The Aquarium Specialist (Wonk) 

“Nitrates will affect your TDS readings - the other damaging effect of nitrates were explained 25-years ago, Takayuki Ezaki MD; Ph.D. Prof. bacteriologist, writing on behalf of the ZNA Research Division, shared his observations of the effect of nitrates on the white ground of Kohaku. He urged that the maximum concentration of nitrate be 15 ppm, stating: "Anything more than this and the skin gradually begins to deteriorate, improving again as the concentration lessens." He recommended that if nitrates can be kept at 5 ppm or less, "the skin becomes so white it virtually shines". He ascribed the negative effect of nitrates on (1) reduced the ability to discharge metabolic toxins from the body, resulting in buildup of metabolic wastes in the skin, and (2) on the pond/aquarium water becoming less receptive to dissolution of atmospheric oxygen into the water. Nitrate levels above 10 ppm adversely affect dissolved oxygen in the water column saturation levels  cannot be maintained.