Just wait a few weeks before you go back and if they bring it up then just say by law they made a mistake and have to deal with their losses for their employee's ignorance accordingly. If they refuse to give you service then, contact the B.B.B. and I'm sure you'll get an apology or the return of service atleast. Either way, keep it. They're in business to sell fish to people who are willing to pay a price for a fish, it's not like you conned them out of anything when you yourself wasn't sure of the specific type of species of bichir it was. Basically, I would never pay for another's mistake in a situation such as this but maybe that's just me.