hi there we are having the same problem with our tang ,and also looking for answers . but would also like to know what others have told you to lowerer the salinity if it will stress out corals and starfish.
inverts and corals cannot tolerate lowered salinaty enviroments if you must use hypo treatment its best done in a sep tank.the sg of hyposalinty is 1.010 inverts and corals will parish in this enviroment as well as sharks rays and eels.
this a very good web site for all types of marine tank related issues http://www.wetwebmedia.com/marine/index.htm