You'll need enough of the carbon to keep the GFO from clumping, so I would do a ratio of about 2-3 part GAC (carbon) to 1 part of GFO(Rowa phos), and run it in reverse, or slow enough that there is no tumble at all, otherwise the GFO will grind the GAC into dust, and that has been shown to be a cause of HLLE.
What do think about this configuration with the Vertex Rx-U 1.5L. Starting at the bottom: 1)Plate (no sponge) Carbon then 2)(Sponge) Rowa Phos then 3) (Sponge)then top plate. If this would work, then what flow rate etc. would you prefer???

