A QT tank shuld be a separate thing. It can be used as a hospital and treated with meds that you don't want in your other tank(s). Besides the meds, whatever is making one sick also should not be shared between the tanks.
Also when you get new fish and QT them (no meds or sickness involved - yet), the purpose is to isolate them in case they have something.
"Life may not be the party we hoped for but while we are here we might as well dance!"