I have API nitrates test 2, you must bang it, shake it stronger you can, else you will have a 0ppm reading, like I always had before...
I seriously don't think amazon swords drain nitrates like that with the lights you have and without CO2
injection... Did you bought the 2x
Do your plants look healty ?
Did you have very much diatoms ?
Is that light you bought ? http://www.amazon.fr/Glo-Galerie-Sus.../dp/B00E4B5GQE
I have the same on my 29g wide but 2x39W, my nitrates dropped at 0 once when I got green water...