Dang!! Sorry to hear about your troubles
Bring up salinity rather slowly for some reason related to osmotic pressure in the fishes' bodies.
Test your source water for ammonia. I recently found out I have .5 ppm ammonia coming out of our RO/DI unit. Filters were changed yesterday as a result of this finding. If you have ammonia in your new water, use Amquel or Prime to neutralize the ammonia. You can also use it in your tank.
Had two fish deaths the other day. Don't know the exact cause, but could have been because of ammonia levels at .5 ppm for less than 5 hours. I'd be interested to know if the fins on your any fish that die in your tank if the fins are open wide as if they are in full display.
Hope your fish don't die, though.