i had a similar issue; one fish would start itching randomly then i would slowly see others doing it. My LFS recommended this stuff -
http://www.jlaquatics.com/product/sc...+-+5+Gram.html
its reef safe for i used it with my corals and cleaner/blood/harlequin shrimp and snails and anenome with no issues. I mixed it in with food for the first bit, then i treated the tank. Make sure you turn off the carbon and dissolve the medication in water first for it takes some time and stirring.
my fish do still itch randomly everyone few weeks but its never been persistant. No fish have died either. I make my own fish food and put a couple cloves of garlic in their to boost their immune systems - also added vitamins.