You also need to take care of your nitrite issues. Discus don't tolerate poor water quality. What is your filtration setup? You may not have enough biological media to house sufficient colonies for nitrification.
And on another note, the discus that LFS come into usually aren't the best quality and they're often not in the greatest health. They may have looked ok to you, but to someone more experienced they may actually not have looked to be in good health. It would help to order from a reputable source. Also you can post pictures on here before you buy if you'd like, and we can assess their quality/health (not as good as in person, but better than nothing at all).
And on another note, the discus that LFS come into usually aren't the best quality and they're often not in the greatest health. They may have looked ok to you, but to someone more experienced they may actually not have looked to be in good health. It would help to order from a reputable source. Also you can post pictures on here before you buy if you'd like, and we can assess their quality/health (not as good as in person, but better than nothing at all).