Chemi-clean will indeed knock it out fast. But it will come back if you don't get at the cause. It takes a while for the GFO to work. Be sure it hasn't clumped in the reactor. It's a bit of an art to keep it gently tumbling, kind of bubbling across the whole surface without throwing out fines, but that's when it's most effective, I think. I don't trust the API kits. They don't start to measure any phosphate at all (in my experience) until it gets to .25. By then the horse is out of the barn. You want a reading below .05, preferably .03 or less. If you don't yet have shrimp in the tank, you can try Dr. Tim's Re-Fresh and Waste-Away. See his web site. As I've noted several times on this site, that regimen is what finally cured my "headache." Good luck.
75 gallon mixed reef
27 gallon home office reef
Nuvo Fusion 10 nano reef