Not to be nasty, but its kind of a double mistake if this is in your 55g, because these are "expert only" fish, that are one of the most difficult to feed and keep alive and much like Tangs, need big tanks (125+). Places like liveaquaria even say they are not good for home aquariums. Www.advancedaquarist.com
has a great article on these fish.
Better to go with Heniochus butterflies. Look kind of the same, but much easier to keep.
For Ich, its the same old story. QT the fish and treat with copper or Hyposalinity. Keep the DT fallow for six to eight weeks to let the It die off. In the end, you have clean fish going back into a clean tank. Anything else is not going to do the trick. Try to feed with garlic supplemented food to boost the immune systems. It helps, but is not a cure. When the spots disappear, understand that its just part of the life cycle, the fish is not cured unless you did copper or hypo.
Is it white patches or little white spits?