We have never used any filtration in our QT tanks. The water is completely replaced after a fish has been in it. The new water is a 50/50 mix of fresh and old tank water. As long as you maintain the temp and have an airstone then you will be OK. When fish are being treated, change 25% of QT water every 1-2 days and replace with fresh (as long as it has been prepared for a coule of days) or display tank water.
Your LFS and importer should quaratine all their livestock anyway, if they are not then get a new one. You should only need to place fish or inverts in a QT tank for specific treatment.
I think this is a bit of a personal one. You can run a fish through 10 different QT systems trying to ensure it is completely disease free and end up killing it through stress.
Finaly, many treatments can affect the bacteria that are present in filters making them temporarily far less effective. This is why we never used biological filtration in our quaranteen tanks.
