The Juwel active carbon sponge cleans the aquarium water of all chemical impurities by absorbing them in ist active carbon. Consequently, these chemical impurities will not effect the bacteria - cultures inside the blue sponges. If the carbon sponge is left in the filter too long, it can release the chemicals again into the water. Consequently, it isrecommended to replace the carbon sponge every 4 weeks.Juwel bioCarb -active carbon filter sponge-bioCarb S - suitable for Super and Compact SbioCarb M - suitable for Bioflow 3.0, Compact, Copmact H and Compact SuperbioCarb L - suitable for Bioflow 6.0, Standard and Standard HbioCarb XL - suitable for Bioflow 8.0 and Jumbo.The package contains 2 pieces of Juwel active carbon filter pads. Please choose the correct one for your filter.