Diane Walstad: "Ecology of planted aquarium" 2nd edition, Echinodorus Publishing, page 62:
"Nitrification is the two-step process whereby ammonia which is toxic is converted to nitrate, which is not toxic. [...] For example Spotte reports that 400 mg/l (!!!!) of NO3 did not affect the growth or mortality of two freshwater fish, largemouth bass and channel catfish."
By me it is one big misunderstanding. Nitrates are NOT toxins, they are only quite fair total water pollution indicators.
Of course discus are not basses or catfish. Of course nitrates can inhibit young discus growth. But nitrates are relatively save for adult discus fish.
So, if all above is true, why your discus become sick after nitrates reach 20 ppm or higher level? Having only 1 year experience with keeping discus in US style (with neglecting water hardness and its pH) I can suspect only- Your fish are infected by pathogenic protozoans and/or worms. If not, your fish exhibit excessive mucus production in hard/basic water. This mucus is a medium for pathogenic bacteria growth which eventually infect your fish gills, skin and fins. It is probably the reason why you can keep healthy discus only using enormous WC.
Hans