Treatment Option 4 - Hyposalinity:
Low salinity has been demonstrated to be an effective treatment against Cryptocaryon irritans (Noga, 2000). A salt level of 16 ppt or approximately 1.009-1.010 specific gravity at 78-80*F for 14 days was reported to kill the parasite. I have never experienced problems when placing fish into a hyposalinity treatment, but have routinely witnessed fish showing obvious signs of distress when brought back to normal salinity levels too quickly. For that reason, I try to limit the specific gravity increase 0.001-0.002 points per day.
One of the alleged benefits of this treatment is the resulting conservation of energy for the affected fish. Reef fish have to constantly drink saltwater and excrete the salt to maintain the proper osmotic balance. Lowering the salinity of the surrounding environment eases this energy demand on the sick fish, thereby allowing them to expend more energy towards fighting the infection (Kollman, 1998 and Bartelme, 2001). On the contrary, keeping fish in low salinity means that they don't "flush" their kidneys sufficiently. After long-term exposure, this can cause kidney failure and kill the fish (Shimek, pers. comm..)
The drawbacks to this treatment are the same as for many of the treatment options discussed above. Invertebrates and certain fish will not be able to tolerate it, so you should not apply a hyposalinity treatment in a display tank. Sharks and rays are two fish groups that do not tolerate this procedure. I would also not recommend this approach in the presence of live rock or live sand. The hyposalinity treatment will likely kill the worms, crustaceans, mollusks, and other life in and on the substrate, causing a severe drop in overall water quality.
I have another word of caution when using this treatment. I would strongly suggest the use of a refractometer or perhaps a salinity monitor. Swing arm style box hydrometers are notoriously inaccurate. The glass, floating style hydrometers are better, but easily broken. An accurate measure of the salinity could mean the difference between being inside the effective treatment range or being too high and ineffective or too low and jeopardizing your fish.
Even given its few drawbacks, hyposalinity is a great method of curing infected fish of ich in a proper hospital tank. Of the treatment options discussed this far, in my opinion, it is by far the safest. While none of these options is appropriate for use in a display tank, and all have their drawbacks, weighing the pros and cons of each leads me to recommend hyposalinity above the others.