I had a blue tang, and a kole tang that had ich. I took them out of the DT, put them in QT, and over the course of 2 weeks, dropped the salinity to 1.009. The ich went away, but now the fish had this black thing all over them, I think it was a fungus of some sort. The Kole tang ended up dieing. At this point, the blue tang looked horrible, was barely swimming and looked like she was about to die. At this time, I heard of a freshwater dip, I did this for 4-5 days, right after the dip she looked horrible, but 10 minutes later she looked like her old self, the FW seemed to be helping alot.
Over the next few days, I raised to salinity back up the normal amount, and treated the tank with copper. A few days went by, and she looked better than she did the day I bought her.
This is my experience, and in my experience hyposalinity did not do much at all for me, FW dips and copper, both worked extremely well for me.