Funny, I was talking to a Marine Biologist today about this same thing. He said there are actually about "25 different kinds of 'ich'" Most will go away on their own if the fish are healthy. The parasite needs a host to multiply. He stated that the parasite is in most tankks-after all, where does it come from to begin with? and when fish are stressed tehy are susceptible to infestation, but most healthy fish can shed teh parasites with their slime coat.
There is a chemical "Chem Marin" which can safely be used in reef tanks-no copper-as a last resort if you have an infestation and no quarantine tank. With large active fish, it is almost impossible to have a second tank set up and waiting to quarantine 1/2 dozen fish for 4-6 weeks.
So, you may never have the problem again-if your fish are healthy and tank parameters stable-but if you do, you have choices-depending on your tank and its inhabitants.
Wait it out, move to quarantine or use a product like Chem Marin.
Good luck. Worst case scenario is all the fish die-you leave the tank empty for several months-(no fun) then slowly add fish back-making sure they are parasite free before introduced to the main tank.