As mentioned, do a large water change and run carbon. This will go a long way towards correcting whatever could be wrong.
Check all the basics:
Salinity
Temperature
ph
Ammonia
Any large fluctuation in any of these, could have caused the fish death.
Also, look into any contaminants that could have been introduced into the water.
Then, also test for:
Alk
Ca
Mg
Any large drop or fluctuation in any of these could be what's keeping your corals unhappy.