waters fine then. i wonder if a parasite could do that? could the parasite have been present when you bought it, and it just took that little bit of time once you got them home to present itself once multiplied enough? be interesting to see what the
LFS has to say and if they've any left in store or if the same wiped out their stocks?
I'm sure its nothing you've done or not done as i've seen you around on here and you've had success with your other fish

i assume they had some natural algae or algae wafers to eat? not that that would be enough to kill it if you didnt as fish are hardy and seem to be able to survive a short while without food anyway. let us know how you get on with the lfs and if they have any insight.