I treat every fish that comes here, I am not one that will put a fish into a QT and wait to see if something happens, I want to nip it in the butt before hand. So I feel my QT is cleaner than my pond. The only problem I would see is KHV, but if I didn't know where a fish came from it would be in the QT for a long time. So even if KHV showed up, the QT is isolated (in my mind of how I would build it), that it could be bleached, dried out, and then started up again.