First of all, get some cork bark and use it as a basking place. You can squeeze it into the aquarium and it will stay put. Then you can raise your water level pretty high, high enough to put in a good fish filter. When looking at filters, it will say on the box, "good for aquariums up to 55 gallons" don't get that one, get one that is used for aquariums over 55 gallons. Turtles are very dirty. RES can live happily with other sliders, yellow belly, red belly etc. Mud and Musk turtles do well b/c they usually stay on the bottom of the aquarium. Just don't try to put a softshell or a snapper in there with your RES. Although, I have had softshells living peacefully with other sliders, it is better to be safe than sorry, they can be quite agressive.