Most filter media have a mechanical function. Settlement chambers allow gravity to drag the solid waste out of the water by slowing the water flow. Such chambers usually come first in a filter.
A vortex unit provides greater settlement, the water moves in a circular movement allowing solids to gather in the center where they can be removed. In addition to baffle plates which slow the incoming water, brushes or matting can be used to strain the water.
This relies on specific bacteria to break down toxic waste products to less harmful substances. There are two stages in the breakdown of ammonia, each stage involving different types of bacteria. The first stage is the breakdown of ammonia to nitrite by nitrifying bacteria, most important of which is Nitrosomonas. The second stage is the conversion of nitrite to nitrate by Nitrobacter.
Both of these groups of bacteria are aerobic (needing oxygen to live), sediment building up in the filter will deplete the oxygen levels so it is important to keep sediment to a minimum by having a settlement chamber first and by cleaning the filter out occasionally (but not using tap water as the chlorine will kill the bacteria).
A variety of different media are available to put in the filter, materials such as gravel, matting, hair rollers, foam, and canterbury spar are all suitable as they provide lots of surfaces for the bacteria to live on.
A biological filter will take weeks or months to mature, cultures of nitrifying bacteria are widely available and will speed up the process.
Activated carbon removes chlorine, tastes, odors, colors, pesticides, heavy metals and other impurities. You should replace the carbon every three months.
Zeolite absorbs ammonia from the water. A good feature of Zeolite is that it can be cleaned by soaking in salt water (6g per litter) for 24 hours and then reused. Never add salt to your pond water if you are using Zeolite since it will release some of the ammonia.
If a large biological filter is present chemical filtration should not be needed, but it is good to use while the biological filter is maturing or isn't big enough for the pond.
Sand filter. Some Koi keepers use a sand filter as a final stage to 'polish' the water. The water is passed under high pressure through sand and comes out very clear, bacterial activity also takes place in the sand filter. Sand filters are expensive though, and you can't make one yourself because of the high pressure involved.