He will chase other cichlids as they are trying to claim territories. SD do not claim territories. They are used as dither fish, helping to keep your other fish out and about more. Target fish are used mostly used for breeding as to help take some of the agression away from the male so its not always on the female. Male will chase the target fish out of his part of the tank. If you want to take agression away from the dempsey use cichlids like convicts, firemouths, and jewels.
What are these other more expensive fish in the tank?