The Fishmate Pressurised Filters feature the ' Powerclenz' cleaning mechanism (except the 5000 model). Just turn the dial from 'normal' to 'clean' & the water flow changes direction back-flushing the filter & the foam layers are squashed which removes the dirt & sludge. The dirty water is then pushed out of a second outlet down the drain or into your flowerbeds. To read more about the Fishmate Pressurised Filters please click here.