Like I said before, negligence is no excuse. I am in a Business Law class right now, and I can tell you, that when they offered you a price and you accepted by paying and recieving the fish, you have encountered a legal contract for purchase. Just because one party was unaware of the actual price gives them no right to rescind the contract. They have NO legal right to ask for the fish back, nor do they have the right to refuse to sell you any more fish. If I were you, I would tell them you have a very good understandig how businesses and business contracts work, and if they want to continue to have their business lisence they need to treat you equally as everyone else after this incident.
Every lfs makes mistakes, this one just happened to catch it and attempt to call you out to make the employee look better. You know your moral and ethical standards, and they are most likely different than mine, but I would keep the fish and tell them to educate people more and prevent it from happening in the future.