What is wrong with my red ear slider if it won't stay at the bottom of the pond?

Red Ear Sliders are not "bottom walkers" so it won't stay on the bottom. They love swimming, and can get quite large so a lot of swimming space is needed. They also absolutely NEED to bask to prevent shell rot and other not so nice medical conditions, including not digesting their food and eventually death. If you are concerned about your turtle, I recommend checking out either the Turtle Forum or the Tortoise Forum (they have a small turtle section), and then deciding if it needs to go to a vet. Most reptile specialty vets will check out your pet for 20-50 dollars, plus the cost of medications and tests.