Just finished reading the posts in a previous "Discus" topic on here from awhile back. There is some information in the thread that I thought I could clarify. (Love the reason that thread was started. Lol.)
Discus can be kept in hard water, just don't expect to breed them in that water.
They require clean, clean, clean water for growing them out to a good size.
Ideally keep them in a bare bottom tank to grow them out to their full potential. (Easier to keep clean)
Water temperature of 80-82 or even 84 degrees is best.
I have well water, pH 7.6, TDS over 300, high KH and GH (forgot the numbers) and have discus. I confirmed their water and care requirements with people who breed, import and show discus. They are a high maintenance fish for sure! I syphon the tank out a few times per day and do daily, aged water, water changes. Once they're fully grown out they'll likely get a twice a week big water change and a tank with sand and driftwood.
Here are mine:
