I'd actually post this in the discus basics for beginner's section. I highly advise against adding discus to a previously established tank. Mainly because of parasite/bacteria cross contamination.
If you are just starting with discus, you want to start clean and new. No other variables. What is your plan? What's your water like? How will you perform water changes? Are you planning to get adults or young fish? Lot's of things to think about before just adding discus to your old tank. Simply replacing the water will not do.