Okay...I had an experience that I will share...I didn't use a QT when I first started, and by the time I actually had ich in my 90g reef tank...unless I had a spare 90g...I couldn't remove all my fish and do as was recommended by everyone on this site. No "reef safe" ich medicine worked, the spots would go and return in worse numbers.
I threw up my hands to give up, and then decided to just let the fish die. Well, I couldn't stand the thought of not trying to do something. I had read garlic will boost immunity, I also read that skunk cleaner shrimp actually picks the parasite off, and out of the fish' gills. So I began to use FRESH garlic juice, and soak the frozen fish food in it. I purchased two cleaner shrimps. All my fish were healthy enough to not get any spots, and my shrimps were taking piggy back rides on the sand sifter cleaning his gills as they went. The other fish would go to the rock the shrimps hung out at and get cleaned up....it was awesome to watch.
I added NO MORE FISH, because I knew a stressed new fish would be attacked, and by sheer numbers do in my survivors. My Hippo tang would always get it first, and all I needed to stress that little guy out was to do a water change. I upped the temps to 82 degrees because I read that was the ideal temp to keep a tang the happiest, and he got healthy and quit getting itchy spots too. Two years after doing that, I moved to a new home and I knew the stress was going to be there ... and I figured my fish were doomed. To my absolute amazement...it didn't happen, no ich outbreaks at all.
I assume that with healthy fish the ich couldn't keep a host, and must have died out...l have no idea when that may have happened. Since then a quarantine tank is the norm. I have never had another little parasite in many years. So if you are willing to keep the fish you have, and not add another one...just keep you’re remaining fish healthy, they might have a chance.
P.S.
I would not recommend anyone doing what I did. I had plenty of fish deaths as the result of ich. I managed to keep the remaining fish alive. That's the reason why I always recommend a quarantine tank; it will save you so much grief. An ounce of prevention is truly worth a pound of cure.