PORO RING biological filter medium. A high quality filter medium in the form of porous ceramic rings whose shape ensures good water-flow and hence an adequate supply of oxygen and nutrients to the biofilter. The effective surface area of Poro Ring M is 600 m2/l by virtue of the complex network of pores, through which water can flow freely, carrying essential oxygen and nutrients to the nitrifying bacteria. The porous structure of Poro Ring M provides an excellent substrate for colonisation by these bacteria, which convert ammonium ions (NH4+) and nitrite (NO2-) (both dangerous to fish) into relatively harmless nitrate (NO3-).