Biological Filters use Bacteria or vegetation to convert Nitrogen wastes and other organic compounds into simpler and less toxic chemicals. These simpler compounds are then absorbed by the vegetation or drained through periodic water exchanges. they are intended to break down the liquid wastes in the water only, solid wastes tend to clog them.

Bacterial filters must have large surface areas for the bacteria to grow on; and can be placed inside or outside the pond. Typically the water to be treated is drawn through or sprayed over the media that the bacteria grow on. The bacteria absorb the waste and oxygen and oxidize the wastes converting ammonia ,to nitrites, and then to nitrates. The nitrates are powerful fertilizers for the plants in the water such as algae. While excess algae is undesirable, they do absorb the nitrates, which if allowed to build up would sicken or kill the fish.

The Bacterial action removes a lot of oxygen from the water, so the water must be oxygenated before it is returned to the pond, fountain heads and or water falls work well. Some installations have specialized aerator devices.

When the water is poured or sprayed over the filter media, such as pea gravel, hair curlers, bio-balls, beads, coarse sand or lava rock, the water drains through the media in thin sheets allowing air to contact the water all the way through the filter. The extra oxygen provided improves the efficiency of the filter.

pea gravel

• fair surface area for the volume
• they are heavy can be washed for long life its
• cheap buy at landscape suppliers

hair curlers

• low surface area for the volume
• light weight for the volume
• can be washed cheap buy at garage sales etc..

bio-balls

• large surface area for the volume
• light weight for the volume can be washed for long life
• expensive buy at pond or aquarium suppliers

beads

• large surface area for the volume,
• light weight (usually they float) can be washed, long life
• buy at pond or aquarium suppliers

coarse sand

• large surface area for the volume
• moderately heavy for the volume can be washed, but beaks down mechanical over time
• cheap buy at building or pool suppliers

lava rock

• large surface area for its volume
• light weight ,and works great until the pores become hopelessly clogged with undesirable anaerobic bacteria and has to be replaced usually within two years so  replace it annually to be safe.
• cheap buy at landscape suppliers