Wow, thanks so much. I will do a few more control tests. I have before, but not so carefully as now.
I'm beginning to think that the KNO3 is definitely not pure. Is there any way to have K (NO3)2 maybe by adding some other element? like KMg(NO3)2 I know that is a silly question.
I know from tests that it does not have Ammonia or Ammonium in it, No Nitrites, No significant Mg or Ca (less than 5 ppm hardness as CaCO3). I know K does not interfere with the test I am running and I think it is calibrated, I will check that.
Thanks again, I'll report back when I figure it out.