Stocking a large amount of filter media ensures your pond water will be clean and your Koi healthy.
Bacteria convert harmful by products from aquatic animals into less harmful nitrates. This process naturally occurs in lakes and rivers. In a Koi pond these bacteria are found in the walls of our ponds, they can also be found on the inside of pond plumbing, attached to skimmer baskets etc.
The reason it is important to consider what filter media to use when we build biological systems is to increase the available surface area for thes bacteria to colonize. The important consideration is to keep the pollutants and unwanted debris out of our ponds, while keeping the beneficial bacteria in our systems.