Id never try keepign a discus alone. I'd say 4 minimum to keep them happy. I do about 1/3 water changes every 3 days and mine are in a 55 gallon. They are young and all about 2-3 inches...but next year I definitely plan on upgrading them. They do grow quite large, 7 inches is common if they're taken care of properly. Also keep them around 84-86 F to be happiest, which most other fish won't tolerate well.
