Why fish need oxygen
Fish need oxygen. To do this, take fish oxygen through the gills and give carbon dioxide away. Fish the size of the red neon need about 10 to 30 mg of oxygen per day.
There is about 30 times less oxygen in water than in air. A liter of air contains about 260 mg oxygen, a liter of water 8.5 mg at 25°C and sea level.
Oxygen gets into the water in two main ways.
- Oxygen diffuses from the air into the water. A surface flow increases the gas exchange.
- Plants produce oxygen during photosynthesis.
10°C 11.3 15°C 10.1 20°C 9.1 25°C 8.3 30°C 7.6
solut_rai / Pixabay
With rising temperatures, the oxygen content of the water drops sharply. Cold-water fish are therefore adapted to a higher oxygen content than warm-water fish.
Many warm-water fish only require a relatively low level of oxygen in the water. These include e.g. B. Fish from standing waters. Fish species such as labyrinth fish have evolved extra organs that they can use to absorb oxygen from the air when there is insufficient oxygen in the water.
Fish from fast-flowing waters require more oxygen. Fish from cold, fast mountain streams need a particularly large amount of oxygen.
According to the magazine “Das Aquarium” 11/00 – 12/00, the spawning frequency of mouthbrooding labyrinth fish from fast-flowing waters drops sharply at an oxygen content below 6.5 mg/litre. In the case of species from standing water, the spawning frequency only drops below 3 mg/litre.
The oxygen requirement also depends on the feed. The fish can only use food if there is enough oxygen available. The more you eat, the more oxygen you need. Therefore, if there is a lack of oxygen, they should not be fed.
If breathing is impeded, e.g. For example, if the gills are slimy due to parasite infestation, fish can suffer from shortness of breath, even though there is enough oxygen in the water. High levels of nitrite, nitrate, CO2 or ammonia also mean that the fish cannot breathe in enough oxygen.
The amount of oxygen dissolved in the blood is determined by receptors in the blood of the fish. If there is not enough oxygen in the blood, breathing becomes more and more hectic until the fish gasp violently under the water surface.
In addition to the fish, bacteria in the filter and on all surfaces also consume oxygen. To mineralize 0.25 grams of dry food, approximately 500 mg of oxygen are used.
Fish can also cope with fluctuating oxygen levels. The guide value for a sufficient oxygen content is 3 mg/liter in the morning and 6 mg/liter in the evening.
In a sensibly stocked aquarium with moderate surface movement of the water through the filter and well-growing plants, there is usually enough oxygen available.
Are oxygen stones useful?
In an aquarium with a sufficient number of well-growing plants and without an overstock of fish, there is usually enough oxygen available. The plants produce enough oxygen during the day through their assimilation. In addition, oxygen enters the aquarium through the surface current of the water generated by the filter. Plant growth can be promoted by good plant fertilizer and the addition of CO2. The operation of an oxygen stone is not necessary in such an aquarium. Even without an oxygen stone, an oxygen content of at least 3 mg/liter of water is achieved. Only when the oxygen content is below this value is an additional oxygen supply appropriate, at least temporarily. At the same time, the cause of the low oxygen content must be investigated and eliminated as far as possible.
An oxygen stone can even have negative effects. Due to the violent currents on the water surface, CO2 can be expelled from the aquarium water. If this leads to a lack of CO2, the plants grow less well and produce less oxygen than is actually possible. Another consequence can be increased algae growth.
However, no concrete measurements are known of how much CO2 is actually expelled from the water. The effect should therefore not be overestimated. However, it is a possible cause when an oxygen stone is in operation and vegetation is not satisfactory.
In special situations, the operation of an oxygen stone can be useful:
- During the use of remedies against diseases
- In the event of a lack of oxygen at high temperatures in summer
- In aquariums with few or no plants
Some fish species, e.g. B. discus fish are often kept at relatively high temperatures of at least 28 °. Since many plants grow poorly at these temperatures, oxygen stones are often used in such cases and considered absolutely necessary. It makes more sense to check whether the fish species you keep really need such high temperatures, or whether the temperature cannot be lowered. Even a small drop of 1° can have a very positive effect on the plants. Many aquarists keep discus fish even at lower temperatures. If it is not possible to lower the temperature, preference should be given to plants that also thrive at higher temperatures.
The same applies to the care of fish species that eat plants or prevent good plant growth by digging in the substrate, such as some species of cichlids. There are a number of plant species that are relatively robust and will also grow under such conditions.
In overstocked aquariums, where there is no way to reduce stocking in the short term, there may be the option of operating an oxygen stone only at night. This avoids the expulsion of CO2 during the day and at the same time bridges the particularly critical hours of the night, when both fish and plants consume oxygen.
While some filter types have proven themselves because they bring a lot of oxygen into the water due to their construction, such as e.g. B. Aquaclear power filter, no additional oxygen should be brought into the aquarium via the filter, e.g. B. by opening the air intake port of the pump. Some pumps and nozzles produce such fine air bubbles that they attach to the fish’s mucous membrane and can cause damage.