I like it, before that I was not able to get low enough reading. My reading from checker has been around 0.03, I have been bothered by Cayno for long time, could never beat it. lately I am trying ULNS to see if I can starve them. I started carbon source in small dosage, after two weeks it drove my reading to zero (yeah myself don't believe ,but tested 3 times all 0). Even that Cayno still regrow back in same speed like before, after I siphon them out in WC. Seems Cayno still grow even there is no PO4 in water volume, it will get PO4 from sand bed & live rocks, or feed on something else.
Don't know about how to calibrate PO4 testing, I just simply tested RO & distill water by the checker all readings came out zero.