Bacteria convert harmful by-products from aquatic animals into
less harmful nitrates. This process occurs naturally in nature's
lakes and streams. In a Koi pond these bacteria are found on the
walls of our ponds. They also can be found on the inside walls
of the pond plumbing, attached to the skimmer basket, and on the
rocks that form a waterfall. The reason it is important to
consider what filter media to use when we build biological
filter systems, is to increase the available surface area for
these 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.