I had an issue with this many years ago, and water wasn't the problem after LOTS of testing (and testing and guidance from a Cichlid expert that had his own exotic store). What we came up with was all of the electrical equipment (ie...powerheads, heater, etc) that was inside of the aquarium were causing something to happen within the water because of a grounding issue WITH those parts. They sell grounding kits at pet stores as well. You do understand that 'Hole in the head' is irreversible, right?
I had a 100 gallon tank back in the early nineties with these fish in it, and they all 'got along' okay for the mix of them. What may have helped was that they were all introduced to each other when they were all around an inch or two in sizes. The only exception was a black-belt (maculicauda) that was special ordered and was a live-caught fish. He had to be moved to a separate 50 gallon tank when he was around 8" or so as he was VERY agressive towards all of the other fish.
The fish list was:
Two Tiger Oscars (male and female)
One Jack Dempsey (male)
One Tri-color [Salvini] (female)
One Tiger Cichlid [Uropthalmus] (male)
One Convict (male)
One Firemouth (female)
One Bifasciata (male)
One black-belt [maculicauda] (male)
One Nicaraguan (male)
One Pleco (male)
........and two loaches that the Oscars ate within an hour or so after introducing to the tank when most of the fish were an average of 8-10" (lips to tail). And yes, that 100 gallons was ending up waaaay too small. It was heavy with filtration as well, with an underground with 802 powerheads, two 500 rear hanger filters, and a Fluval cannister.
So...to attempt an answer, I think I got lucky as I really do not think that in most cases that all these breeds would have "gotten along"