It works as a biological filter media, aquarium biofilter is excellent interaction between air and water and gas exchange. The bio-balls can give a nice living environment for beneficial bacteria where they can settle and be fed by water and oxygen. They will also promote gas exchange and increase dissolved oxygen levels. So as to form a good ecological cycle. By the way, material superior at effectively removing ammonia & nitrites that hurt fish and cause unwanted plant life. You'll be surprised how clean your tank or pool is when using this aquarium filter. Bio-balls do not require replacement unless damaged or destroyed, use the scientific method, reasonable filter material, the filter media works better.