Definately set up a QT
if your in need of treating a fish. Do not
mix treaments though, one or the other. Combining copper meds and hyposalinity will typically result in the fishs' death. Be sure you run a polyfilter or carbon as well as several water changes to reduce/eliminate the copper from the main system depending on type used.
As far as lowering the salinity, you can actually lower it much faster than it can be raised. You can check out step by step instructions on hyposalinity >>here
Moved to the sick fish/coral forum.