just run GFO and test the tank and effluent of the GFO reactor and replace the GFO as needed. I'd recommend you use 1/2 the required amount and change often, testing the effluent. I wouldn't drop the PO4 to 0.00, but try maintain it so it doesn't go over 0.10 ppm.
Also, if you have alot of PO4 bound up in rocks, it may see-saw as the levels go down, and release, etc. You may have to use alot of GFO or you can bring it down with lanthanum chloride and then maintain with GFO
Also, if you have alot of PO4 bound up in rocks, it may see-saw as the levels go down, and release, etc. You may have to use alot of GFO or you can bring it down with lanthanum chloride and then maintain with GFO