Oxygen O2 and its importance
The oxygen content in water depends on many factors: the depth of the tank, the temperature of the water, the presence of plants and animals. Water that is in motion is generally better oxygenated than that standing. The dependence between the temperature and the degree of oxygenation is inversely proportional – as the temperature rises, the oxygen saturation in water decreases. Water in the aquarium is oxygenated mainly by the air. If it stays still, only its upper layers are oxygenated. An additional producer of oxygen are healthy plants grown by us. Oxygen recipients are fish and other aquarium animals, microorganisms living in water (mainly in the substrate, in the filter) and at night (in the dark) also plants. Hence the necessity of additional airing at night (putting water in motion). In a well-oxygenated aquarium, there is an oxygen decomposition of organic pollutants and oxidation of organic compounds (proteins, carbohydrates or fats) to simple compounds easily absorbed by fish and plants. The processes of anaerobic decomposition (oxygen deficiency in the aquarium) lead to the formation of poisonous substances harmful to the inhabitants of the aquarium (eg: hydrogen sulfide, organic acids, polycyclic aromatic hydrocarbons), which are the cause of, among others, unpleasant smell.
To increase the amount of oxygen in the aquarium, you should install an additional filter or aerator, replace their parts or introduce more plants to the aquarium.
CO2 carbon dioxide and its importance
During the day, carbon dioxide is excreted by fish and other aquatic animals, arises in the processes of decomposition of organic compounds, and at night is also excreted by plants. Excess of this gas in the water is favored by the aquarium with a small amount of plants, lack of ventilation and excessive fertilization of the substrate. Excessive amount of plants in relation to fish in the aquarium is also not recommended – in the day it leads to a deficiency of carbon dioxide and, at night, of oxygen. It is also the cause of biological decalcification of water (plants draw carbon dioxide from calcium bicarbonates and calcium carbonates). Only an aquarium that maintains biological balance (the amount of fish in relation to plants, optimal temperature, pH, water hardness, amount of light, systematic cleaning and replacing of water, suitable filters and aerators, loose substrate) is able to function efficiently for the benefit of our fish and plants. It should also be mentioned that excess carbon dioxide can cause a decrease in pH. If we exaggerate with the amount of this gas when dispensing to the aquarium (above 40mg/liter), the pH may drop to the levels at which the fish can start dying.
The most recommended dose of carbon dioxide is 20-35mg/liter and this must be maintained to ensure plant growth. It is also important that the amount of lighting is 0.5W/L or more so that plants can consume CO2 in the process of photosynthesis.
O2/CO2 balance
Water is a good solvent for many compounds, including gases in the air: oxygen and carbon dioxide.
In an ideal gas exchange cycle between plants and fish in the aquarium, the fish draw oxygen contained in water, transform it and release carbon dioxide (CO2). Then, plants under the influence of light (in the process of photosynthesis) absorb this carbon dioxide and convert it into pure oxygen (O2). Thus, in a continuous cycle of gas exchange and transformation, this oxygen is re-used by the fish and the microbial fauna responsible for the biological balance of the aquatic environment. However, in order for this balance to be truly autonomous and functional, its three most important factors must be maintained: a small amount of fish + a large quantity of water + a large number of plants. Such an excellent combination is, however, hardly achievable in normal breeding conditions, so it is often necessary to install an additional oxygenation system.
In practice, oxygenation can be the result of various technical or chemical processes. The most frequently used is the gas exchange process taking place during the mixing of water masses (the combination of water and air).
The air consists of 21% oxygen and 79% nitrogen (neutral gas). Thus, oxygenation of water during its mixing with air allows to enrich it with oxygen necessary for the life of fish and bacterial fauna responsible for biological purification (see the transformation of waste by aerobic bacteria).
The optimum oxygen content in the aquarium water is about 8 mg/l at 25°C. The ability of gas absorption of water to achieve the ideal oxygen level in an aquarium depends on three criteria: mixing force – physicochemical quality of water – temperature. Regarding this last indicator, it should be noted that the solubility of oxygen decreases markedly as the temperature rises; the dissolved oxygen concentration indicator is therefore smaller at 30°C than at 20°C.