I battled HA for a year and finally won a week after I starting running GFO. Everyone else is right, unless you're using an electronic tester, chances are you'll never be able to get a good read on very small amounts. Beside, all the HA will suck up the phosphates so you'll probably test zero or low anyway. The GFO will suck them out faster than the HA will, so they starve. In my case, it was not "truly" hair algae, it was that rough green turf algae. Nothing ate it, not tangs, not turbos...nothing.