Food, tank dominance and personality. Certain type of fish (clowns & tangs) are territorial and once established will protect their tank space by attacking/bullying any intruder that's smaller and similar size. Tangs are same, which is why traditionally it was recommended to have 1 tang/tank depending on tank size. These days u can overcome this w a large tank and as long as Tangs r introduced at the simultaneously so one fish doesn't have time to establish territory and then they also r fed enough. Tang's should be the final addition to a tank bc they'll harass any new comer. However each fish has it's own personality and may exhibit different characteristic, i.e. I've had a kole tang that was very shy and never bothered tank mates or new additions. Plenty of food will make fish less competitive/aggressive also. When fish shopping, I observe each fish's personality at the LFS and try to stay away from bullies.
