if you think your test is messed up then dont assume nitrates are 0 out of the tap. like asked above^ all the details from your tank are necessary.
IF the test are correct, something is adding to your nitrate total. Sticking to the assumption your test is accurate, tap water is 0, youre either over stocked, over feeding, under filtered or combo of all three. but if heres the kicker.. if your test are accurate, and your tap is 0, and you do a water change.. and your nitrates dont change.. then there HAS to be nitrates in your tap. and to further prove that thoery go buy some RO water, wc and test again. or your simply reading your test wrong and have much higher levels than you think. maybe youre up at 80+ and cut it down to 50? deep red and not quite deep of a red are pretty damn similar looking in a test tube..
test your tap for ammonia/nitrites. also find your city water report. i learned of a originally unknown source of ammonia in my tap from their treatment process that was slowly causing algae, killing fish and giving headaches.
if not^ then buy another test kit and test again.
my tank is dirted and is still leaching heavily into the water column and i consistently find my nitrates well over 80 and i hardly dose N at all.