Do you use Seachem Prime? I've read that it will alter your nitrate readings. Look in the seachem forum on apc for a few threads about it.
Yes I do, but only for my tank water. The test solution I mixed up was made up of RO water only. The nitrate test kit (API)is crap. Now I wonder about the other tests (API master kit). Guess I'll have to make known solutions for those tests.
I'm just going to pay close attention to what my plants and critters "say" to me about my tank. At least I know the levels of ferts that I have been dosing is ok for my tank.