Just my 2 cents, but could it be possible that the bio load was just too much for the existing bacteria to handle? There may have been a couple days of extreme ammonia and or nitrite spikes that did them in. My understanding is that a bacteria colony will gradually adjust to increased bio load. When I started my 125 gallon, I only added 4 fish at a time, waited a week and my bacteria reestablished to handle that load, then I added 4 more, waited another week then added my final 4. They were all juveniles. I hope you get it figured out, it sucks to lose fish.