You don't see too many max size dempseys, and even then would take years to get near that point. Dempseys don't make good solo fish, they will be somewhat skittish like that. I'd look into some non-cichlids for something a little more interesting
some dempseys are good solo fish... I have 1 that loves being a single(with a catfish)... dont count out the solo jack ...you just need to find a good one