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Plant fertilization with CO2 in the aquarium

Why is fertilized with carbon dioxide in the aquarium

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Carbon dioxide plays an important role in the fertilization of aquarium plants and thus in the overall ecology of an aquarium. Many aquarists bring carbon dioxide into the aquarium with more or less effort, other aquarists can do without this effort.

Fertilization with carbon dioxide is intended to prevent plants and animals consuming less carbon dioxide in the water than is required by the plants. Otherwise, the plants get the required CO2 from the carbonate hardness.

This leads to rising pH values ​​and lime deposits on the plants, the so-called biogenic decalcification. The gills of the fish can be corroded by the high pH values.

Where does carbon dioxide (CO2) come from?

During photosynthesis, plants use carbon dioxide and give off oxygen. Plants and animals use oxygen when they breathe and give off carbon dioxide. Plants need more carbon dioxide during the day than they produce and produce more oxygen than they use. The extra carbon dioxide that plants consume is produced by animal respiration. At night, plants also consume oxygen and produce carbon dioxide.

So carbon dioxide is produced when you breathe. Carbon dioxide and oxygen form a cycle consisting of photosynthesis and respiration.

The amount of carbon dioxide and oxygen in the aquarium depends on the number of plants and animals. Therefore, some aquariums with many fish and few plants often need air pumps to bring oxygen into the aquarium. In aquariums with few fish and many plants, it may be necessary to supply carbon dioxide from outside. It is then often spoken of CO2 fertilization. The type of plant also plays a role. Cryptocorynes consume e.g. B. relatively little carbon dioxide.

There is a certain equilibrium between the carbon dioxide in the air and the carbon dioxide in the water. This equilibrium is around 5 mg/liter of water. Plants need a level of between 10 and 20 mg/liter of water. In this area, carbon dioxide is constantly released from the water into the air. In addition, the plants consume carbon dioxide faster than it can be absorbed into the water via the water surface, even if the water surface is heavily agitated. Due to rapid consumption and the slow establishment of an equilibrium between water and air, many aquariums experience a constant shortage of carbon dioxide. Only in aquariums in which a lot of carbon dioxide is produced is there enough carbon dioxide available for the plants.

light and carbon dioxide

The need for carbon dioxide can be partly controlled by light. If the aquarium is illuminated more intensively, more intensive photosynthesis takes place and the plants consume correspondingly more carbon dioxide. Plants also play an important role in this regulation. Due to the intensive photosynthesis, they grow correspondingly fast. This creates a lot of new plant mass, which decomposes again and generates additional CO2. Under favorable circumstances, consumption levels off in such a way that the CO2 content is just as high after the increase in illuminance as before. In many cases, however, more CO2 has to be supplied with stronger lighting.

With the double lights normally used, no CO2 fertilization is usually necessary with normal fish stock. If the lighting is stronger, if there are few stocks, etc., fertilization with CO2 may be necessary.

Plants grow more slowly in dimly lit aquariums. Therefore, the fertilizer introduced indirectly with the feed can be sufficient.

When fertilization with carbon dioxide is necessary

Whether CO2 fertilization is necessary depends on the relationship between CO2 producers and CO2 consumers. The CO2 production is determined by the size of the stock or the amount of feed.

CO2 consumers are mainly the plants, which consume carbon dioxide during assimilation (photosynthesis).

If enough carbon dioxide is generated in the aquarium so that the plants do not suffer from a lack of CO2, CO2 fertilization is not necessary.

In well-lit aquariums with many plants and few fish, on the other hand, additional CO2 fertilization is often necessary. If not enough carbon dioxide is produced in the aquarium, the plants extract carbon dioxide from the carbonate soil. In this so-called biogenic decalcification, lime is precipitated. The pH value rises sharply to values ​​between 8.5 and 9. In extreme cases the pH value can rise even higher.

The guide value for a reasonable CO2 content in the aquarium is a value between 10 and 20 mg/liter of water. If the CO2 content is within these limits, no additional carbon dioxide needs to be introduced. If the CO2 content is below 10 mg/liter of water, CO2 can be added.
Alternatively, plant species that require little CO2 can be used. These are usually plants that get by with little light.

