Would a treatment with chemi clean suck out all phosphates to give a reading of 0.00 on a Hanna 713 4-5 days after treatment? Never used a Hanna before and was surprised with that reading. I assume it's because of the treatment?
Right. I don't see how it could directly affect any test either.
Assuming you previously measured high phosphates or had an algae outbreak, I cant imagine a bag of CPE could bring PO4 down like that in a 180g. Maybe I'm missing something but I think a testing error of some kind occurred, assuming you had measurable PO4 in the first place.
Unless your tank was totally nitrate blocked, or a swamp, phosphate readings are pretty rare. This stuff is for algae what candy is for kids. Even if your phosphates are through the roof your test will often still read zero since algae mops it up so fast. Almost none of the commercially avaliable tests check for organic phosphate. It is one of those things that I never test for. Checking for a small patch of hair algae or cyano is a far better "test". Once you see one of those it is time to change your GFO.
Btw, did you get the Hanna checker because you did not trust your old test?