It's hard to be sure on why the fatality was so quick but in general terms, stress and an enclosed (untreated) environment for the most part. Most LFS
keep a certain level of copper in their systems so some fish actually give the appearance of being healthy when indeed the problems only being slowed down. Keep in mind with velvet, the treatment only affects the free swimming dinospore so while on the fish, it is unaffected by treatments and can easily be carried into your QT
. I am by no means advocating treating prophylactically, just a possible explaination. I do not think anything you could have done would have been successful anyway. I would speak with the LFS
about a store credit though. This fish was sold to you sick.
Since it's near impossible to be sure what the tang actually died from, it's best to tear down the QT
, clean well and re-establish before getting a new addition.