set up. If fish gets eaten, then big whoop! .. the turtle is only the size of half dollar right now.