Ok, then explain why I got 0 ppm on a solution of Green Light Stump
Remover? As I said, either the Stump Remover is not KNO3 or the AP
nitrate test sucks. I was ready to blame the Stump Remover until I got
another negative on a diluted solution of Seachem Flourish Nitrogen
solution. According to the label the Flourish solution is suppose to
contain nitrate. Incidentally, the stump remover label stated that it
contains KNO3, it just didn't say whether it was pure KNO3 or not. If
pure KNO3 is suppose to register zero in the AP nitrate test, then it
is pretty much useless for my needs.

The only time the AP nitrate detected nitrate was when I aerated a
diluted solution of lawn fertilizer (Scotts (or Scott) Turf Builder).
The lawn ferilizer contain Urea as I recall. So after several days, I
suppose the Urea was converted into nitrate. Of course this means that
the solution is likely to contain ammonium and nitrate as well.