Hey Doc,
I'm guessing you mean aquarium when you say cage. lol RES turtles need water, as much as possible really. I'm new at this, but I'm happy to pass along what I know

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Okay so your tank size should depend on the turtles shell size. The rule is 10 gallons of water per inch. The heater is an awesome start. Try to keep the water between 70 and 75 degrees, give or take. Turtle's have to bask to be able to produce vitamin D3 and take calcium in. Basking is when a turtle takes in sunlight and raises is body temp and what not. Your floating rock should have a heat lamp pointing at it so the temp on it is about 10 degrees warmer than the water. Other than that you need a good filter as water quality is important when it comes to turtle health. The only other things I can think of are the UVB lamp that should go over the rock and of course turtle food. In summary it sounds like you need turtle food (pellets), a filter, a heat lamp, and a UVB lamp.

Hope this helps.