A pair of breeding discus can go in a 18" cube, but if its a community tank you need a larger gropupd as Jayjat said to spread the bullying out.
I would say 50L per fish is better, although I have eight in 300L as I have two but eheim pro II filters and do 50% water changes at least every week.
Just a note to anyone wanting to keep discus. They are not hard to keep at all, but I would reccomend a LOT of research on this forum, the net, books and specialised dicus sites before getting any. I spent six months learning about them and making sure the tank, filter and water was ready for them before I got them. A firend of mine has a 150 Gallon tank, he knew what he was doing and that took a year of planning and preparation before any discus were put in