[tlo]

I qt everything b/f it goes in my tank. I have lost some fish in qt, but I think I would have lost them anyways if they went straight into the system. This way I can catch it soon and hopefully treat it. I keep my water parameters the same as in the dt (unless I am treating ick w hyposalinity) and this way it is less stressful on the fish when it's time for transferring.

Do you test the sg of the water of your lfs? Some stores keep their water sg pretty low- if there is a large difference b/t your water and the stores this could account for some deaths.
What size tank is your qt? Depending on the size and type of the fish - if the tank is too small - the stress could lead to death.
Do you have hiding places for the fish?
Are you monitoring water quality? -In a small tank ammonia and nitrates can build up fast.
another thing to think about is the quality of the suppliers that the lfs uses. If a fish is cyanide caught it will appear healthy, then die a few weeks later. Other than buying MAC certified fish, there is really no way of knowing for sure . Your best bet is to buy from high quality fish stores.