So the solution is quite simple, make regular water changes. As much as you can. See previous answer. However, I have a job and a family so this water change schedule is my compromise. But that's not enough.
Discus produce slime on their body.
This slime is shed constantly and coats the inner surface of the tank to promote bacterial growth. I try to wipe down every discus tank every other week. Others breeders wipe down their tanks weekly. Discus also prefer soft water, but this preference is far less important than cleanliness. In fact, achieving optimal conditions is far less important than maintaining constant water conditions.