From what I have read and seen personally with my tank, you will see high aggression with same species, ESPECIALLY if one colors is brighter then the other! You can tell if one cichlid is the dominant one if its color is brighter than the other. Typically other Cichlid fights other cichlids for territory purpose. Usually in its natural state, the losing fish would swim away if defeated. Since they are placing in the tank and have no other place to run, the it will continue to get picked on. EVEN if is shown to be no threat, it will still be picked on.
That is why alot of people say to build lots of caves even plants so that they could claim a place as theirs or even have a place for them to hide.
Hope this helps!