Why did my fish die in 3 hours? (Fish bought from walmart)

I have purchased Wal-Mart fish before, and also fish from another big box store, but I've never had to replace them quite that quickly. When you see a lot of fish in one aquarium your brain should go, "oh look, there's too many fish... they're stressed." If you still want some of those fish, look carefully at them for clues to their overall health. Clamped fins and tails means very stressed or sick or both. Drooping tail (unless the breed calls for it) also signifies a sick fish as most healthy fish hold their tails erect and spread out. White spots all over the fish is indicative of "Ich" and that fish should not be purchased. A bloated looking fish with pronounced scales is suffering from air bladder disease, and while it might not die, it will probably float upside-down as if it were dead. A fish with red spots on its gills is probably cursed with a condition that eventually will cause the poor thing's body to rot away while it's still alive. Watch out for ragged fins and tails, and weird "flashing" characterized by rubbing the body against objects or against the glass walls. Finally, if the fish you desire is difficult to catch... you've got a winner. Most sick fish will move sluggishly if they move at all when threatened. If the fish is too easy to catch, tell the store associate that you've changed your mind and pick out a different one.