[Delphinus]

This might be a question of perspectives. But to me "quarantine" implies action on a newcomer which may be introducing hitchhikers of a sort. I.e., you QT a new arrival into an environment.

So. Being that you already have your fish, and you've had them a while, and they're healthy, there's no need, in my view, to QT them all of a sudden.

But. !! It depends on how you're going to do the tank switch-over. If you move your rock and fish in one day, you haven't "introduced" anything new (except stress). However, if you buy new rock and let your new tank cycle before you start moving over the livestock, then, although this will likely be far less stressful on you and your livestock, you've introduced the new rock, and thus from THAT perspective, there may be an advantage to "QT'ing" your new rock (but not so your fish that you already have).

Just my $0.02....

PS. (Edit - I just saw your second post). You've kind of hit the nail on the head. Any "new" fish gets ick. This is basically because, it's probably coming in with that new fish. (Stress in the store, sharing a tank with other stressed fish, etc.) So you would need to QT that NEW fish before introducing into your tank. But if your current fish are healthy, then I wouldn't be thinking of them as culprits for that ick you see on the newcomer. It's come in with the newcomer, even if the newcomer didn't appear to have ick at the store. Hope this rambling makes sense.