i work at a petsmart and i understand the frustration with that kind of stuff. but do understand, we could separate goldfish and tropicals with a concrete wall ten feet thick and people are still going to get to the other side to mix fish. i routinely refuse people fish (which i am allowed to do) when i know they are keeping them in a tank too small or with unsuitable tank mates, but understand they will just come back when im not working or go to another store that doesnt ask questions about their tank.
so its not entirely on the store. believe me, i could tell someone 'this is a saltwater fish and will DIE in your tank' and if they could get it themselves, they would.
i get people constantly going 'oh thats ok, ill take him anyways' and that is when you know its out of your control because theyre idiots.