If you want to use it as a hospital tank right now, add some filter media from the other tank to the new filter, and you can add the fish right away, just keep an eye on your parameters. If you want to use it as a QT in the future, I would wait until you need it to set it up, then do the same thing. If you want to leave it up all the time, you'll need something in it to keep it alive. Or when the QT is not in use you can tranfer the QT's filter over to your DT and run it there to keep your bacteria alive.