Beneficial bacteria colonise the ceramic media and remove toxic fish waste from the water as the water flows through it. biOrb ceramic media is full of pores for bacteria to live on, just 900g of ceramic media has the surface area equivalent to a football pitch. You can add more media to your biOrb to increase the area for filtration. Over time, these pores can become blocked with dirt and limescale reducing the surface area available. Replacing some of the media every year or so can help to keep the filtration efficient. Never replace more than 1/3rd of your media at one time and wait at least a month before replacing any more.