My partner's tank was specifically to contain the fish that couldn't get along with discus, AND for a betta pair. Well, that didn't work. The fish (synodonts) that go after discus also like betta fins.

So to save the life of the male, we moved him to the discus tank. Fondness for 80 degree weather? Check. Fondness for tubifex? Check. Quiet, bothers nobody that isn't another betta---or food---? Check. Moved his mate over. Both are doing fine in the deep water, and are unbothered by discus, or by the tetras. All is peace and quiet. The bettas are doing great, and nobody's bent out of shape.

I only have a 52 gallon tank (but being a curve, has a 48" swimming run) but the 3 discus are getting along, after the initial sort-out. No fights if I keep them all well-fed. Thanks to recommendations on this site, they're on Cobalt and Hikari discus formula dry food, dried sanitary tubifex cubes, and the occasional treat of bloodworms, and only have issues of temper and pecking if their food runs short. I'm new to discus. Never seen fish so placid about my working with the tank.