The basic concept of Nature Aquarium, that is, making use of diverse living organisms including  aquatic  plants, mud  snail and Cardina japonica for water purification. Although  it  is  impossible to reproduce  the natural ecosystem  exactly as it  within  the limited  space of a  tank, the essence of the ecosystem can be extracted and rebuilt in an aquarium.Biological   filtration, which is regarded as the most important process in the filtration system of Nature  Aquarium,  is a method  of filtering  water  with the help of microorganisms, a representative  decomposer in the ecosystem.

In a natural aquatic environment, microorganisms  such   as   bacteria   and   protozoa   are present on the mud and sand at the bottom of the water as well as on stone surfaces. In the aquarium  tank,  too,  they  are found  on the surface of the substrate and stones, but their water   filtration   function   is   not   powerful enough since the tank area to microorganisms can attach themselves  is very restricted.  To solve  this  problem, an independent  external filter is used for the aquarium to produce more bacteria  so  as to  achieve  a  high  filtration capacity.

The  external filter  contains   filter media such  to which microorganisms such as bacteria  and  protozoa  adhere. Since  more bacteria  can adhere to a  larger surface area, materials with large surfaces and good water permeability are ideal for biological filtration. The filtration capacity of microorganisms declines if the filter media are too fine or stuffed too much to the extent that water permeability is affected.

To perform biological filtration, it is necessary to ensure good water permeability so that the filter media surface is exposed to the water containing rich oxygen in order to optimize the important functions of microorganisms that need oxygen such as aerobic bacteria and  protozoa. In nature, too, water  quality can easily deteriorate  in  the aquatic   environment  with  stagnant  water where oxygen is lacking. To keep the water in a  good  condition, it is crucial that the water contains adequate amounts of oxygen and is kept in motion