Painted Turtle Question

Any thoughts on why out painted turtle refused to bask anymore?

For awhile he was only "basking" at night. He would swim all day and slept in his dock. Now he swims all day and sleeps under water.

Background:
Leon was a rescue. We got him in 2012. He is 16yrs old. Old neighbors gave him to us when they could no longer care for him. He was in a tiny tank for his size. We bought him a 50g stock tank, built him a ramp/got him a heat lamp and he would bask like normal during the day and sleep in the water at night.

We upgraded him to a 60g aquarium so he could really swim as the stock tank wasn't very deep. Maybe he is just enjoying all the water? I'm really concerned he is going to get shell rot.

We have tried several different watt heat lamps. Tried reptile and no reptile carpet on his egg crate basking platform.

Aside from not basking, he eats, poops and otherwise acts exactly the same.