I have an oscar & a common in a 72 gallon. I've found the opposite of Pimp, if I leave the room totally dark at night mayhem ensues. I'll find a good gallon of water splashed out in the morning, with either an oscar with missing scales & scrapes, or a plec with torn up fins. Luckily I have a basement light above the tank on a dimmer, leave it on about 1/4 of the way, and they only pull this about twice a month. They also tear off filter intakes, and try to shove the spray bar out of the tank.
These are rather large fish, 10" oscar, 15" plec. The only decorations are a large river gravel for a substrate, and a few silk plants. I had to pull out all the driftwood, they smash it against the side of the tank while skirmishing. I couldn't imagine a gt & a dempsey in the mix.
The oscar is the instigator, he spits gravel at the plec, the plec really doesn't like this, and does a bull charge at the oscar.