By the way, the reason I recommend tank transfer above all other treatments for ich (cryptocaryon irritans) is because of the nature of the life cycle.
The front end of the life cycle has the least variation of time for each stage and it begins with the trophont.
The stage where the parasite is attached to a fish is called a trophont. 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 what is called a protomont. The dropping off of the trophont is relatively predictable from a time perspective. However neither the protomont not trophont is affected by copper.
This protomont travels to the substrate and begins to crawl around for usually two to eight hours, but it could go for as long as eighteen hours after it leaves it's fish host. Once the protomont attaches to a surface, it begins to encyst and is now called a tomont. This too is a relative predictable time period but again is unaffected by copper. However, the front end of the life cycle is highly predictable and tank transfer eliminates the tomont before tomites can be produced.
Division inside the cyst into hundreds of daughter parasites, called tomites, begins shortly thereafter. As a consequence, this explains the exponentially increasing effect of ich in a closed system such as our aquaria. This noninfectious stage can last anywhere from three to twenty-eight days in general, but the longest known period in the marine biology literature is 72 days. During this extended period, the parasite cyst is lying in wait for a host. However this illustrates why ich seems to disappear even though it is still present in the tank.
After this period, the tomites hatch and begin swimming around, 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 also the only portion of the life cycle that is vulnerable to copper, chloroquine phosphate or hyposalinity.
They prefer to seek out the skin and gill tissue, then transform into trophonts, and begin the process all over again. What this means is that when your tank is infected, you can actually see symptoms during a very small part of the life cycle, and it why your tank is infected even though your fish are resistant. It will also explain why symptoms come and go.
