if you recently purchased the fish, i would march it right back to the LFS and demand the manager refund your money on the basis that their employees are ignorant, or plain lied to you about max size.
otherwise, you may have a hard time getting rod of a pacu. their one of the most posted about fish, and hardest to get rid of because of their max size
I'm in a unique position on that one. I won't go into it too much but there are reasons I can't take him back to the stor I got him from. I am posting the guy on a few boards hoping that someone will take him. I am going to put him on craigslist today too. I just wish I would have researched outside of the store prior to my purchase.... Lesson learned.