Sounds like typical clown behavior, my 2 clowns sleep at the top of my tank and then swim middle to top the rest of the day. If you've only had him for a day I'd give him time to settle in. He doesn't have ripped find or any sign of disease does he?
20 Gallon SPS reef
29 Gallon FW "fat catfish" tank
20 Gallon Long Dart Frogs "Zig" and "Zag"