Jay for most fresh water fish 80ppm is the the upper most level ...beyond that its considered by most to be toxic..
With Discus... I think they do best with nitrates under 20 ppm.. 5-15ppm is a very good range.
Discus can get very stressed as the nitrates climb which in turn affects their behavior and overall health.
Heres a decent article at TFH..
https://www.tfhmagazine.com/articles...p-with-nitrate
Hth,al