I'm going through the same thing with my coral beauty angel at the moment. I bought a couple true perculas from a LFS that I swore I'd never buy from but the regular place I like hasn't had true perculas in weeks. So after they were in the shop for a week I bought a pair and foolishly added them directly into my show tank (90 gal , 4 hermits, 1 coarl beauty angel). They must have been hosting some ICK, im assuming. The coral beauty certainly wasnt distressed with the newfound tankmates. they were swimming with eachother in minutes and continued on for the remainder of the angels duration in my display tank. So i came home from work a few days ago and saw the tell tale spots on my angelfish, SIGH !
Who knows were it came from I guess.
In the past I have used the freshwater dip with no success and eventually lost fish because of it. I haven't tried hyposalinity, curious to see how your results are. I'm currently trying a copper treatment called Cupramine and it seems to be doing very well, the ick is mostly cleard up already (i count 3 spots on it now) but I isolated the angelfish in the QT early on.
I just notices a white stringy feces about 1/2 long hanging from on the the perculas in my main tank. I hope he(smaller one) doesnt have a parasitic problem too
Anyways, keep us informed I'm certainly curious for your results, best of luck!