Tropical Fish Aquarium Composition of Water

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.