To customize your Marineland filter we suggest separating the blue floss from the plastic backing (it is glued on) and removing the carbon inside. Take your plastic backing and create a cardboard template for future filter cutouts. Then, place FilterFirst against the backing with the looser weave side facing towards the back and the denser weave side against the backing. You can also place our Polishing Pad behind FilterFirst for even more clarity. Lastly, use our 3"x4" FINE mesh bags fill half with our Activated Carbon, and place on the right side of the filter (see attachments for reference).