dead fish need help

Perhaps the naso and the foxface fought all night....the fox face has poisonous spines.

The anglefish could have died from all the ammonia in the tank caused by the dead fish.

I wasn't there so obviously take this with a grain of salt.

 

Low oxygen level? May be possible depending on tank size, qty & size of fish, water circulation, surface agitation, etc.

 

I would start out with checking the basics.

Ammonia, nitrate, temp, salinity.

Could be stress induced if everything else was ok, I would look at low oxygen levels, but if you have plenty of water movement, that should be ok.