Take it from one who knows Jon has been thru it pretty safe to say will never put anything str8 in the tank againYes and No. IMO everything, fish, coral, inverts should be put in a QT. This will save you headaches in the future. That being said it depends on what you are trying to quarantine the fish for. If it is only for Ich and you already have fish in your tank that haven't been quarantined then it is most likely already in your tank and I wouldn't bother with a QT. If you keep losing fish in your QT you might be having ammonia spikes. Depending on how you have it set up you will need to do a combination of prime (ammonia remover) and water changes. I learned the hard way about quarantining everything and am doing it the right way now.
+1, Also some fish are far more sensitive to copper and need a slow build up or a different method entirely. Any other medications? Using DT water isn't a bad thing, but most of your bacteria populations are not free swimming, so this won't be enough to act as your biological filter. 25% water changes every two days might work, but that depends on like Eric touched on the amount of fish and size of tank. I know when I was doing the Tank Transfer Method and dosing prime I needed at least a 50% water change every two days to keep the ammonia low (I decided this last round to do a transfer every two days instead of three and avoided the water changes all together and got more transfers in, while using the same amount of water in the end). Also depending on where you bought your fish from they might not have been healthy to begin with and nothing you could have done would have changed them dying.If there isn't ammonia then it should be fairly easy to keep fish alive in qt. How big is your QT and how many fish did you put in it? Were you running copper and, if so, did you test to make sure the concentration wasn't too high?
Winner Winner Chicken Dinnerit would be easier if you cycled you QT, then your new fishes have better chance of staying healthy. just put some live rock in there for instant cycle.