Hurray for not being fired and being a real manager!
Since I've been working at a LFS I've knowingly sold fish to there death. Don't get me wrong, I try to teach them and lean them in a different direction but they don't care. There are customers who come in and will leave with a fish no matter what! They just want to learn the hard way. There is also the customer that will strait up lie to me to get what they want.
The thing is you can only prod at them so much before they just don't go to you or the store again because they know that they will be put in fishie store court. I just say, "Hey, that fish will kill/be killed maybe you should get (insert fish here or leave your stock)." Customer replies, "No, I've done this before, I want this fish." Then I sell them the fish. If the customer doesn't want to listen to me fine, he can learn the hard way as so many of us have... Atleast I know I tried.