i'm sure members have read my threads before they know my tank is young @ 4 months. I have the Hanna phosphate checker. It's not the ultra low just the marine. i'm very careful about my testing procedure but sometimes even within a day I notice quite a bit of variability. I use a turkey baster and make sure I use the same column of water for both control and sample vial. for example, a few days ago the sample bottle immediately turned blue when I put the reagent in it. It was a high rating. "I said geez, that's very high." I repeated the procedure exactly, with a turkey baster, making sure I use the same column of the water. second time reagent did not turn the vial very blue and the phosphate value considerably lower. i make sure to use all the reagent and equal volume of water in each vial, clean the vials. IDK. one day it's high, next day low.