Looks most like a Sergeant to me, though the only species signed is the Indo-Pacific Sergeant and they look a lot brighter than this guy. There were a couple of these in the tank
Could be Indo-Pacific Sergeant (Abudefduf vaigiensis) but hard to rule out other species as it is so dark, it's hard to see the markings one would usually use to identify. I've seen similarly dark specimens before, I believe it has something to do with spawning and/or egg guarding?
