ok.....
First, don't use crushed coral!!! Crushed coral will cause the sharks underside to be scraped up, which will lead to disease. Also, sharks require good water quality and CC will only raise the Nitrates. Before going out and simply buying a shark, I suggest you read this book on sharks. I think its something like "Scott's Shark and Ray Guide", not to sure though, so I suggest you wait. Also, what size is your tank and what other fish do you have in there? Angels, puffers, triggers, and most tangs deffinetly cannot be houseed with sharks. They will bite at the shark's skin and eyes, causing stress and disease. Smaller fish and almost all inverts will be eaten or very stressed out.