I used to manage a local fish store. I had first dibs on every shipment at wholesale prices... i used to try a fish out.. if i didn't like it or had issues with agression, i would take it back to the store and resale it. i've had yellow tang, purple tang, sailfin tang, powder blue tang, blue tang, naso tang, scopas tang, niger trigger, picasso trigger, crosshatch trigger, bursa trigger, 2 emperor angel, blueline angel, mejestic angel, queen angel, banna wrasse, red coris wrasse, lunare wrasse, coral hind grouper, copperband butterfly.. scott's fairy wrasse, dogface puffer, 2 golden puffers, s&s puffer, tobies, 4 different lions, zebra moray, 3 snowflakes, goldentail eel, peppered moray, purple firefish, royal gramma, 5 different pairs of clown ETC i can actually go on for days but you get the picture. see the problem was i would go reef, then agressive, back to reef and so on. I Believe in natural immunity. 90 percent of fish i have encountered have developed imunity to ich. anytime i would add a new fish, the new fish would get ich... no other fish in the tank would get ich. i have only lost maybe 5% of the fish over the years...
i'm not saying that you shouldn't quarantine your fish... but for example i wouldn't pull all your fish out for a mild outbreak. it make make it worse