Are you in Lancaster, PA? If you want to come down to me, I'll swap rock or media from my sump with you. It's cycled already. Test ammonia, api isn't great, but it's safer than not knowing.
What's your fishkeeping experience? Good things happen slower in saltwater. Bad things, from my experience happen just as quickly.
Always wait for ammonia to be 0 before adding fish. Since fish are in the tank with ammonia, add more bacteria. It'll take a couple of days to drop, but it'll go in the right direction. If the fish were fine, but died quick in your tank, sorry but bits your tank. Make sure there isn't something else going on there too, salinity and temp are the two big ones outside ammonia.
Damsels could have fought too. There are any number of reasons outside a rare disease.