OASE BioCompact filters use a combination of filter foams and carbon filter media for optimal water quality and clarity. Each filter model has its own replacement filter foam set. BioCompact 25’s carbon filter media is in foam form and is included in the filter foam set. BioCompact 50 uses carbon filter media in a granular form. Timing to replace the filter media varies upon aquarium size, number of fish and plants, and a decline in filter performance.