i wouldn't suggest dropping the salinity below normal with the SPS and snails.
I have had hermits in hyposalinity before, but not other things.
good luck.

I would feed the rest of your fish nutritious selcom and garlic soaked food(pe mysis), as long as they are eating, they are fighting and that should be half the battle. best of luck.

garlic, fresh garlic, put it in a blender pour the juice in the tank and soak the dry food in it.
a cleaner wrasse works wonders too.