Biological filtration is a naturally occurring method that occurs when bacteria break down ammonia and convert it to nitrite, then nitrate. This process is known as the nitrogen cycle. Biological media, an inert material that houses bacterial colonies, is placed in the filtration system and functions when water is forced through, which brings the oxygen and nutrients necessary to complete the nitrogen cycle.
It is important to check your filter and the media regularly to ensure they are not clogged. If you see a film or discoloration on the media, this just means that there are healthy bacteria residing in it. If the media is clogged to the point where it can no longer function, instead of immediately replacing it, first try flushing the media with aquarium water.
The media never should be replaced; doing so removes the bacterial population that is essential to the filtration process. To reduce the chances of clogging within your filter, it is a good idea to incorporate mechanical media; doing so will greatly reduce the amount of particulate that reaches the biological media.
Temperature is very important to bacterial health; bacteria need temperatures above 55 degrees Fahrenheit to flourish. Once a healthy colony of bacteria is established within your biological media, only excessive cleaning, chlorinated water or certain medications can damage it.