Not sure honestly, i've never had sharks. Is this something he's done before or is it new? Is he kicking/rubbing his tail against a rock? The pic of the pleco that you posted a few days ago that had all of the gold dust on it -- do any other fish in the tank look like that? Sometimes it can be hard to see with the lights on; you can turn off the tank lights and make the room as dark as possible then shine a flashlight in the tank and see if you can see any dustings on the fish.
Just so I can review, what fish do you have left now and what symptoms, if any, are they showing? Have you tested water lately? If so can you list your params (ammonia, nitrite, etc). Also list any fish that died and their symptoms and any treatments you've tried so far. Seeing it all written out in one post will help me review it more clearly.