I have 2 Yellow-bellied Sliders which are a subspecies Trachemys scripta, just like RESs (sorry if you know this already). I have tried both HOB filters, canister filters, and in-tank filters. They all do a decent job but, as I'm sure you've concluded at this point, turtles create a lot of waste in the water so will need to be regularly vacuumed. If you can get yours to eat in a separate enclosure I would definitely recommend it as it can cut down on food waste in their water. I've also used sand, rocks, and gravel as substrate for my turtles and have found no substrate in the tank is the easiest to keep clean for the sliders.