+1 with plantkeeper. Here's the reason- they can't "talk" to each other. Cichlids from south america all use a similar method of communication, species from Africa all use another. So when one fish says- "back off, this my cave" that signal is sent differently depending on which continent the cichlid is from. (This is a generalization the more closely cichlids are related, or the closer the region, the better they communicate with each other.) Your dempsey won't get the aggressive signals from an african cichlid, and vice versa- so you end up with a ramp up in tensions, and usually the death of one of the fish. Now is that 100%? There's no such thing, of course. Some people get lucky and can mix small amounts of cross continental fish successfully. But usually it's a recipe for disaster.