Fish die they are wild animals some make it some don't. You are always rolling the dice on a wild caught fish. I buy them, but i know its 50/50 chance at best. I worked briefly in the industry and when we transshipped fish from the islands there was always a minimum %20 die off out of the box, usually a quite bit more than that after the first week. I wouldn't be the least bit surpirsed if the majority of lfs's have to work just to break even on fish.
That being said everyone knows how I feel about quarantine. In short, it believe it to be much more stressful and detrimental to the overall health of the fish. If the fish's immune system can't fight it off on its own, well thats the nature of the beast.
Good luck, I wish you better luck than I have had!