OH GREAT... NOW WHAT

[George]

What is your quarantine protocol? I have seen all kinds of of so called "quarantine" methods. Some are very effective against ich, some are not.
If you really want to get rid of marine ich in your tank, get all fish out into a quarantine tank and follow a proven protocol against marine ich. The protocols I recommend are: tank transfer, copper and chloroquine phosphate. Some may suggest hypo-salinity also but I don't like it personally.
Leave your tank fallow (not hypo) for 72 days to get rid of marine ich.
That would be my course of action if it was my tank.
Good luck!