Marine Ich has a life cycle of a month or more, hence the general recommendation to leave the tank fallow for a 4-6 weeks. . .
From Bob fanners article at http://www.wetwebmedia.com/ichartmar.htm
Cryptocaryon irritans is a parasite with a direct life cycle, i.e. requiring no intermediate host like an invertebrate to complete its life cycle. The time per generation is temperature dependent, ranging from a few days for tropical to a week or more. If one considers the possibility of "resting stages", marine ich can wait out weeks to months before seeking out fish hosts.
A) Starting with a stage feeding (call trophonts) on its fish host. These are embedded below the epithelium (upper living skin layers) of host fishes, under copious amounts of mucus, not affected by chemical treatments.
B) Protomont stage leaves the fish, drops to the bottom and forms a resting/developmental cyst (tomont) stage persisting for 3-30 days generally. Becomes attached to and transmissible by any wet object. For about a day at 78 degrees F. reproduction occurs by binary fission; that is, by each cell dividing into two, possibly producing two hundred individuals (then called theronts). These encysted stage individuals are not affected by chemical treatments.
C) After 3-7 days, as tomites or theronts they break out of the cyst (typically at night, when reef fishes are often "sitting on the bottom") and swim into the water in search of a host fish, in a to several hours to a day or two at elevated temperatures they must find a fish host or die. If the parasite is lucky (and its host fish not so) it will find a host and burrow into its skin or gills. This "free-living" swimming stage is the opportune moment for chemical treatment.