anyone use Phosphate RX?

So Ive have a small problem with hair algae in my tank and only have a little bit left. it took months to get to this stage. I try to pull it out manually but cant get it all and after a week it thickens as it grows. I have tangs and snails but nothing eats this stuff. My PO4 was reading .07 and I have a 100 gallon tank with a 36 gallon sump. after LR and sand I have aprox 100 gallons of water.

I was at .07 and put 65 drops of Phox RX in and the next day it read .05. I added another 30 drops and today its still reading .05

I only feed 1 cube of brind shrimp and half a sheet of Nori for my tangs.
I do 10% water change every 2 weeks

would like to get my PO4 down to .01 and not sure why this didnt budge at all.

my nitrates at 0
DKh 8.4
mag 1280
cal 460
PO4 .05 as stated above

thanks in advance and love this site