Do you have fish in the tank? I would be very surprised if your nitrates were zero if you did. I suspect your test kit is wrong.... try calibrating it Calibrating Test Kits - for non-Chemists
, or test another tank to see if you get similar readings. I am doing ei daily, and have been slowly reducing the amount of nh3 (about 1/4 reccomended amount) because of high nitrates I would have by the end of the week (= or >40ppm). Most of it comes from fish waste/food.