What salt are you using for water changes? There was a bad batch of Instant Ocean with alk levels through the roof (I assume that your tank is testing at 10-11 dkH, no meq/L). I have heard of one guy in Victoria loosing all his fish. Try mixing up some fresh saltwater and testing the levels in it.
I seriously doubt that isopods are killing all your fish. I also have to disagree that you need a quarantine tank. It is an issue of hot debate lately on reef central as to the benefits/merits of a quarantine tank. Besides, if you are going to use a QT tank, you should really keep new fish in it for 4-6 weeks as this is the amount of time it can take for ick to show up due to the parasite's life cycle.
Just my .02
