I just added a jack dempsy to my 55g cich. tank . the albino African cich. Whom dominated the tank previously is now getting pushed around by the new jack. And when he gets chased he shakes violently like he is having a seizure.
Territorial dance/shake. You will see it in a number of cichlids. Even piranah.
It's a show of dominance, and usually ends up witht he fish sucking eachothers face in a nasty lip lock. Eventually one wins and becomes the alpha.
Often the alpha kills the other fish if there's not enough fish to distribute the aggression or if there's not enough hiding spots to hang out in and rest.