Replacement filter cartridges for the Ehiem Pickup 45 for mechanical and biological filtration. These filter cartridges work both through mechanical and biological filtration. This means that the cartridges will filter out dirt and waste but during this process the filter media\'s surface area becomes colonised by useful bacteria. Even after a short maturation period the foam cartridges perform not only mechanical water purification, but also biological decomposition of harmful substances with the bacteria that has colonised. The cartridges are reusable several times over. To clean them, simply rinse them and squeeze them, so that the bacteria cultures are not completely destroyed. It is advised to use water other than tap-water to rinse out as tap water will kill a high percentage of the useful bacteria. The filter cartridges will eventually over time require replacing and your water quality will reflect this.