The main parameters determining the usefulness of a deep sand are grain size, surface area, depth and availability of food for the micro-fauna. If your QT tank is, for example, ten gallons and is only rarely occupied...I'd guess it would be too small an area and not enough nutrients to keep the fauna going. IMHO...a waste of time. Much better to keep a sponge filter in the sump of your main tank that you can easily move to your QT tank when you are hosting a new arrival.