Sadly, curing marine ich is much more difficult than preventing it. I once lost an entire tank of fish to it (barring a chromis).
I like to buy cleaner animals that can directly treat the fish. Cryptocarion (marine "ich") is very easily picked off by a Labroides (avoid that genus unless you find a specimen that is very clearly accepting food), Lysmata, or Gobiosoma. Unfortunately these animals cannot not stop all disease; their services are limited to the removal of relatively large external parasites such as Cryptocarion irritans and small Lymphocystis.
I am sad to say that you may be better off not taking my advice as I recently lost four of my fish to an array of diseases, including one that I cannot identify and have never seen anything like. The other diseases the fish got I could loosely identify, but were far more virulent and acute than I have ever seen them. Three of my fish may not be out of the woods yet. Worst of all, none of these were ich or any other disease my Wrasse or Shrimp were adept at removing.
The moral of the story is to prevent disease by feeding Spirulina (or other immune-booster), having a UV sterilizer (with a new bulb), keeping upkeep, and having plenty of "doctors". Not to say that you did anything wrong, 'cause you didn't.
-Lynden
P.S.
Freshwater baths are a good way to save a fish that may be doomed otherwise; chemical treatments are (to me) a last resort.