You don’t have to pour a bunch of chemicals into your koi pond to keep it free from bacteria. A biological filter is a medium of some kind that at converts harmful nitrogen by products from aquatic animals into less harmful nitrates. This type of bacterial occurs naturally in nature’s lakes and streams. It usually attaches itself to rocks or stones.

These bacteria are already noticeable on the walls of most koi 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 we build biological filter systems is to increase the available surface area for these bacteria to colonize. It is simply not enough to dump a plastic liner into the ground and surround it with dirt. Rocks help grow the bacteria that eat nitrates that are harmful to koi.

In an effective koi filter, two, three or four chambers of suitable media, on which our nitrifying bacteria can grow and thrive, normally represent the biological stages. These chambers can be any shape. They should also be benched steeply towards a central drain valve in the base of the chamber, so that any solids that do collect in the chambers can be easily be removed by you.  If you own a biological pond filter it is important to unclog it to maintain superior water quality.

This green stuff that grows on biological filter mediums is called a biomasse. Bimoasses of friendly bacteria need both an adequate supply of ammonia oxygen and nitrite to survive. Again if the mechanical stages of your filter are not working effectively, the biological media will gradually become clogged with debris and your good bacteria will die.

In order to ensure the biomass has an adequate supply of oxygen, air should always be pumped into the filter, by placing air stones under the media, on the filter grids and air introduced by way of an air pump. Without an air pump, a biological filter will never perform biologically as it should.

There a wide range of medias to use in the biofilter. Gravel is the most popular choice but synthetic medias are also a good choice.

Synthetic medias don’t have a lot of the problems that gravel does. Being lightweight and less clogging makes synthetic medias easier to handle and clean. Most of the synthetics have a large surface area per cubic foot for growing bacteria. This attribute gives a bigger biofilter in a smaller space. However synthetic medias may not grow the “algae lysing” type of bacteria that naturalizes very easily in genuine pools and steams. The “algae lysing” bacteria grow better in slightly stagnant and low oxygen zones created by a rock, gravel or stone based filtration media.