What is the reason that some Discus fade?

I have a 150 gallon tank. Bare bottom. I do water changes 3x a week 80%. My temp is kept at 84 degrees. Water parameters, ammonia, nitrate, nitrite are zero, My Discus 12 of them 8 adults 4 4" are all doing well. Accept for what I call a fading syndrome. Currently 2 fish are dark, almost black. Still eating, still active but continue to fade. They get thinner and thinner over a period of a few months until they die. All the other fish seem fine. IN fact are thriving by all standards. I feed a beef heart mix, algae mix flake and New Life spectrum. 12 black neons are always healthy and a bushy nose pleco healthy. This current tank and group have been together for almost 2 years.

Filtration is done by 2 FX6 filters cleaned alternating months and an internal sponge filter just as a bio back up.

Is this fading just a part of keeping Discus? They never seem to come back from it although they act healthy, eating swimming etc.

Maro1