Ich is like a cold.......when we get stressed, we get sick.....ich rears its ugly head when fish are stressed. Fish are REALLY stressed when collected, shipped, transfered and finaly dumped into a small tank,(compared to the ocean), with possibly some unsavory tankmates.
Ich will go away on its own as the fish fights it naturally. Some fish do a better job at this because of thier slime coats. Some fish, tangs especially don't do this very well and CONSTANTLY have ich.
Some say ich is always present in a tank and only appears when fish are stressed, when adding a new fish can stress out the others just as bad as the newbie, hence the appearance of the new fish contaminating the tank.....I tend to believe this myself.
If the params of the tank are in check it should subside, cleaner shrimp can help....and the "reef safe" treatments can help, however I found they just mess up the quality just as bad.
However, it is a judgement call, for extreme cases you may want to pull the fish and QT them and treat with copper.....but who want to rip thier tank apart? You can try a "reef safe" treatment but are generally expensive and take longer to treat than copper, or you can just get some cleaners, and wait it out....usually a mild case will subside, but be prepared to lose a fish if its massive......
Food for thought, it has also been said that ich rarely causes the death rather an infection that results from the scratching on the rocks.......
However, if you really want to be ich free, take out the fish QT and treat with copper, and let the main tank run fallow for about 2-3 months, the time for the lifecyle of the ich to run through and die, with time for overlapping generations to die off also.
Good Luck.