Well considering they are quite sensitive and need to be in groups as juveniles (at least until they start pairing up), I'd say 55 gallons. An established pair will work in less than that but you need a group to raise and then rehome all but the pair. With discus, the bigger the better. Keep in mind that you'll need room for otehr fish if you want it to not be a species-only setup and discus grow large (long and tall. Ask in the new world cichlid section as well - that's really where all the cdiscus keepers are.
