Cryptocaryon irritans; here we go again. A few misperceived points need clarification as usual.
This parasite may live many tanks but it doesn't have to be there. It can be eradicated and prevented. Ich free tanks are possible with quarantine and preventative treatment,preferably via the tank transfer method . Infected tanks left fishless for 72 days will be ich free going forward.
Ich is an equal opportunity killer ;attacking both weak and strong fish. If folks choose not to quarantine new specimens then ultimately, sooner or later most will find they actually chose to have ich in the tank . It's pervasive in the hobby as the fish we acquire are generally exposed to other fish with it somewhere along the line in the chain of custody.
Garlic ,cinnamon, pepper , herbal medications , cleaner shrimp, more food , etc. are useless hope and cope actions. Many who choose not to quarantine or treat effectively ,thus, choose to keep tanks with ich in them and may also use these ineffective methods with false hopes often reporting poorly perceived results not really seen with clear eyes seeing success when it's not be there at all.
If ich doesn't kill the fish in first infestation , it offers a respite for a few days,a week or even a month or two ; sometimes several months may pass as a fish develops partial immunity .But then; one day as tomites hatch out in large numbers it strikes and may do so again and again over the course of a year or years. It can persist unseen in a partially immune fish indefinately.
These respites foster a perception that the ineffective methods products help . Manufactures of these products take full advantage of the placebo effect in their marketing. Caveat emptor applies here.
In brief ,there are no reef safe effective treatments but several out of tank effective methods:tank transfer; copper; fomalin; quinine meds are new and appear useful.
I help Mr Turkfish and Snoravitch from time to time on the Fish Disease forum. Most of the info there is solid,imo. It's a good resource. A search in this forum will also offer lot's of information as well for those who want to learn more about this fish killer.
I know Norm( salty scottsman) knows what's up and will do the best he possibly can.