Why do you want low nitrate ?
A very good question, indeed. If I were asked, my reply would be following:
(-1) The toxicity of nitrates is very mild, yet scientists discovered that even plants with good resistance to eutrophy (
Vallisneria sp.) activate certain enzymes and produce certain compounds which are considered unmistakable
signs of stress when nitrate content is elevated (roughly 5-10 mg/L). Admittedly, it has little practical significance in our hobby.
(0) Plants take up nitrates not only for assimilation but also in order to maintain ionic balance and osmotic pressure of their fluids. If nitrates are in short supply (which is natural), plants take up chlorides in their stead. Chlorides have been proven to provide certain protection against parasites, namely fungi. Again, this is of limited importance in our hobby, as far as I (we?) know.
(1) In sparsely mineralized ('very soft', below 50 µS/cm) water, elevated nitrates can easily lead to deficiency symptoms of other macronutrients.
(2) If nitrates are abundant, the suboxic zone (where microbes respire nitrate - denitrification zone) in the substrate is thick and may reach the very bottom of the tank. If this is the case, the zone where iron is reduced may be completely missing. This is important because iron, phosphorus, and other transition metals remain in their oxidized state, i.e. precipitated and buried useless in the substrate eternally. I believe this partially explains why those who lavishly fertilize with nitrates have to dose incredible amounts of phosphates and especially (chelated) transition metals.