The life cycle of this parasite is interesting and is important to understand when evaluating a treatment because different treatments work on various aspects of this life cycle. The stage in which the parasite is attached to a fish is called a trophont. While it looks like a grain of salt sprinkled on the skin, it is actually under the skin, making it inaccessible to cleaning animals such as cleaner wrasses, gobies, and shrimp. The trophont will spend three to seven days (depending on temperature) feeding on the fish and that is what you see symptomatically when you see "salt sprinkled on the fish".
After that, the trophont leaves the fish and becomes a protomont. This protomont travels to the substrate and begins to crawl around for usually two to eight hours, but sometimes as long as eighteen hours after it leaves its fish host.
Once the protomont attaches to a surface, it begins to encyst and becomes a tomont. Division inside the cyst into hundreds of daughter parasites, called tomites, begins shortly thereafter. This noninfectious stage typically lasts anywhere from three to twenty-eight days, however, the longest recorded period is 72 days. During this extended period, the parasite cyst is lying in wait for a host.
After this period, the tomites hatch and begin looking for a fish host. At this point, they are called theronts, and they must find a host within twenty-four hours or die. This is the most vulnerable stage of the life cycle. Theronts prefer to attach to skin and gill tissue, then transform into trophonts, and begin the process all over again.
Thus, when your tank is infected, you can actually see symptoms during a very small part of the life cycle, or, possibly, not at all. Also, this is why the symptoms seem to come and go.
Many hobbyists are fooled into believing they have cured their fish of the parasites when visible symptoms disappear, only to find marine ich present again on fish a few weeks later. Don't be lulled into a false sense of security. The parasites may be in a stage where they are merely regrouping and multiplying for their "next offensive." In the wild, this sort of massive reproductive phase ensures that a few will find a suitable host to continue on the cycle. In the close confines of our aquariums, though, it means comparatively massive infection rates and exponentially increasing infection.
