This is such a pet peeve (pardon the pun) of mine, but why does Wal-Mart sell live fish? Everytime I go to any Wal-Mart with tropical fish, they look sick, many having ich (white spot disease) or even worse - dead floating in the tank.
But still, they always have rows of tanks full of dying fish. And they keep buying more from distributors since the tanks are always full of semi-alive fish covered with ich or fungus.
Adding unhealthy fish to your community tank can effectively wipe-out all the other fish in you have. Before adding any fish to your tank, inspect it closely for torn fins, fungus, and white spots. Make sure the fish is alert, active, and swimming around while not at the surface gasping for air.
Most likely, you won't find any fish like this at Wal-Mart. And even adding one unhealthy fish to your aquarium can introduce diseases that can destroy your community.