Why overstocked aquariums are still not good

In overstocked tanks, the many fish produce a lot of carbon dioxide. During the day, this carbon dioxide can possibly still be processed by the plants. At night, however, the plants can no longer carry out photosynthesis due to the lack of light. At night, the plants produce additional carbon dioxide and consume oxygen. This is why the oxygen content in such aquaria can be so low in the morning that the fish suffer from a lack of oxygen and, in extreme cases, die.

Why understocked aquariums can also be unstable

Some aquarists have found that problems can arise in aquariums with no fish or only a few fish. If the fish were removed from established aquariums, after a while the plant growth stagnated and algae spread, even if the water was changed regularly. Conversely, in some aquariums with poor plant growth, growth could be improved by adding more fish. In such aquariums, there may be a lack of organic substances, such as fish excrement and uneaten food, which produce CO2 when they are broken down by bacteria. So there is not enough CO2, the plants take care of it and die. The organic load increases again. The trace elements released when the plants die and the additional carbon dioxide produced by bacteria when the organic substances are broken down benefit the algae, which then multiply rapidly.

However, there are also contrasting experiences. The plants grew at least as well as before after the fish died in a previously moderately stocked aquarium and no new fish were introduced. In other cases, certain plant species grew particularly well in very sparsely stocked aquariums. Even without additional fertilization with plant fertilizers or carbon dioxide. In such cases, the metabolic processes in the overall aquarium system may adjust to a lower level overall. Perhaps the plant species also play a special role. Cryptocorynae were affected in two specific examples.

An exact prediction of how an aquarium will behave when there are changes in the cycle between carbon dioxide and oxygen cannot be made. An aquarium is not a self-contained system. Gas exchange on the water surface, snails and other organisms in scum, plants, etc. also influence the carbon dioxide and oxygen content.

What a stable aquarium looks like from the point of view of carbon dioxide

In a stable aquarium there are neither too many nor too few plants and neither too many nor too few animals. The system is so balanced that as much carbon dioxide is produced as is consumed on the other side.

Align the stocking of fish and plants with the natural CO2 balance?

In principle, it would make sense to align the stocking with the CO2 balance. For this purpose, at least the lighting, the planting and the fish stock would have to be precisely adapted to the water volume and to the production and consumption of carbon dioxide and oxygen. In addition to plants and fish, invertebrates, algae, bacteria, etc. also play a role. It is hardly possible to create a functioning circuit in an aquarium. However, there are a number of other factors that also need to be considered. In nature, CO2 production by fish is relatively low. By far the largest proportion of CO2 is generated by bacteria. In bodies of water, most of the CO2 is generated in the sludge. There, leaves and other organic matter are decomposed by bacteria, releasing carbon dioxide. However, a layer of sludge 20 to 30 cm high is undesirable in aquariums.

In natural bodies of water, there are very few fish and plants relative to the amount of water. According to this, there should only be one or two fish and a few plants in an aquarium. As a rule, there are fewer plants in relation to the amount of fish in natural waters than there are in aquariums. Plant growth in aquariums is usually unnaturally strong. Much more carbon dioxide is required than in nature. Even the high fish stock compared to nature is often not sufficient to generate enough CO2 for the plants. In order for the plants to grow well, a lot of fish would have to be used. This in turn makes extremely large filters necessary to break down the excrements of the fish. Above all, the fish did not have enough swimming space.

Generating the required carbon dioxide by correspondingly high stocking of fish would lead to extremely overstocked aquariums. It is impossible to imitate in an aquarium a natural body of water that runs stably without external intervention. Such an aquarium with practically only water is also not desired by most aquarists.

The only alternative is to supply the necessary carbon dioxide from outside. The CO2 fertilization can be switched off if there is too much CO2 in the aquarium. Too high stocking cannot simply be reduced.

How much carbon dioxide a fish produces

The CO2 production of the fish depends on the oxygen uptake, which in turn depends on several factors. These include fish size, activity of the fish, diet or fecal production, water temperature, etc. Herbivores and algae eaters produce e.g. B. more droppings than carnivores. Fast swimmers have different gills than slow swimmers. They use more oxygen and produce more carbon dioxide.

Fast-swimming fish therefore prefer to live in cold, oxygen-rich waters, while slow swimmers like to stay close to the bottom or in