rubbing

Hey, i have a columbian shark that always swims directly under the other fish, actually touching them... he always does it to all the fish. Any ideas why?

Several of the fish in my 180 CA/SA community tank will occasionally rub up against rocks/driftwood in the tank. The worst of them is my synspilum, who has actually scraped several scales loose from the side of his head. At first I thought this may be ich coming back (I had a pretty bad case...