I am a few days late to this thread, but I am sorry for your loss.
My experience with some chain stores is that roughly 50-75% of the fish I get from them tend to drop dead for no apparent reason within a month or so. Sometimes they only last a few days, sometimes more, but they are not usually healthy in the long run. So, since your params were good, I am guessing that was the case here. I have not had an different luck when purchasing just one fish, or purchasing a group. Its good that they refunded you though.