Transfering the water from the main tank is a good start but you should still cycle the QT
like any other tank. Once accomplished, you only need to place small amounts of food in it 1-2x a week to keep the bacteria fed. If the QT
is needed in a hurry, simpley transfer some biomedia from the main tank. Filter pads, bioball/bale or the like. Do not use rock or substrate though.
You do not need to do water changes with it while there are no fish but I would suggest a small 10% change with newly made SW
that has been prepared in advance. A day ahead of when new fish will be added. That will help ensure the chem is up to par.