I had the same problem, never any nitrates showing up in just one of my planted tanks. I read on here about adding KN03, which is the ingredient in stump remover. (There is a thread further down here on that in particular.) I know it's just a fertilizer and the concentration is what makes it a "stump remover" but it was still hard for me to add it into my tank last week! But I did to get a small reading (less than 5). Today, before my water change, my reading was back to 0 so I added a granule more than last week.
Just as an interesting side note, when I did a search on potassium nitrate (KN03) to add to my tank, a couple of good reading sites that came up told me various methods of using it to make smoke bombs, LOL. (Oh, boy, fun at the cottage next summer!) KN03 apparently is the same as saltpetre.
Anyhow, I'll let the experts explain the "whys" I just thought I'd chime in my experience.
