You can't blame Aqua Giant for that. EVERY lfs gets a fish here and there that can be carrying something you can't see. Nobody can say they have never lost a fish or two from a top shelf retailer.
The healthiest fish in my tank came from Aqua Giant. And I know others with nice fish from there as well. The fish must have looked ok as you purchased it.
I am sorry this happened to you, but that is more a reason to set up a hospital tank.
Aqua Giant is a sponsor on here as well. I find it funny that nobody says anything when this sponsor gets bashed publicly. If this were any other SPONSOR the bullets would be flying.
Maybe consider using their forum and making your comments. Or call the store and perhaps talk to them. They are easy to deal with.
Looks like you had a dying fish issue in Feb as well... A few weeks back.
http://www.canreef.com/vbulletin/showthread.php?t=73142
Good luck. Hope you do not lose anything else.