If your tang has ich, then your display tank is infected and your tang will eventually come down with ich again. You should consider QT all your fish and let the display tank go fallow (no fish) for 10-12 weeks to try and iradicate the ich. As far as Hyposalinity goes, if you read the literature you will see that it is not very effective.
To answer your first question about the ammonia in the QT tank, you must do daily water changes and use carbon if not treating with chemicals or antibiotics.
Good luck, I battled ich for over a year trying not to use copper and trying all the anecdodal remedies to no avail. Finally I used copper and removed all my fish from the display tank for 3 month and am ich free. _________ 75g reef with Nova Extreme 8 bulb HO T-5 lighting, refuge, Remora skimmer, DSB, Seio powerheads, Acro's, LPS corals, assorted zoo's & mushrooms, Yellow Tang, Maroon Clown, Blue Cleaner Goby , Cleaner, Fire and Sexy Shrimp, Coco Worm, T.Crocea Clam, Derasa Clam
120g FOWLR, 260w Orbit power compact lights, Remora Pro skimmer, 30g DIY Sump/Refuge, DSB,Seio Power heads, Powder Blue Tang, Purple Tang, Flag Fin Angel, False Eye Puffer, Copperbanded Butterfly and a Moorish Idol  | 
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