They fight to establish dominance. If they already have established that, then the bully might kill the other. Isolate the bully for a day then reintroduce. One of my clownfish developed a sore on his chin during the fighting (white spot with a red center. Center got more red over a week.) They stopped fighting after a week or so and then the sore cleared up over a month. You might try feeding more so there isn't food aggression. Also feeding more will give the injured clown energy to heal. You could soak food in selcon for an immune system boost as well.
