But sadly, fish deaths happen to everyone, from the most experienced aquarist on down. I lost 3 cory cats in one week and it took me a long time to figure out why. It happens.
Petsmart really does cram the fish in, don't they? I saw a tank at my store that had so many tiger barbs in it, I couldn't even see the back of the tank for all the fish. And even though the water and tanks are clean, there's got to be a lot of ammonia in there because of the overcrowding.
Good luck finding your new platies.