This is one reason I never buy fish from pet stores or 95% of fish stores.
I will buy from breeders, home based importers/sellers who actually quarantine stock before selling it or else as direct imports. The latter has the most risk, but it also means paying around 25% (or less) than retail if one can handle the quantities needed to get such a price.. The risk here is the fish come in with issues one has first to diagnose and then to treat with the right medication.
Bear in mind that selling live things carries the risk that they can die. They also require more complex care than dry goods on a shelf. Most stores which sell fish want to turn them over as fast as possible. They also want to spend as little as possible either medicating or feeding them.
Not all stores are this way, but my belief is many are. The ones which are not tend to be more expensive.
The best part of the above post is that railer20 knows the problem is not because the tank is not properly cycled
Keep an eye on things. Also, were there any external signs/symptoms which might help figure out why they died? Were theySsollen, discolored, fin loss etc.? Did they hide, did they poop?