American Marine Pinpoint Nitrate Calibration Fluid Set - Calibration Fluid for Pinpoint Nitrate Monitor - 0.6 Pounds - Aquarium Test Kit

don't work at all

Stock solution of NaNO3 or KNO3 should be 1,000 ppm or stronger of nitrate nitrogen, then diluting as necessary (100 ppm, 10 ppm) but made up daily as it's not stable at those dilutions. Even the stock solution shouldn't be kept beyond 6 months, unless preserved (such as adding chloroform).