our local petco is really good. the guy running the aquarium section knows alot about fish, and ive yet to see one dead (except in the feeder fish tank) on otherhand i had the misfortune to wander into walmarts "aquarium" section - and i do use the term loosely there were dozens of fish in little tiny tanks swimming amongst sooo many dead fish. i counted 15 dead in one tank. 2 tanks had whitespot, one tank had at least a dozen green puffers that had obviously been beating up the weaker ones. algae in the corners - list goes on it was as if someone literaly dumped the fish there and walked away. I went back a few times that month, and each time there were more dead ones and even sicker live ones. i think if the pet store is good with maintaining their tanks and sells fish fast there is nothing wrong with overstocking for a short period.