If you're going to QT, leave the tank bare bottom and put some PVC
pipe coffee mugs etc for the fish to use. Get some perigean or other ammonia detoxifier, get a lot of WC
's ready. And yes, I'd use tank water and slowly drop the salinity over a few days.
But if you're not going to QT EVERYHTING in the future, algae, inverts, fish before adding to the DT etc, I wouldn't move the fish. Moving sick fish into a small cramped space is only going to add stress and maybe kill the fish.
, I've read TONs more threads of people beating Ich keeping the fish in the tank, lowering stress levels, feeding a quality varied diet soaked in selcon and garlic, than I have from people moving their fish.
Granted a proper QT method and sticking to it in the future, is the best option, however, managing Ich once in the tank is easily doable. Just keep the stress low and food quality high!