I've seen many zoos and public aquariums offering such attractions as overstocked pygocentrus tanks. I can only hypothesize that the fish are so worried about another fish attacking while their guard is down start a fight.
Just make sure you have plenty of filtration, and all should go well. I will warn you, in my experience with overstocking, if they do start to fight, they will usually all gang up on one. You might not even realize that there is one missing for a while!
Congrats on the success of this project.
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