Filter media plays an important role in your ponds eco-system.  There are different properties in which media can perform, Mechanical (solids removal) and Biological. Most types of media on the market can perform both tasks. Without enough Biological media your filter and ponds eco-system will not be able to handle a large amount of fish. Biological filter media is where your bacteria that control water parameters are held. After solids have been removed the bio media takes affect in the nitrification process. Bacteria turns the harmful Ammonia from fish waste to Nitrite and then to Nitrate. The vital aspect of having an area in the filter that can capture those toxic chemicals and grow excellent bacteria is key to help maintain the waters proper balance to ensure the health of your koi.