Each molecule of water is composed of one atom of oxygen to every two atoms of hydrogen. This is expressed chemically as H2O. Fishes use oxygen, but they cannot use the oxygen that is chemically a part of water. But air, including oxygen, will dissolve in water - just as sugar or salt will - and it is this dissolved oxygen that fishes use.

The major source of usable oxygen in an aquarium is the air above the water. It is therefore at the surface of the water that the major interchange of gases takes place. Carbon dioxide is released by the water at the surface and oxygen is absorbed there. One can drive all the gases out of water simply by boiling it: the warmer the water the smaller the amount of gas it will hold in solution. It is also possible to have an excess of CO2, enough to cause the death of fishes even though there is an ample supply of oxygen present in the water. It is not enough to supply oxygen to the fishes; a means for disposing of the CO2 must also be provided.

In practically all natural bodies of water the surface is proportionately many times greater than the depth. In such bodies of water there is also usually a movement of the water caused by wind, current, temperature changes, and so on. These factors assist in the rapid interchange of gases at the surface so that normally water-dwellers do not suffer from a lack of oxygen or an excess of carbon dioxide. In addition, fishes are seldom as crowded in nature as they are in an aquarium.

Tropical Aquarium Aeration and Filtration

Water is the fishes’ atmosphere. It is the medium in which they normally exist and through which they travel, and from which they extract the oxygen so necessary to existence. Just as our air must be pure and contain sufficient oxygen, with no excessive amounts of harmful or irritating gases present, so must the fishes’ water be clean and well oxygenated.

Provision must be made to prevent the accumulation of harmful materials in the water; whether gaseous, solid, or liquid; whether in solution or suspension. The fishes’ water, just as the air we breathe, always contains a certain amount of harmful material; that is, material which would poison the fish if allowed to accumulate above a certain minimal amount.

The atmosphere over every large city contains tons of waste matter such as factory gases and products of combustion. When weather and geographic conditions prevent their dispersal, they can accumulate in such quantity as to inhibit human life. It is this sort of situation which is described as “smog.” The same sort of thing, on a lesser scale, can occur in our aquaria unless we guard against it.

The problem is twofold: to insure a proper supply of oxygen for the fishes to use, and to dispose of the products of respiration, metabolism and decomposition.

Oxygen is the fuel that stokes the furnace of life. This is true of fishes as well as of higher forms of life. The majority of fishes breathe by taking water in through the mouth. The mouth is then closed tightly and the water is forced outward over the gills and through openings at the back of the head. Each of the openings is covered by a flap - the operculum - often incorrectly called the “gills.” The gills are located under the opercula. They are the red branching members seen when the gill flap is lifted.

A few varieties of fishes, such as the Bettas and certain Catfishes, have specialized auxiliary breathing organs which enable them to extract oxygen directly from the atmosphere. However, such fishes are in the minority. It is the gills of the vast majority of fishes that are analogous to our lungs. As the water passes over the blood-rich gill surface, the dissolved oxygen is extracted for the fishes’ use, and waste carbon dioxide is given off.

A fishes’ nostrils are not used for breathing. The nostrils do not connect with the mouth and are used solely as organs of smell. There are a very few exceptions to this, but they are not included among the fishes kept in home aquaria.