Are you sure it is not the test kit? Nitrate test kits have a very short life, and I have foudn some to be awful. The Aquarium Pharmaceuticals nitrate test I had showed 80-100ppm on my frogfish tank 18 hours after setup. I tested it on fresh RO+salt water and it still read that high.
Using the same product (but different actual bottles) I tested my largest FW tank with two samples at the same time. One came in at 0ppm and the other at 160ppm.
See if you can't get a different test kit done on your water to confirm it is actually correct.