The JBJ Aquarium HB-45 Reaction HOB Replacement Filter Media is a complete replacement media set that includes the Carbon Filter Cartridge and Coarse Sponge. This replacement filter media set fits perfectly in the JBJ Reaction HOB filter (sold separately) to keep your water healthy and clear. The set provides the media for all three types of filtration—mechanical, biological and chemical filtration. The coarse sponge provides mechanical filtration, trapping free-floating detritus to keep your aquarium water crystal clear. The sponge’s coarse and porous surface also provides biological filtration by allowing beneficial bacteria, which remove ammonia and nitrates, to grow. The carbon filter cartridge absorbs impurities and removes odors and discoloration. To replace your filter’s media, simply slide out any old carbon filter cartridges and slide in the new ones—depending on the model, your Reaction filter may have one or more. Then, remove the old coarse sponge and replace it. It’s that simple!