Aquaclear Activated Carbon Insert, 20-Gallon Aquariums, 3-Pack (3-Pack) Convenient 9 pack. (3) 3pks Adsorbs odors, discoloration and impurities Designed to fit the AquaClear 20 filter Suitable for aquariums between 5 to 20 gallons 3-pack; Economical way to purchase inserts