I don't think the chainstores, (Petsmart, Petco) carry tanks that can support Pacus, Silver Sharks, Grouper, etc.) and therefore should not carry those fish. If tehy can't sell a proper home, they shouldn't sell the fish.
They need to be more responsible since a lot of their customers are impulse buying with no regard to the actual need of the fish. Also, the chain store don't take the time to train their employees to educate the customer and turn away those customers that can/will not care for them properly. And like he said, if they don't sell a large enough aquarium for a particular fish they have no business selling that species.