I have a 60 gallon with an old emperor400 and 280 that's setup in my garage. A 6" angelfish occupies the tank and I usually leave her in unless Im using it for meds, which is very rare. I also have a 90 that has ben used for qt purposes and the angel "Lucy" will occasionally spend time with a fish in the 90. If I put the angel in the 90, I move the biowheels from the filters on the 60 into the sump on the 90 t keep some bb alive....
been setup and usually occupied by something for around 5 years now.....
since Ive used the qt setup, I haven't had to treat any illness in my displays, other than occasional salt from fish being rough on each other