Biological filtration is based on the natural breakdown of pollutants carried out by aerobic bacteria, which can convert proteins & their by-products into less harmful substances. To maximise this process quickly & effectively, a highly porous filter medium is required to give the bacterial cultures the greatest surface area possible within the space available. Eheim Substrat provides a huge surface area of 450 m² per litre, offering optimal conditions for micro-organisms to flourish & delivering fast, effective filtration for salt & freshwater aquaria. Eheim Substrat at a glance: Standard bio-filter medium Highly porous material made from sintered glass Excellent conditions for bacteria cultures Reusable (rinse carefully to clean) Suitable for fresh & saltwater Tip: Never clean or change all the filter media together in one operation (and never clean too rigorously). The bacteria cultures must not be completely destroyed, otherwise re-colonisation will be retarded. The filter baskets in the external filters & filter modules in the internal filters aquaball an biopower are ideal for partial cleaning.