I must say, I don't keep discus.
I just don't see why discus are kept at such high temperatures... They come from the same areas as angelfish, yet angelfish have lower reccomended temperatures. Is it because the Discus' water is more shallow?
I bet it's because discus are so expensive, people reccomend keeping them at high temps to prevent disease. Not to mimic their natural climate, more similiar to how pet stores add salt to their freshwater tanks.
I someone knows this to be wrong, let me know. I might keep discus one day. This is just what I've gathered from my own research. Discus do perfectly fine at 80 degrees or so, and that will free up tankmates. Just keep the water pristine.
Probably the fact most angels sold in shops here are bred in captivity, so have adapted to whatever conditions, where as a lot of discus are still being caught wildly, so they can't adapt as easily as other species :)