I think once a well set up tank is established our test kits for N and P will read zero anyway, so it is almost a waste of time to even use them. I never test for nitrate (after about 6 months) and test phosphate maybe every 2 or 3 months just to be sure.
Chasing magical numbers will only get you into trouble. Watch your corals and respond to their needs. If they start become too light add amino acid to increase available nitrate. If everything looks good, leave it alone.
My 2c, your results may vary
