Chances are, what you are referring to is just still a QT tank, but one that you can keep running all the time, instead of just when you need it. Theoretically, you could have a tank that could be set up at a moments notice with spare media for a sick fish ( a hospital tank ). And you could have a tank you leave up and running all the time with some hardy fish where you keep new arrivals for 2+ weeks, but then if the new fish get sick you've exposed the regular fish from that tank to the disease and would have to treat the whole tank anyway. :sick: .... which in turn kind of elimates a hospital tank in the first place!!
Your best bet is to just buy a cheap 10g tank and only set it up for new fish or to QT sick fish. Not to say you couldn't just get another tank and buy a QT tank as a third tank.... :thm: