Well this issue has been debated. Some people say..gfo and biopellets is a no go..because biopellets will take care of both. Some other people are saying..that some phospate should be present so that bacteria don't starve to death...and some other people say to add a tiny amount of gfo to the carbon and run it together with biopellets...so obviously many opinions out there...I am just curious who makes the best biopellets...
My understanding is that the presence of both is required for one or the other to be reduced. Some has tossed around the Redfield ratio, but also read it's the improper gauge/ratio. If you're NO3 deficient, you'll be left with PO4; thus some would recommend GFO to remove the remaining PO4.
Have a read, particularly post #10.
http://www.reefcentral.com/forums/showthread.php?t=2109768