At my store we offer a 1 week guarantee, if they bring back their fish body + a water sample, if they don't buy the minimum required for a fish we stamp a "final sale" stamp on their receipt, and at least we can refuse to sell anything if we see the person is not going to take care of them.
But i know how you feel, i almost feel like a cheap bastard just showing them the strict minimum like, how to do the water changes with a small vacuum instead of only removing the top water, and they are like "well i always did it that way and never had a problem, i kept my fish for 6 months and more " and i always make this face without saying anything -.-... Some stores just dont care though and that is why a bunch of client have no idea what to do.
One example of that was a woman who came few weeks ago, she had bought for around 800$ worth of saltwater stuff and fishes, and she never ever had an aquarium in her life, another store wanted to make some quick money and just sold this poor lady all that, because she wanted a "nemo". When she came looking at the saltwater part she went "i want this fish and that fish, and this thing here" "well mam what do you have in your aquarium, how long has it been running" oh i just started it last week. She proceeds to show me some pictures and the damn live rock they had sold her, where bleached out corals lmao. I laugh but when we started telling her what is wrong, the poor lady wanted to cry. Some stores suck and don't guide the clients and give them a bad base to start. Other people are just..cheap as hell, if you don't want to spend money for the requirements of an animals don't even bother to buy one. My girlfriend works in the rodent dep, and she was telling a woman, how she should give some vit c to her guinea pig, and green vegetables from time to time, and vary her diet, instead of only giving her some Verselaga mixed shit, the woman went apeshit and screamed at her "well if i don't give vegetables to my kid, i wont give that guinea pig any"..like seriously wtf kind of excuse is that lol. Anyways, working in petstores you sure see a lot of crazy people...or in any sales place i guess.