Penn Plax 12-Pack Small World Filter Units and Replacement Cartridges for Small Tanks Convenient 12 pack of filter cartridges Easy to install replacement cartridges for your Small World filters High quality carbon/zeolite cartridge for chemical filtration and a foam block filter for mechanical & biological filtration Clears discoloration and removes harmful contaminants & odors Carbon cartridges should be rinsed before using to remove any carbon dust from handling.