Of coarse I don't know how they keep fish in Petco's and Wallmart's (I live in Holland
) and looking at the reactions of everyone on this forum, it usually is pretty bad
But let me just say in the defense of pet shops, that sometimes it is very hard to keep all fish healty. Even when discarding the amount of work the tanks need to stay clean, to feed the fish, clean up the shop and help the customers, they sometimes receive fish that are sick from a wholesaler or importer of fish. And not everyone can spot a sick fish between healthy ones, and the sick fish spreads the desease to the rest of the fish without notice until it usually is too late.
Some shops also don't have a quarantine tank that customers cannot see, to put sick fish in. It is very possible that the employee of a pet shop cleared out the dead fish in a tank with sick fish in it, and moving on with his or her tasks of the day, only to see more dead fish in the tank a few hours later. And how many customers have seen these dead fish before he or she did without even saying? They might not come back anymore because of this! But it's impossible to sit next to the tank and keep an eye on fish that might be dying...
So don't give a pet shop a bad name if at some time some of their fish aren't healthy or even dead, it could happen to any shop even with the best of caretakers!
But of coarse, it is obviously the right thing to say when the fish are sick constantly and you can see that there is nothing being done to cure it, prevent it or to stop it from happening again!