I might get flamed for this but in my experience I would add more fish......try over populating the tank.....just don't have a pair of the same species keep single specs.
I've had moderate luck removing the offending fish and placing in isolation sometimes for a few weeks. Redecorating supposedly helps, but I think it's mostly short term.
Keeping cichlids has alot more to do with " managing behavior" than just keeping the fish. It's just their nature really.








Even the smartest person has STUPID questions and answers...that's what this site is for. We're all just people bound for mistakes and blunders, even stupidity
...and about your problem, LOL, its everyone's problem as long as they're keeping cichlids. It is the very nature of these animals. Even a bonded breeding pair, even brothers and sisters, even the mother and own frys, will turn on or against each other in a matter of seconds.