If you not test your NO2 (Nitrite) - how do you know your real NO3 (nitrate) if you not know how much the NO2 interference with your NO3 readings. This is especially important in the start because - depending on NO3 test kit - nitrite (NO2) can interfere with 50 to 100 times the read NO3 concentration. Rather important in a start when NO2 around 0.5 not is unusual - it means that you NO3 test will show between 25 - 50 ppm to much (depending of which test you use)
If you not believe me -
see here Scroll down for English. The video is not in English
Sincerely Lasse