Key is to start with good, high quality stock...as well as having husbandry practices to maintain their quality and health. This involves warm water, frequent water changes, and good food. Discus do not need soft water with a low pH. My personal hatchery has been running for many years with a pH above 8.0 with moderate hardness. Clean, stable water is what's most important. ONLY reason for soft water and low pH is when you have a breeding pair (spawning/hatching issues). Water changes are dictated by multiple factors. Stocking density, filtration, foods fed, size of discus (juvenilles vs adults). If you were to have a large tank, and understock it, overfilter it, feed clean foods (no beefheart) and have adults....you could get away with water changes once every week or two (50%+). Opposite is true if you have juvenilles. They need far more care and attention. Much more frequent water changes. There are many variables to discus husbandry, but the basics are all the same....clean, warm water, and good food.
-Ryan