Biological filter is a filter that works with the help of microorganisms, especially bacteria from the class of decomposing ammonia. So that microorganisms can live well in the filter and perform necessary functions with optimal media and a suitable environment for the growth and development of these microorganisms. The main function of the biological filter is to reduce or eliminate ammonia from the water.

Ammonia is also produced by the metabolic waste water organisms, including bacteria, fungi, infusoria and also the rest of the fish feed. The filtering process ammonia in fish farming Nitrogen follow the laws of the circulation element in nature. Two groups of bacteria play a major role in the biological filter, namely bakery Nitrosomonas sp and sp Nitrobakter bacteria. Nitrosomnas role oxidize ammonia to nitrite, while the role Nitrobacter oxidize nitrite to nitrate.

Nitrosomonas and Nirobakter living attach themselves to solid objects in the aquarium, therefore, that the purpose of life (residence) to fulfill their need to be provided a place to attach themselves. All types of solid objects, as long as it is not toxic to the bacteria, will be used as the residence of the bacteria. Factors to consider in selecting the “residence” or media for bacteria is its association with the contact area between water and bacteria.

For water to be filtered properly by bacteria, the water needs to be in contact with the bacteria in question. Therefore, media selection must take into account this broad contact area. The wider the contact area it will be more effective biological filtration takes place. Extensive contact area is closely related to the size of the media used. In general it can be said that the unity of volume, media which have a smaller grain size will have extensive contact area or a larger surface area.

Here is a simple illustration of the relationship between the grain size of the surface area, or wide contact area. To simplify the illustration used cuboid objects.

If we have a cube with sides length 1m, then, the surface area of the cube is 6 m2, while the volume is 1m³. When we split into 2, then 2 m2 surface area increases, so that the total surface area is 8 m2, while volume remained.

When a cube sides length 1m split in horozontal into 8 small cubes of the same size, each small cube will have a surface area the size of 0.5 x 0.5 m2 x6 = 1.56 m2. So the total surface lua 8 small cubes to 1.56 m2 x 8 = 12.48 m2, the total volume of 8 cubes remain 1m3. Thus an object when we split into objects that are smaller in size, the surface area of the object be increased while volume remained.

In the same way we can calculate if the cube is divided so that each berkukuran 1cm. So we can easily know in volume 1 m3, we’ll get 100 x 100 x 100 = 1,000,000 cube. Each cube of the surface area is 6 x 1 cm² = 6 cm². So that the total surface area is 1,000,000 x 6 cm ² = 6,000,000 cm² or equal to 600 m².

With this illustration we will more easily determine the grain size to be used for a biological filter media. The smaller the grain will be widened so it will be more extensive surface contact area between the water and the bacteria that live on the surface. Nevertheless, if we go back to the principle of a mechanical filter, then there is a tendency that the filter with fine grains would quickly clog.

To avoid this it is necessary a good mechanical filter is installed before the biological filter. Thus, the incoming water is already a biological filter gets prefilter water, ie water which had previously been pre-filtered mechanically so that no longer contain solid particles that would clog. Some accessory manufacturers have made the aquarium filter media that is expected to reverse the process of blockage, such as: cicin (tube) ceramic or bioball, however these materials have side effects such as reduced surface area of contact.