How fix my turtles shell rott.?

I have 2 aquatic turtles, one of them is doing fine. The other is having shell problems. The lower half of his shell seems a little warped, and there are two spots that rotting away. I have a UV light and I feed him a good diet. One thing i've noticed is that he never basks. The other turtle will all the time, but i have never seen the one with the rott get out of the water. I even put an extra rock which really makes it easy but he still wont bask. Yesterday I dropped the water level in the tank down to next to nothing so that he would be forced to bask under the light, and let his shell dry out the spots seem a little better. Is there anything else I can do.