No experience on the Cuban but an adult male Dempsey is one tough fish. My 9-10 year old male has survived some of the most murderous fish my father and i have owned including large male jags, a breeding pair of jags, a very mean blackbelt, multiple midevils who killed everything else but it and two separate evil Buttis. I really don't understand how he's survived as long as he has lol
Cuban's are a whole different kind of aggressive. They are hyper aggressive. Once they hit the 6 inch mark there is no stopping them sometimes. I have a 7 inch male Cuban in with three jag's a kraussi and a red terror and he is the boss of the tank.
Let me clarify my thing. Dempseys are tough and resilient but in no way a aggressive fish really. Mine won't start anything but will stand its ground. From what I know of Cubans they are right up there with worst of parachromis and amphs in regards to outright aggression
out right aggression cubans are the best, but trhey may start something that they can't finish. When I was a kid, my dad's cuban got killed by either a catfish or this characin, I cant remember for starting a fight. But jacks typically don't start much and will last through almost anything.