Fresh Water

This type of water is also known as Continental water, a more accurate term from the scientific point of view. Continental water accounts for only 2.6% of the Earths water, the rest being made up of seas and oceans. Of this volume, 98% consists of sterile water, in the form of glaciers and underground water, leaving only the water of rivers, lakes, and ponds as shelter for living organisms- barely 2% of the total volume of Continental water.

• The use of a heating kit allows the water in an aquarium to be maintained at an almost constant temperature.

An important parameter for aquatic life, the temperature regulates the growth of animals and plants and exerts an influence not only on oxygen levels but also on many other factors.

Whereas mammals have a regulated and practically stable internal temperature, that of fish and other aquarium creatures varies according to the temperature of the water around them. They can survive only at certain temperatures and some species are more sensitive than others to variations in this parameter. The temperatures of fresh tropical waters, ranging from 20 to 30°C, are characterized by less significant variations than those found in temperate regions. In some places the shade provided by the tropical forest cools the water, while in calm water the temperature goes up under the direct influence of the sunlight. The mean temperature most often recommended for aquariums is 25°C, and variations of 1 or 2° are of little consequence. Fish are even capable of withstanding even more significant variations for brief periods (under 24 hours). On the other hand, their metabolism (i.e. their general bodily functioning) is in danger of serious disturbance over any longer periods, and sooner or later they may die. It must also be noted that excessively low temperatures sometimes favor the development of certain diseases.

OXYGEN AND CARBON DIOXIDE

Since air contains around 20% oxygen, even the most oxygenated water rarely contains more than 1% dissolved oxygen. Fish have special organs - branchiae -which allow them to extract most of this (see Anatomy and Biolo Oxygen contributes, in addition, to the respiration not only of plants but also of organisms which are invisible to the naked eye and often forgot-

Simpleagitation systems stir the water, enhancing the diffusion of the oxygen required by fish

Examples of water temperatures in aquatic settings (in°c)

temperate zones

tropical zones

Mountain rivers

Sea-level rivers

South American, Amazonian rivers

South-East Asia, marshy zones

Africa, the Great Lakes

4-14/15

6-18/20

23-28

26-28

25-27

ten: the bacteria. The latter transform the organic matter emitted from living beings (excreta and various other residues), and these chemical reactions similarly require oxygen.

The oxygen in water comes from the dissolution of the oxygen in the air, a process enhanced by movements in the water produced by wind, currents, or downward flow. The more water is stirred, the more it is oxygenated. Plants also provide oxygen, which they produce through photosynthesis, although this process occurs only by day. The maximum amount of oxygen that water can contain is determined by its temperature: the higher this is, the less oxygen the water can contain (at 25°C there is 18% less oxygen than at 15°C).

Oxygen is measured in mg/liter, and its control is quite a complicated matter. The most turbulent, and therefore the most oxygenated, water contains 8-10 mg/liter, while the most deficient water sometimes has less than 2 mg/liter. The oxygen content in an aquarium is usually at its maximum, providing the recommendations for stirring the water are followed. The rare problems which do occur are the result of negligence as regards the overall balance of the aquarium (overpopulation of fish, small number of plants), or non-functioning of equipment due to forgetfulness, breakdown, or a power cut.

Carbon dioxide derives from the respiration of fish, plants, and bacteria. Stirring the water enhances its oxygenation, thereby reducing the levels of carbon dioxide in the water, and passing it into the atmosphere. Carbon dioxide is quite rare in an aquarium, and this can, to some extent, prove prejudicial to plants, as they absorb it by day through photosynthesis to extract the carbon they need to grow. It is therefore vital to establish a permanent equilibrium between oxygen, carbon dioxide, plants, and fish, although this balance changes at night, when plants stop producing oxygen.

Carbon dioxide is also one of the main factors affecting the pH.

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