I have a 55 gallon community tank with mostly small, peaceful schooling type fish along with 1 female betta and a couple baby angelfish.
A couple days ago I brought home two dwarf cichlids, they are about the same size and shape as my female betta. I don't know if she though they were other betta or what but she started harassing and biting the male. She would alternate biting at him and hiding in fear, going very dark in color. She was even flaring, doing the I'm tough dance at the poor little cichlid.
I ended up moving her into her own tank, didn't want to risk her injuring my new babies. Has anyone else ever seen a previously peaceful betta turn on just one species of fish?