The Carbon Pre-filter Foam Set for the BioMaster 250, 350, or 600 includes six carbon replacement filter foams. The combination of activated carbon filter foams with coarse and fine filter foams 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 pre-filter foam set varies upon aquarium size, number of fish and plants, and declines in filter performance.