Howdy,
I am not saying it will change the world. But maybe there is a competent manager, who notices that a) they lose more fish than normal b) they sell fewer fish than normal c) thay have more sick fish in their tanks than normal ("normal" compared to other stores). The manager may not worry about fish, but about sales.
Lastly, it's a question of conscience: Yes, d20monsteroscar20 saved on fish. But due to his action and the action of others like him, more fish will be put in the same conditions. If d20monsteroscar20 really wanted to help fish, he'd keep his money in his wallet and walk out.
Just my $0.02
(which I don't spend on sick fish)
HarleyK