Eheim ehfisynth wool is a fine loose filter wool that can be used as a mechanical filter it will over time become biologically active can be used in any type of filter r great to use as a fine polishing media the eheim ehfisynth filter wool mechanically polishes aquarium water before it is introduced back into the aquarium r eheim ehfisynth removes fine particles of dust from the aquarium which are frequently the cause of cloudy water r place eheim ehfisynth as the final media in a 10mm laye