Replayfine filter mats provide an optimal habitat for important microorganisms with their highly porous surface, which break down pollutants biologically. At the same time, Replayfine filter mats mechanically trap fine dirt and tiny suspended particles, ensuring crystal-clear pond water in conjunction with the prior cleaning by the Replayraw and Refinecoal filter mats in your Eheim pond filter. The result: fewer algae and healthy fish!.