I cut donut-shaped filter pads to place above and below the foam pads at the top and bottom of the foam pads that come with the reactor, and these pads ALWAYS fill up with GFO particles (even though the flow is slow enough that the GFO doesn't tumble). I also run a second phosban 150 packed full of Kent marine carbon after the GFO reactor to catch microparticles of GFO (and of course other impurities). I highly recommend doing this, and recommend making new pads for each GFO change.