Why did all of my fish in the aquarium (tank) die? Guppies, catfish, tetra and glofish.?

My son got a 10 gallon fish tank for his birthday. We've had them a week and yesterday all the fish started dying off. He started out with 12 and is now down to 3, all dying within a couple of days. He had male guppies, a small catfish, red eyed tetras and glofish. All that are surviving is three of the glofish out of six of those. The fish started growing almost what looked like tiny crystals on them and they looked like they just kind of crystalized. So I'm wondering what we did wrong. We set the tank up and added the water purifier and let it run 24 hours before adding the fish. We have a small heater in the tank that is supposed to keep the tank at a correct temp. We fed them twice a day.the water isn't dirty it still looks clear as can be. Also, we did not overfeed. Any ideas? Thanks in advance.