Fluval Pre-Filter MediaFluval Pre Filter Bio Media consists of inert, solid ceramic rings used to trap large and medium solid particles before they clog the finer sieves of the modules above. For all external filters. Inert, solid ceramic rings Allows biological activity on the surface of the rings Trap large and medium solid particles before they clog the finer sieves of the modules above The diversified water flow allows establishment of biological activity on the surface