Oase BioMaster 250 External FilterBioMaster 250 External Filter This new external filter from Oase offers crystal clear water through a combination of mechanical and highly effective biological filtration, this is achieved through fine 45 ppi pre filter foam for optimal mechanical filtration of the coarse debris, which ensures a longer service life of the biological media. With an EasyClean pre filter module, the filters can simply be removed, rinsed out and reinserted. The automatic