You need to seperate both media in the FR. Pack the carbon at the bottom, then place filter wool above the carbon, then put the GFO (Rowa Phos etc) above it. Reason for this simply is that you do not want the carbon to fluidize because this will create carbon dust and you need the GFO to fluidize in order to have max effectiveness. For me I put carbon and GFO in 2:1 ratio meaning 1L of carbon and 500ml of GFO and I change them out every 1-2 months or so.