@taricha I reran the test with a 1/10 dilution with RODI as directed. Hanna checker display 47. Throwing that into the equation (47*0.00765)+0.1169 returns 0.47645. In your first post, you said this "gives the sample NO3 concentration in ppm". Is this the concentration of the sample from my tank or the concentration of the 1/10 diluted sample so I would need to multiply by 10 to get the actual tank water concentration? Thanks!