For sure, I would also lean that way to the OEM reagent. If this was Alk we would be speaking much differently. I am okay if my nitrates are less than 20ppm so this looks like it may work for the purpose of checking a parameter that has lots of wiggle room.It’s cool that you’ve ran a long term test using both reagents. I agree that testing with the Hanna reagent then testing with the ATI reagent and having different values doesn’t mean that the API reagents gave the wrong value. For any of these tests to be valid these tests really should be performed on a sample with a known value, or at least be ran with tests from other manufacturers to see how they compare.
I will say that when there is a difference between the tests using the two reagents I tend to trust the reagent that was made to be used with the tester, although that doesn’t mean it to the one that’s the most accurate.