Absolutely not;-) If I am simply switching MY fish around, yes, I will use the same water, but anything new coming in gets a completely cleaned pond and filter media ... I want to know that the incoming fish are truly in QT. If something goes wrong with the new fish, then I know EXACTLY where to point the finger. A great example would be that batch of babies I got last year, they came with anchor worms ... I didnt catch it for a long time as the temps were down, and when I did, I only spotted, by luck, one attached worm each, on two fish. out of just over one hundred fish. What if I had missed them? Then anything else coming in would have ended up with them.
CW, If I see anything then I break it all down. Too many times I am simply moving a fish that I already know a history on and I feel like my chances of stressing it out are far worse in a newly set-up system instead of an established on. I'm not saying it's the best way to do QT it's just what has worked best for me. I do realize there is a certain amount of risk I do take doing this, but I have not lost a single fish other than by predator, or suicide in the last four years. IMO most of the reason I have been this successful so far is two factors, less stress while moving, and less stress while QTing and making sure everyone is clear to go before adding. We are now scraping and scoping both before adding new fish to the QT and again before adding to the display pond.