Some folks will tell you the smallest tank a shark can be kept in is a 180. The beasts of the sea are a tough species to keep in an aquarium much less with little experience. Gasguzzler and some other experts will be able to help you. I am guessing the lfs said it was okay to keep the shark in your tank.
Sounds like he is stressed. Please tell us what type filtration you have, substrate, etc. Also, have you done any water testing today? If not...test and post the readings.
How long has the tank been set-up? Did the tank cycle? What else is in the tank with the shark? How long have you had the shark?
I sugest posting this on the Agressive Board.