Not to sure about corals, but maybe he got stung by sweeper tentacles?
Each packet contains 400mg Sodium sulfathiazole, 100mg Sodium sulfamethazine and 100mg Sodium sulfacetamide.
Looks like a lot of salt...whats your salinity?
Actually, I have been looking everywhere and it seems these two meds are used in conjunction:
I have not seen anywhere that the folks have used them in their main (I know you don't have a QT), but I did see mention of this possibly killing off some good bacteria.
It could be possible that the combination of the two could be the cause.
Does any one else have definitive answers?
Maybe you have one of these in your tank:
should be carefully monitored for the presence of a small parasitic snail, Thyca crystallina.