What kinda fish are his tank mates ? Tetras I'm assuming right ?
Angelfish work nice, a couple of them would be good if you buy them small and have good plant cover.
Dwarf Gouramis
German Blue Rams
Kirbs
Apistogramma's
Mollies, platies, sword-tails and all the other fish made by Fisher-Price.
Edit: Ya Discuss are ok in a community tank, once they get really large, small Tetras like Neon's will go bye bye though. Also fast moving fish may spook discuss, they usually get used to them though. A 55 would be good, I would just make sure you double filter it for Discuss. As you know they are not the easiest fish to keep.
Lots of Options really. Good Luck