Well, I'm not going to be much help on exactly why your fish died, though something toxic to them in the water would probably be my vote, since it looks like the rest of your water parameters look just fine, especially for fish. Could be an aeration problem too, and they just suffocated to death. Do you have a significant amount of microalgae in the tank. I believe that can deplete the oxygen supply.
Anyway, the real reason I posted is to tell you that if you haven't already, try to find a store that specializes in fish rather than someplace like Petco. I don't know about where you live, but around here (and I'm right by the water), they don't concentrate in saltwater fish, and the staff is usually less than knowledgeable about them. It seems unlikely it would be the case with all of them, but perhaps the fish you got were already sick.