sera Pond Crystal ClearHigh performance filter medium for mechanical filtration The innovative, patented 3D fiber structure of sera pond crystal clear Professional very quickly removes even smallest particles above 10 m (such as floating algae or sludge). Unlike common filter wool, the filter balls keep their shape and can be washed out several times. They are 80% reusable after washing out. Maximum filter performance can be achieved by combination with a UV C lamp and or