Still makes no sense cause a QT tank should still be cycled and ready to go at any time.. sounds like a bad way to QT IMO. Fish already stressed from new environment and small tank at that.. (is tangs are bad at this they will super stress in anythjng smaller then 3’ Long) stress weakens immune response and then bad water conditions only add to this.. what’s the point to even doing a QT if the tank isn’t cycled and ready to handle the fish load? Best case you have fish with ammonia burned gills that live half their life.. worst case they never come out of QT cause of ammonia.. if this is what anyone is doing this is **** poor husbantry and you’re making your fish sicker and weaker by putting them into Qt ca directly into tank. Even more so tangs who need and want room to free swim, are easily stressed and susceptible to lots of diseases when stressed. If this is how you are QT fish and corals you’re either going to spend a lot replacing fish and corals or have lots of DT issues. Likely both..
Your QT should be fully cycled and running ready for fish 24/7.
All that said.. if one were to be running this kind of QT spur of the moment setup I would advise to be using prime at the 4x dose to nutrize ammonia as that will have way worse effects in any specimens then some disease or oarisite they may or may not have.. best way to kill a sick fish is introduce ammonia even in small amounts.