Had a Tomini Bristletooth Tang come in with Ich and was treated, as are all our new fish livestock, with hyposalinity! It, along with a Flame Hawkfish, and a Black Sailfin Blenny (Atrosalarius sp.) went into the 20g long QT! The salinity was kept a 1.011 for 30 days, then we spent another 7 days raising it to the level of the DT, 1.0255 !
There were no visible signs on the tang after only 4 days, and no re-appearance throughout QT or since while in the DT ! Neither of the other two QT tankmates has ever showed any signs of Ich! Copper is hard on their vital organs and smooth bodied (scaleless) fish like the Blenny will suffer from Copper treatment!
Just my experience for what its worth!
John