I recently bought 3 young bala sharks from the local petco. I have a few weeds, and a set-up of driftwood in the tank, however; the sharks like to keep swimming upward towards a certain corner of the tank. They seem healthy and do not appear to have ich(ick) or any other sickness. Is it just that my sharks are retarded, or is there something i can do to make them more inclined to roam the whole tank?