pH, Alkalinity, Ca
and Magnesium are all related and should be in certain ranges. You may want to consider testing for them too.
From my readings I discerned that it is possble to show near zero or 0 PO4
and still have PO4
present. It's just that the hair/other algae are in equalibrium and using it as it becomes available. Once you remove the algae the test kit will then show the presence of of the phosphate.
On my last oubreak of hair algae (probably due to my overfeeding) I got some turbo snails. They are still in the tank, knocking over anything that is not glued down (small rocks, some corals, clams) and look about ready to be called escargot