I understand where the kid was coming from...because I too have read not to keep neons with rainbowfish...but I took that to mean the larger rainbows..not celebs. I also understand your point of view on not liking to be told "No..no you're wrong" when you know that you aren't. I could read up on all kinds of fish, but that does not replace hands on knowledge of them and make me an expert. I think perhaps you could have handled it a bit differently though. You told him to shut up, put his head down and bag your fish. I'm sure he felt he was right. What happens when someone purchases Red Bellied Pacu and he asks what tank size...they respond 10 gallon. He may question that customer, that customer could become as defensive as you. He remembers you and perhaps getting in trouble like when he questioned you. He then says nothing, drops his head and bags the fish. He then becomes the the horror story we hear on local fish store employees.
I think by mirroring his poor attitude back to him, you lost a valuable opportunity to educate him. I think I would have said something along these lines "Yes, it was often written in books that neons and rainbows don't mix. This information has now been proved false especially in the case of Celebs. I know where you are coming from with the information but unfortunately it is really outdated now. I could give you the name of a couple good books with more current info to read or even some web sites where you could catch up."
I could be wrong...just IMO
I think by mirroring his poor attitude back to him, you lost a valuable opportunity to educate him. I think I would have said something along these lines "Yes, it was often written in books that neons and rainbows don't mix. This information has now been proved false especially in the case of Celebs. I know where you are coming from with the information but unfortunately it is really outdated now. I could give you the name of a couple good books with more current info to read or even some web sites where you could catch up."
I could be wrong...just IMO