Excessive stress just doesn't kill fish overnight, especially if they were eating normally the night previously.
You definitely have a rapidly killing disease in your system. If you didn't quarantine your fish yourself, assume they weren't quarantined. Even purchasing pre-quarantined and prophylactically treated fish should be put into a separate system and monitored for a few weeks at least prior to putting into your main system. Also, if you've added coral to the system, there is also a possibility of a hitchhiking disease on the coral plugs, etc... Also, when purchasing pre-quarantined fish, you want exact details of how the fish were quarantined and if treated, what meds and time frames! There are many different opinions on the definition of quarantine and pre-treatments!
My concern at this point would be Brooklynella or Velvet in order to kill so quickly. These are two rapid killers. The photos aren't totally clear to make a definite determination. So, Velvet typically starts with rapid breathing and fish can be seen swimming into high flow areas. Brooklynella typically effects clowns and other damsels most but can hit all others as well. It typically looks like a fading of color on the head and back of clowns and then turns into looking like mucus sheathing coming off the fish.
If it were me, I would treat for Brooklynella first by what I've read in the thread so far. A formalyn dip for 45 minutes followed by 10 days of Metrodonizole every 48hrs for all the fish...and in a separate system from the main tank. If during that 10 day treatment you start to see dusting on the fish...then possibly switch it up and go to copper quickly. Then, keep your current tank fallow for 45 days for Brooklynella and 76 days for Velvet.