Fish such as plecos, bristlenose catfish, and mystery snails are great options for cleaning a red-eared slider tank. These fish help in keeping the tank clean by eating algae, uneaten food, and other debris. They are also compatible with the red-eared sliders and won't pose a threat to them. Adding these fish to the tank will not only help maintain a clean environment for the sliders but also add variety to the tank's ecosystem. Ensure proper space for the fish to swim and hide, and maintain good water quality to keep everyone healthy.