Another way to treat ich is to gradually reduce the salinity of the aquarium by about 2 ppt (parts per thousand) per day until the salinity reaches 18 ppt (about half the normal salinity of seawater). Maintaining the salinity at this reduced level for a month or so will kill off any free-living parasites.
Again, this is normally only an option in tanks containing bony fish. Corals, living rock, macroalgae, and most cartilaginous fish and invertebrates will not adapt to reduced salinities.
Monitoring and controlling water chemistry is very important. Use chemical buffers to ensure pH and alkalinity remain constant, despite the reduced salinity. Also note that temperature affects specific gravity, not just salinity. So, while a salinity of 18 ppt corresponds to a specific gravity of 1.012 at 77 degrees Fahrenheit, the specific gravity will be closer to 1.010 at 86 degrees.
Some strains of Cryptocaryon irritans are more tolerant of low salinities than others, in which case reducing the salinity further and/or extending the period of hyposalinity may be required.