Hyposalinity in your reef tank would be a bad idea, the odds on loosing multiple corals are pretty high... I'm with Whoyah on this one... do whatever you have to and remove all of the fish from the tank and run a hypo treatment on them as space permits, sound like multiple hospital tanks is in order here. But don't leave a single host in the tank or you will not be able to break the ich's life cycle.

It really sucks rocks not having any fish in the tank for 6 weeks, but the investment alone is enough to warrant it the necessity.