For BioTec 5.1/10.1 and BioSmart 18000/30000/36000 The heart of a filter is its filter sponges: Green filter sponges have the finest filtration properties, as they develop their full function at a low water flow rate. This means that even very small dirt particles simply get caught in the sponge during the flow and are thus removed from the pond water. However, filter sponges not only perform the classic filtration function, but also provide plenty of living space for the smallest bacteria. This small biological oasis ensures that algae-stimulating substances are converted into naturally occurring gases (denitrification). Together with the red and blue filter sponges, they ensure clean filter performance.