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Water properties for aquarium – movement, temperature, gas exchange

Reservoirs inhabited by tropical freshwater fish can be very diverse in their dynamic parameters – from slow-flowing forest rivers and ponds with standing water to fast-flowing rivers with rapids and large freshwater lakes, sometimes actually being inland freshwater seas with waves and surf. True, the fish living in the depths of such lakes, can almost not feel the movement of water, because only their surface is in constant motion.

In general, the nature of the movement of water and the need of fish for oxygen are interrelated.

It is necessary to come to terms with the fact that fish originating from water bodies that have a diametrically opposite nature of the movement of water in the aquarium are usually incompatible with each other. In this regard, it is impossible to simultaneously satisfy the needs of both.

For example, the intensive movement of water needed to meet the oxygen demand of fish in nature living in turbulent rapids can cause considerable discomfort and stress in fish that have a flattened lateral body and long veil fins — all of this has developed in them evolutionary development so that they can slowly swim in the calm water of some forest pond. Such fish will painfully feel the blows of water streams and will not be able to swim comfortably. Too small fish in such conditions, too, will not be able to swim normally.

The same applies to fry of all species, even to fry of reophilic fish (i.e., fish preferring a rapid flow) and lake fish, since in nature fry usually live in closed microareals inside a common biotope. It will be difficult for such fish to get the required amount of food, as they will not be able to swim fast enough to catch the pieces of food that are spinning in a turbulent stream.

Fish, forced to constantly be exposed to unnaturally fast (for these species) movement of water, will soon begin to experience severe stress. Over time (perhaps very soon) it can have a negative impact on their health.

In addition, too rapid flow can adversely affect the growth of plants – especially in combination with filtering, arranged by the type of false bottom.

As with other factors of the natural environment, tropical freshwater fish are adapted to life in a specific temperature range, which can vary significantly depending on the type of fish. The water temperature in the tropical zone is very variable.

The sunlit stretch of the river can be much warmer, especially at noon, than the shaded stretch that is very close upstream. In addition, water temperature varies with depth. Of course, in all the cases described, fish living in a river, unlike aquarium fish, have a choice.

The temperature of the water can vary depending on the time of the day, as well as on the season – not necessarily due to significant changes in the local temperature, but more often due to the influx of cold water after heavy rain or melting snow in the mountains somewhere far upstream . On the other hand, the annual and daily water temperature in some large lakes remains almost unchanged, with the exception of water at the surface itself.

The reason for this consistency is that it takes a very long time for an external phenomenon to have a significant impact on such a huge amount of water.

Daily and seasonal variations in water temperature in many natural reservoirs give us some degree of freedom in maintaining the temperature of the water in an aquarium if it contains fish originating from such reservoirs. Other fish in this regard may be more demanding. It should also be remembered that, although some fish are able to withstand a short stay in icy melt water or in overheated and rapidly evaporating ponds at the height of the dry season, they will not be able to endure such temperatures throughout their lives.

What is important is the temperature at which the fish lives most of its life. For many fish species, suitable temperatures or temperature ranges are given in the literature on aquarism.

Tip: if you keep fish of this type, for which you can not find the necessary reference data on water temperature (or any other biotope data), look for such data for other species of fish that live in the same biotope.

(cold water)

Together in the same aquarium, you can keep only those fish that need approximately the same water temperature. The normal working temperature of the water in the aquarium should be within the range acceptable to all species of fish living there. Too low a working temperature can make fish lethargic and inactive, and in the long run lead to a serious deterioration in their health.

Such fish are unlikely to breed. However, too high a working temperature is also undesirable, since as a result fish’s need for oxygen will increase, which will create additional stress on their gills and may lead to permanent damage to them. In addition, fish may become too active, nervous and constantly under stress.

A temporary increase in the temperature of the water in an aquarium is sometimes used in the treatment of certain diseases, but it should be considered only as a short-term measure.

In addition, it should be noted that due to improper water temperature, fish may experience many acute and sometimes deadly health problems.

Some people keep tropical fish at an unnaturally low temperature and low light to save on electricity consumption. This is absolutely unacceptable.

If proper fish keeping is too expensive, then they should not be kept at all.


Sources of water for the aquarium.
Type of water – Possible problems.

Plumbing – Chemical composition may not be suitable for fish kept in an aquarium. Water may contain chlorine or chloramine.

Water may contain copper from the aqueduct, especially in areas with soft water. There may be a high level of nitrate or phosphate in water due to pollution by agricultural fertilizers or because the water passes a repeated cycle.

Rain – It may be polluted by industrial emissions into the atmosphere, materials from which the roof is made, gutters and pipes, as well as dirt and detritus accumulating on them.

Distilled – Pure, however, requires saturation with oxygen and minerals.

Obtained by reverse osmosis
– Pure, however, requires saturation with oxygen and minerals.

Water is formed from two gases – hydrogen and oxygen. It is able to absorb free oxygen (which does not have a compound with hydrogen when H2O forms a water molecule) and other gases, such as carbon dioxide, nitrogen, sulfur dioxide, ammonia and chlorine.

All these substances affect the fish. The effects of ammonia, chlorine and sulfur dioxide (in rainwater) have already been discussed in this chapter.

Excess nitrogen in water can cause embolism.

Fish, like people, breathe the oxygen they draw from the water and exhale carbon dioxide. Fish of some species are able to breathe atmospheric oxygen.

In nature, they usually live in water bodies where there is a seasonal or constant lack of oxygen (that is, there is too little dissolved oxygen in them).

