Hagen 12-Pack Fluval Aquarium Filter Bio-Foam for A237/306 and 406 Models Convenient 12 Pack Includes 24 bio-foam pieces that fit Fluval 305/306/405/406 filters Enhances biological filtration Adds a second layer of mechanical filtration Fluval A237 306, 406 Bio-Foam 24 pk (12x2 pack)