[George]

How big is the tank? How many fish and what type? How much do you feed them? Is it possible to cut back the feeding?
GFO works. You might not use enough of it. Increase your GFO. measure the effluent from GFO, if PO4 is not 0, change your GFO.
Grow macro algae in ref or building a algea scrubber.
keep up with water change.
Your rocks may leak phosphate if they were in high PO4 water previously.
liquid or solid carbon dosing will help, but they need nitrate. You don't have any nitrate. Is your nitrate value correct?
Skim wet.
Some use Lanthanum Chloride with success. Google it because you have to be careful using that stuff.
Good luck.