Different people have different ideas regarding this, but I personally prefer letting a fish try to fight off ich by itself and using garlic extract and Selcon in the food, and minimizing stress instead of quarantining and medicating sick fish. Quarantine tanks are too small for tangs and will only stress them out and make them more sick, in my opinion. For most types of fish, if you fortify their diet, boost their immune system and eliminate stressors, they are usually able to fight off ich on their own. I have had fish get ich, but I have never lost a fish to ich, and I have never quarantined or medicated a fish for ich.
That being said, I have never had a regal tang, which are notorious for being very very very susceptible to ich. My strategy may not be the best approach for such a sensitive fish that doesn't really stand much of a chance at fighting off the parasite on its own. Regal tangs are disease magnets and get sick very easily.