Before
looking at specific filter designs, you should understand aquarium aeration and
surface agitation. At the surface level, water and air undergo a natural
exchange of gases. Oxygen goes from the air to the water, and carbon dioxide
goes from the water to the air. This is how the oxygen that fish breathe enters
their habitat and how the carbon dioxide that they produce by respiration is
removed from their habitat.
When
the surface of aquarium water is disturbed, the rate of gas exchange between
the water and the air is increased; more carbon dioxide is released into the
atmosphere and more dissolved oxygen is taken by the water. The surface tension
of the water must be broken for sufficient gas exchange.
Fortunately,
creating surface agitation is easily done with aeration, or pumping air into
the water so that it forms bubbles. The bubbles rise to the surface and burst,
thus breaking the surface tension.
This
also creates water movement in the tank, in effect stirring the aquarium ever
so slightly, so that all of the materials and compounds in the water -- both
the beneficial ones and the harmful ones -- are evenly distributed throughout
the tank.
One
way of providing the necessary aeration in an aquarium is to use air stones
connected to an air pump. The air stones can be made from wood or other highly
porous materials. When air is forced in one end of the air stone by the pump,
it is released as bubbles from the other end.
