Large Box Air Filter. Place your own rocks or other heavy materials in the bottom for weight and filter material in the top. Attach airline and filter pulls water into the top of the filter through the slits and through your filter material then exits the large tube. The rocks at the bottom hold bacteria and act as a weight. We recommend red lava rocks for weights because the red lava rock is natural (black lava rock has been painted) and it has a lot of surface area. This ad is for the filter only. We sell the filter with rocks and filter (foam) material for $13 here: BOX FILTER, AIR – LG w/ROCK & FILTER MATL.