IMHO, a QT should not have any livestock in it except the animal in quarantine. That way if that new arrival, or otherwise sickly fish or invert does not affect your healthy livestock. it can even be as simple as a small tank with a sponge filter.
I just had a few trouble maker starfish and two battered clowns in there my goby was having trouble with the flow in the large tank I think he just didn't look happy in there. I took down the whole QT tank tonight. It was commonly referred to as the swamp by my wife; it has a small problem with algae. Oh well. Live and learn.
275 gal, loving my new stand.
Always looking for big fish.
CARTER