Or maybe two males that are competing? Hope I'm wrong, when I first started out had 4 chromis, then one by one they picked on one till it died. Hope I'm wrong and you're right.
I don't know, sometimes fish start attacking each other for reasons known only to them. Maybe mating instinct, competition for mates, territory. Try moving some of the rocks around if it's not too much trouble... even the playing field.
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