I know what you mean, sometimes you think you have everything figured out and a problem still sneaks up on you. I had three yellow tangs in my QT tank that were doing great with each other, until all three died overnight of what appeared to be deoxygenation. I felt horrible. Two weeks later I saw another three tangs all in one tank with eachother, so I decided to bring them home and try it again, this time with a large airstone and with the fish room door open to the rest of the house at all times. Again, the fish did great with each other and in general, but a week afterwards I watched all three succumb to labored breathing and bloody bruises on theirs sides within the course of twelve hours. I feel atrocious at this loss of life, but all tests on the QT turned up negative for any warning signs. At this point it must be something toxic or parasitic in the water, but since theres no way to test for that I am taking a page from your book and fallowing my QT for eight weeks...