I used to do 8 weeks in a 30 gal QT tank but found it a pain to wait that long.
So now, only corals, LR, snails etc go into the QT tank now.
I now use the tank transfer method for fish, where every 3 days you transfer the fish to another tank for a total of 15 days.
I use 15 gal pails for this, each with its own heater and air hose.(3 pails) Mind you, I have not done a tang yet, this would require a larger pail/tank.
I also use prazi-pro for 3 transfers. No copper
Read up about this
http://atj.net.au/marineaquaria/3daytransfer.html
I mainly use this method to eradicate ich and worms/parasites
Luckily I have not had to deal with marine velvet yet.