Sounds like you're trying to "manage" ich rather eradicate it.If that's the case, your reasoning makes a little sense; the fish is going to get ich again. But ich is only one of many reasons to quarantine ALL new fish. What if the new fish has velvet? That parasite can wipe out your tank in a few days. If you are living with a known ich-infested tank, why in the world are you buying a new fish? I'd get down to the job of eradicating ich from your DT ASAP, the ich won't stay quiet forever.