JBL CombiBloc is a set of high quality foam pads of differing pore size for the Combi filter basket. The large-pored pads serve as a mechanical pre-filter insert. The T-profile retains coarse pollutant particles, whilst maintaining the ideal flow rate. This protects the underlaying biological filter materials from becoming unnecessarily soiled. The medium-pored JBL CombiBloc pad filters biologically through colonisation by cleansing bacteria. The fine-pored pad retains any particles of dirt which might damage the pump, as well as filtering biologically. Package content: 4 pads 10 ppi, 1 pad 20 ppi, 1 pad 30 ppi suitable for the external filters JBL CristalProfi e700 and e900 and e701, e901.JBL recommendation:- Cleaning: large-pored pads approx. every 8 weeks, medium-pored pad every 6 months, fine-pored pad about every 4 months- Replace: after cleaning two or three times