Join Date: May 2008
Location: San Francisco, CA
20H is if you only want to keep one discus and even then, that's kind of iffy. Unless you only want a small discus of 3 inches or so, then 20H is fine. But for adults? You definitely want to go bigger. Not only is it better for the fish but it looks better to the keeper as well. A good adult discus in a 20H will be incredibly cramped and they won't really been seen in their true beauty unless given good amount of space to swim in.
29G would be more suitable for a minimum tank for one or two discus.