Pangasius hypophthalmus, commonly known as a Irredecent shark. Grows to nearly 4 feet and requires a tank of around 1000 gallons.
Ask if the lfs to take it back, and tell them to put a sign on the tank, stating the fish requirements.
Give them back to him, as you didn't know what they are, and he fooled you. Or you can always try to rehome them.
Go to your lfs and explain whats happened and they will more than likly take them off your hands