800L is a bit excesive, but if you can afford to go that large go for it, as sorcha said the bigger the better for the fish. My dad has three oscars, a dempsey a red devil and a quetzal and they are all healthy and get on fine. I think his tank is 600L (6x2x2) he did get em young, they went through a stage where they fought a bit as they were maturing and that was it. Oscars and jacks are fine from my experience.