I have one discus in a twenty gallon, he will eventually need a deeper tank though when it is larger. I would say min is a 40 gallon for an adult 6 inch pair. His companions are neons,ottocinclus, and the occasional mirror friend
Anyway, discus seem to do best with a Ph of about 6.5-6.8 many claim though that you can have discus in 7-8 if they were raised in that high of a Ph. They like soft water with the Dh around 4-5. I do about 50 % water changes but, make them frequent not in one big water change at once. You may feed them special discus dry discus food but you should give them live and frozen food for a variety in the diet of the fish. Temp should be around 84 F. Background is a must because usually the discus will hide without one. Filtration is of great importance, on my twenty i have one HOB aquaclear 150 and one aquaclear mini. Good luck, hopefully you will start to love discus as much as i do ;D
*celebratesmiley*
Feno'stol
Daniel