As said above, do some research before the hyposalinity treatment, but a frshwater dip is very stressful for your fish, and stress is the main cause of the ich in the first place. Not only is the dipping in frsh water by it's self stressfull, but the capture of the fish to do this is very stressfull too. If you don't have a quarantine tank to do the hypo treatment in, try soaking it's food in garlic before feeding. Garlic is like a natural anribiotic for fish, and can work wonders on treating ich by boosting the immune system. Don't waste your money on any of the ich medications, as 95% or more of them don't work anyway. Copper does also work, but it also must be done in a quarantine tank. Copper will kill your reef and ruin everything in it. If your live rock it contaminated with copper it will only be good for fish only tanks from now on. Copper soaks into the rock and is next to impossible to ever get out.
If you just have a minor case of ich, try the garlic out first. It won't hurt the reef at all. If you have a bad case, i highly recommend the hypo treatment. Just research it first and follow the directions exactly. You can lower thier salinity in 24 hours or so, but when the treatment is over in 6 to8 weeks, it takes about a week to raise the salinity back to normal levels safely. If you raise it too quickly, the fish can't secrete the salt from it's body quick enough and will die.