The ceramic media in your biOrb forms the main part of filtration and is important in keeping the water clear and healthy. 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 field. 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 now and again can help to keep the filtration efficient.