GFO will work in a bag, especially in a small tank. Just less efficiently then in a reactor. Since water may channel in a mesh hatband it will be more prone to trapping detritus than a reactor with foam sponges, I suggest lifting it and rinsing it with RODI water and moving the m dis around gently without pulverizing if. This will help with efficiency & make it last longer.
BTW, .20 ppm phosphates is an ENORMOUSly high reading if accurate. Was that a typo? If accurate you have some fundamental issues to look into first.