What is a biological filter?
A biological filter or "bio filter" is simply a home for the beneficial bacteria that perform the nitrogen cycle. The filter provides surface area that the bacteria can live on and a recalculating water pump ensures that water is constantly flushing over the bacteria so they can obtain their necessary nutriment and oxygen.
Where do the bacteria come from?
The bacteria occur naturally in a pond. They live on fish and other underwater surfaces such as plant stems and rocks. In order to get the bacteria established in a filter or new environment, aquarium and pond owners used to borrow some gravel that had bacteria on it from an existing system. This cumbersome procedure is no longer necessary as there are now available bottled viable bacteria cultures in either living form or freeze dried spores.
If the bacteria occur naturally, why is a bio filter necessary?
Most backyard ponds have a much higher concentration of koi fish than would occur in nature, and the fish are usually fed a high-protein food. These facts result in a higher concentration of organic waste, i.e., ammonia,than the naturally occurring bacteria can deal with. A bio filter houses more bacteria, hopefully enough bacteria, so that ammonia and the other nitrogen compounds are completely eliminated.
