Oxygen

Oxygen  is a chemical gas with the chemical symbol O. Oxygen is one of the two major components of air (the other is nitrogen) where it consists of 21%. It is produced by plants during photosynthesis, and is necessary for aerobic respiration in animals. At standard temperature and pressure, oxygen exists as a diatomic molecule with the formula O2, in which the two oxygen atoms are bonded to each other with the electron configuration of triplet oxygen.

Oxygen is only slightly soluble in water (Oxygen is approximately 95x slower at dissolving in water than CO2), but naturally occurring dissolved amounts support all ocean and freshwater animal life.

O2 is the common allotrope of elemental oxygen on Earth and is known as dioxygen.

O3, Ozone is 3 atoms of Oxygen and is a poisonous gas at normal pressure and though rare in nature is found mostly in the stratosphere (see Ozone Hole), it can be produced by machines and is used in small amounts in aquariums for its disinfecting effect.

Oxygen is the most common component of the Earth's crust (49% by mass) the second most common component of the Earth as a whole (28% by mass), the most common component of the world's oceans (86% by mass), and the second most common component of the Earth's atmosphere (20.947% by volume), second to nitrogen.

Elemental oxygen occurs not only in the atmosphere, but also as solution in the world's water bodies. At 25°C under 1 atmosphere (unit) of air, a litre of water will dissolve about 6.04cc (8.63 mg, 0.270 mmol) of oxygen, whereas sea water will dissolve about 4.9 cc (7.0 mg, 0.22 mmol). At 0°C the solubilities increase to 10.29 cc (14.7 mg, 0.460 mmol) for water and 8.0 cc (11.4 mg, 0.36 mmol) for sea water. This difference has important implications for ocean life, as polar oceans support a much higher density of life due to their oxygen content.

Oxygen in the aquarium
Most life in your aquarium uses oxygen to live. Not only the fish, frogs, snails, shrimps, etc. but also the nitifying bacteria, fungus, mulm consuming bacteria and not forgetting plants and algae).

The total amount of oxygen that can be dissolved in water varies by temperature and is called the 'dissolved oxygen' (DO). See this DO article for more detail on this and the amounts of oxygen required by aquatic animals.


 * Note: You often see air pumps and tubes with air stones pumping air into the water in the form of air bubbles. But a typical air bubble would need a minute and a half to dissolve into the water. Far too slow a time to indeed be dissolving any appreciable amount of oxygen. But what the air bubbles do achieve is agitating the water surface, effectively increasing its surface area and so dissolving more oxygen into the water (and releasing trapped CO2) that way. So if your aim is to increase [DO], then agitate the water surface.