had this once where my whole population of fishes got ich (also from a PBT bought from CF!)
i ran UV lights (big wattage)...dose Aquapharm Cure ich (makes the water blue but at least i did not lose any corals). took me about 2 weeks to go back to normal but still lost a flameback, a adult ABT but my YT,PT,Flame, clowns and flamehawk survived...
i never fiddle with hyposalinity etc.. and also did my regular water change.
expensive lession for me after which i decided not to touch PBT ever even though the temptation is great!
you have to have the mindset that prepare for a wipeout and whoever survived is a bonus.........