This is just a suggestion (and someone, if you know any better, please jump in here), but I would get a couple of cleaner shirmps or wrasses to help get the ick off the fish. They are not known to live very long so don't be surprised if they die after the ick is gone but they will clean it off the fish. Kind of a natural process instead of medicating the tank.
I don't know about the whole tank and having the ick in there for 4-6 weeks, so can't help there.
Billy & Mary
155 Bow front Reef tank w/
sump w/ refruguim, 20# of live rock, and protein skimmer and pump
200+ of Live Rock, with a 5-6 in sand bed.
3 Pendent Lights w/ 250K each
wave machine w/ 6 power head
2 P. Clowns, 1 Tomini Bristletooth Tang, 4 Shrimps, 4 emenaral crabs & lots of other snails & hermit crabs