more than likely the Ph difference was to much and shocked the "life" out of them. Also 40minutes is no where near enough time for them to get accustomed to 90* water, considering the water in the bag was probably around 80* to begin with, temperature fluctuation can be a real killer. so either one of those or both could have been the killer. Next time you buy fish, simply open the bag, add an airstone and use the drip method, save yourself some money. To quote the kid saying he just throws his fish in and never has a problem, I highly doubt that, Maybe you're in fact telling the truth, but your luck will run out eventually and just pray it's not a 300$+ fish.