There is no tank big enough, within reason, to change their behavior completely unless you introduce them into a community of fish. Some pairs live together well (quite a few in the Parachromis group actually), but some specimens of the very same species will never get along --- especially when isolated. There are several biological and behavioral reasons for this fact, and they explain why so many people struggle. A solid understanding of the gonadal cycles and the behaviors related to those cycles and to territoriality in general is necessary if one is to have realistic expectations.
IMO, the best and simplest source for this information is in Loiselle's book, The Cichlid Aquarium. It should be required reading for all cichlid keepers!
IMO, the best and simplest source for this information is in Loiselle's book, The Cichlid Aquarium. It should be required reading for all cichlid keepers!