How do your discus look? Have they gained/kept/increased their coloration since you bought them? This is an essential diagnostic tool, as discus that are kept in water which is below their *very high* water standards (too many toxins or too high a water hardness) or which are fed an improper diet will lose their coloration.
From 'Fish Health' by Dr. Chris Andrews et al. "Water quality can have noticeable effects on fish health and appearance. [*Discus*], for example, are clearly paler in unsuitably hard water."
And: "Nitrite is less toxic than ammonia, being lethal at levels of 10-20 mg
/litre. Again, this toxicity varies with species. Guppies, for example, can cope with levels up to 100 mg
/litre, while some fish, such as *discus* (Symphysodon discus) suffer increased disease susceptibility at levels as low as 0.5 mg
/litre." (Andrews et al: 'Fish Health.' Toronto, Ontario: Firefly Books. 2003)
P.S. BrianNY will almost certainly explicate this further, but carbon filtration is bad for discus, as it stunts their growth.