Oxygen and carbon dioxide
While the air contains 20% oxygen, the most oxygenate waters rarely contain 1% oxygen. The fish have some organs, branches, allowing them to extract the most oxygen dissolved in water. The oxygen is also used in plants respiration and other invisible creatures from the aquarium, such as bacteria. These convert the organic compost resulted from organisms (excrements, food, other); the chemical reactions for this conversions also require oxygen use.
The water oxygen results from dissolving of air oxygen, favored by water motion: wind, current, flow. The water is more oxygenate if it is tumultuous. The plants also produce oxygen by photosynthesis, but only while daylight. The maximum quantity of oxygen in water depends on the temperature; as the temperature increases the oxygen level decreases (at 25° C there is 18% less oxygen as at 15° C). The oxygen level is measured in mg/l and the dosage is quite delicate. The most oxygenate waters contain about 8-10 mg/l oxygen, and the most pauper ones 2 mg/l. In aquariums, the oxygen level is usually at maximum. The problems appear only if the general balance is not respected, (fish overpopulation and few plants) or when the materials don`t work (interruption of energy or oblivion).
The carbon dioxide results from the fish, plants and bacteria respiration. The motion of water, which increases the oxygen level, decreases the carbon dioxide level, which goes in the atmospheric air. In aquariums, you can rarely find important quantity of dioxide, which could affect the plants. In day time, he plants absorb the dioxide and use it for photosynthesis in order to produce oxygen, indispensable to aquatic life. A permanent equilibrium should be settled between oxygen, dioxide, fish and plants. This equilibrium changes during night when the plants stop providing oxygen. The carbon dioxide is one of the important factors which affect the pH.