Aquatic plants (including algae), like terrestrial, absorb carbon dioxide in the daytime or when light is turned on in the aquarium. They use the carbon contained in it to produce nutrients, and free oxygen is released into the water.

In bright light, tiny bubbles of oxygen can be seen on the leaves of aquatic plants. However, at night they absorb oxygen and emit carbon dioxide.

Requirements for fish to oxygen content

Fish are not able to split the water molecule into its constituent hydrogen and oxygen. Therefore, if there is not enough dissolved oxygen in the water, the fish may suffocate. This condition is called hypoxia.

Fish that receive insufficient oxygen usually move their gills too often, open their mouth and concentrate near the surface of the water, where the oxygen content is higher.

Oxygen requirements specific to particular fish species usually depend on the oxygen content in their natural habitat. For example, fish from biotopes with a high oxygen content — for example, from fast-flowing rivers or large, open-wind lakes — require more oxygen than those in slow-flowing water.

In individual fish, oxygen demand usually rises when they feel unwell, under stress, more active than usual (for example, during spawning or when they are pursued), or if they are kept at a higher temperature than nature intended. Similarly, fish need less oxygen when they are inactive (for example, day-living fish at night) or if the water temperature is lower than necessary.

However, this does not mean that lowering the temperature is an acceptable way to combat the lack of oxygen!

Generally speaking, most of the free oxygen enters the water from the atmosphere, although in the daytime some of it can provide plants.

Water can absorb oxygen from the air only where these two elements border, namely on the surface of the water. Similarly, carbon dioxide is emitted into the atmosphere also only on the surface of the water.

The larger the surface area, the more oxygen it can absorb and more carbon dioxide is released. This fact is very important for the maintenance of fish, because the amount of fish that a given aquarium can withstand depends on the amount of oxygen, as well as for choosing the optimal shape of the aquarium.

The process of absorbing oxygen and releasing carbon dioxide is called gas exchange. To a certain extent, this is really an exchange, since the carbon dioxide content in water limits its ability to absorb oxygen.

Excitement on the surface of the water increases the effective surface area. The surface covered with ripples has a larger area than a flat surface, thereby increasing its gas exchange ability.

Water circulation is also very useful, as it brings carbon-rich water to the surface, and water that has just been saturated with oxygen carries to the bottom layer. The joint process of surface movement and circulation of water aquarists call aeration.

This process can also be used to remove other gases from water, such as chlorine and nitrogen.

Aeration can be carried out using a filtration system designed to ensure maximum circulation of water, and if possible, excitement on the surface of the water. However, you should not get too carried away and create in the aquarium a real whirlpool in miniature. Simply filtration equipment should be suitable for this tank in terms of capacity and sustainability.

In addition, it should be set in an optimal way. Aeration can also be performed using an air pump (microcompressor and spray). This forms a stream of air bubbles, also promoting the circulation of water and the formation of ripples on its surface.

Contrary to popular belief, air bubbles alone do not add relatively little oxygen to the water. It is their effects on the circulation of water and surface gas exchange that bring benefits.

Aeration plays a very important role in providing the necessary amount of oxygen to fish originating from reservoirs with a high concentration of oxygen. In addition, some aquarists use aeration so that they can hold more fish in an aquarium than with normal oxygen content.

All this, of course, is very good, except for related problems, such as high levels of organic waste. Therefore, such a system will last only until the day when the microcompressor fails.

Aquarists holding fish with a high oxygen demand need to have a spare aerator on hand so that their fish do not suddenly die from lack of oxygen.

Factors affecting the oxygen content in water

Temperature. The temperature of water affects the oxygen content in water:

The warmer the water, the less oxygen it contains, and vice versa. In addition, the increased temperature accelerates the metabolic processes in fish, with the result that their oxygen demand rises precisely at a time when its content in water decreases.

This problem can be overcome by more intensive aeration.

Plants. Aquarium plants are often appreciated for their ability to produce oxygen. However, it should be remembered that at night they themselves consume oxygen and produce carbon dioxide.

Thus, although plants can really help to meet the oxygen requirements of fish during the daytime, at night, all living things in an aquarium compete for oxygen, the content of which at this time of day is decreasing. Therefore, in aquariums, densely planted with plants, at night there may be a lack of oxygen.

Top dressing of plants of CO2. In recent years, some aquarists have used carbon dioxide to accelerate plant growth.

At the same time, it is introduced into the aquarium from special cylinders. However, the introduction of CO2 should be done with great care, and perhaps this should not be done. Do not forget that a high level of CO2 can lead to a decrease in the amount of oxygen absorbed by water, and then there is a risk of hypoxia in fish – especially if the aquarium is densely populated or there are those among the fish that have high oxygen demand.

Some aquarists try to circumvent this problem with additional aeration. However, although this measure will certainly increase the oxygen content and benefit the fish, at the same time it will contribute to the removal of carbon dioxide, and this will make the introduction of CO2 completely meaningless.

The CO2 generator must be turned off at night when the plants do not need this gas.

Snails and other creatures. A large population of snails can have a significant effect on the oxygen content of an aquarium.

Bacteria can do the same. Oxygen consumption by aerobic bacteria involved in the nitrogen cycle is permissible because, instead, they bring significant benefits. However, if an excess of organic waste is observed in an aquarium (for example, due to regular overfeeding of fish), the bacteria population will grow and absorb more oxygen than when rationally fed fish.

Snails, of course, also increase the content of organic waste.

With regard to the oxygen content in the water, here, as in many other aspects of the maintenance of the aquarium, the most important thing is to achieve a reasonable balance between the needs of all the inhabitants of the aquarium.

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