there is a black/dark brown spot on my fish. It is on his tail a little under the posterior side of the dorsal fin (not on the tail fin, but on the tail). He has also been rubbing himself against a rock in the tank lately. any ideas what could be wrong or what I could do to help him?

You'll have to get us a picture dude.

A little more info will be needed. a close up clear picture will be best. At lease a give the size, shape, texture, raised or flat.

my first thoughts would be parasite or heater burn but with so little to go on i wouldn't put a penny on it. )