I have a Nutrafin Nitrate test kit. Have always got zero or very low NO3 readings. I decided to verify the readings so I tested immediately after dosing KNO3. Zero nitrates. So I mix a much higher concentration of KNO3 and guess what? Still zero. Maybe I mislabelled the KNO3 bottle (highly unlikely since the plants pearl soon after I add it). So I use Ca(NO3)2 instead. Still zero.

Any recommendations on the Nitrate kit I should use to get reliable readings?

Thanks.