phosphate at 0 use GFO or not??

My phosphate level reads 0. The water didn't even turn a shade of blue. I have a fuge, but if I run my reef lights HA will grow. There is currently no HA in my DT or fuge. I use chetto in my fuge along with whatever other algae grows. I would like to make it a reef tank very soon, as I have reef lights. So I'm thinking of putting some GFO in a sock and putting that in the sump. Any suggestions as why I'm getting a reading of 0 with no HA, but a day or two with lights on HA shows up. Is it that the chetto is taking in the phosphates? Should I bother with GFO? Tank is over 7 months old, 75 gal DT, 30 sump/fuge, 150 reef octopus skimmer.

Thank you!

Oh! I should add, the HA only grows on one side of the tank and down the center in the sand, front to back of tank...odd

 

What is your Nitrate level? GHA utilizes mainly Nitrates, where, IIRC, Chaeto needs a balance of both Nitrates and Phosphates (don't recall the actual ratio); if it runs out of one, it stops absorbing the other.

BTW, Cyano (red slime algae) feeds mainly on Phosphates.

 

Thank you for the info. My nitrates are at 0.