Aquatop Aquatic Supplies' Flex-300, an internal hang-on-back style filter, is designed for smaller aquariums and hard-to-keep-clean turtle tanks. The filter provides unparalleled mechanical and chemical filtration and leaves plenty of room for the addition of biological filtration media, the company states, and is suitable for both fresh- and saltwater aquariums up to 20 gal. The filter features an 80 gal. per hour flow rate, installs easily on smaller aquariums and includes a waterfall return, which promotes gas exchange at the water's surface, the company adds. The filter includes a replaceable filter cartridge constructed with poly-fiber for mechanical filtration and includes AquaTop's Premium Activated Carbon.