AquaTops Flex-300 Hang-On Internal Filter provides a calming, relaxing waterfall return and promotes gas exchange at the waters surface. The Flex-300 includes a replaceable filter cartridge loaded with Activated Carbon for crystal clear water and offers generous room for the addition of Ceramic Rings or Bio-Balls (sold separately). The versatile filtration media compartment allows for the enhancement of the Biological Filtration, which will help maintain safe living conditions for your aquatic creatures. Designed for easy placement, each unit can be mounted with the included Hanging Bracket or Suction Cups. The Flex-300s innovative and flexible design helps eradicate and control free-floating debris and harmful waste from both fresh and saltwater aquariums.