Here's my plan. I use a 10gal that is empty all the time as I haven't needed to buy and QT anything for a long time nor do I plan to buy. For emergency like a Hospital Tank I take water from my main tank, mix with some stored I keep for water change, and go from there (I have filter, heater, etc). I will rely on frequent water changes to keep down all the bad numbers. Once I am done with the HT I clean it all out and wait for next time.
Should I plan to buy and then need the QT I would do the same but wait longer for things to settlle and again, rely on water changes.
Its a pain but usually only for three week periods.