You have to take the sick fish out and use the transfer method:
Remember that tank should just have an air stone no sand; etc..
You can also try lowering salinity but has to be done slowly and don't have experience in that. I do with the transfer method.
Using the transfer method
The transfer method is perhaps one of the oldest and still remains one of the best ways to treat marine ich.
The basic premise with the transfer method is that you move the infected fish to a clean, disinfected tank every few days. After the move, you clean and dry the old tank, removing any cysts and after a few more days, move the fish back to the first tank. When you do this, the parasites that fall off the fish never get a chance to reproduce and reattach to the fish, so after a few cycles of this, once all the parasites fall off and get removed, the fish have been cured.
In 1987, Colorni wrote about this. If you want to use his method of
saltwater ich treatment, simply transfer your fish to a clean tank on days 1, 4, 7 and 10, cleaning and drying the alternate tank for at least 24 hours in between uses.
The reason I consider this to still be one of the oldest and best methods for saltwater ich treatment is that even if your fish are infected with a persistent strain of the parasite, like the parasites in the studies that survived as a cyst for 72 days and 5 months, respectively, the saltwater ich will all be removed and either cleaned out or killed during the 24 hour dry period. This mitigates the advantages that even the most persistent strains would have.