I am having the same problem. I checked the water temp at the bottom of my tank, it's at 78 degrees. Right where they like it.
I do have some other fish in my tank. A Six Line Wrasse, Flame Hawkfish and a Blue Spotted Puffer. Yet my Clown was the first fish in and had two weeks on his own, did not leave the top of the tank. Still does not leave the top of the tank. He eats well and sleeps fine, so I am not too worried.
Fish Sticks, the best thing you can do with your dead fish.