You don't have to use water from main tank. When I hypo'd all my fish in a 29, I dropped the salinity over about a 24 hr period. With the White Face Tang, I dropped it in 12 hrs. Fish can handle the lowering of SG much better than when it is raised.
So for hypo, do I put the fish in QT filled with water out of main tank and in the course of a couple hours slowly lower the S.G., or does it take a couple days to get them acclimated to it?