Perhaps I am lucky but I had a bad Ich breakout about 9 months ago. The 270g tank was 4-5 months old and had a 2 Heniochus butterflys, 2 clowns, 3-4 gobies, 2 Cardinals, and one fish that I cannot recall. I could not QT all of the fish so I made my own probiotic foods and saved all but one of the Hennies and the mystery fish. It took about 6 weeks but the ich finally died out. I think I could have saved the henny but he jumped through a very very small opening when no one was around.
I know I was too aggressive and my mistake was buying a henny that was too big for my QT tank. Since then I add only 1 fish every 2-3 months and now I am just one fish left (Mandarin) before I am done. Ich is an evil thing.