James my wife is able to somehow make api test kits work and match the other kits. You must follow directions to the T. Mix for allotted time. Add when directed to and mix again and wait for allotted time. Be very careful not to get seepage from under the cap. Shake chemical bottles as directed as well. So there's a ton of waiting and shaking going on doing that Nitrate test.
That being said your system is absoloutely not devoid of nitrates. Look at all that green on those rocks
This is also where its super critical for you to clean them up so you have an accurate actual number your looking at when your taking those tests.
Remember most of your nutrients are sitting in on the surface of your rock and sand bed
Clean that rock and then test again after a day or 2 of feeding thats going to give you your numbers your looking for