The Carbon Filter Set 4 BioPlus for the BioPlus 50, 100, or 200 includes four replacement activated carbon filter foams. The addition of activated carbon to your filtration system allows you to customize to meet the filtration needs of your aquarium. Carbon filtration transforms unpleasant substances without endangering the aquarium inhabitants. Activated carbon impressively absorbs unwanted substances in aquarium water, such as medication residues. The filter foam of activated carbon makes replacement much simpler than common granular forms. Timing to replace the carbon filter set varies upon aquarium size, number of fish and plants, and declines in filter performance.