I think there is a little bit of play with the results, even with the hanna checker. But you never have zero phosphates, much less do you truly want zero. technically you want to be shooting for 0.03. But I have spoken with Hanna, and they told me at such low levels, you could get a reading that is 0.05 off, up or down. I thought this to be a little sad considering we are trying to test at the lowest range possible. But either way, as much as I feed my tank, I am always wanting to know my phos levels. And still would buy one if I were you.