Plants need CO2 as a nutrient. When exposed to light, plants absorb CO2 and release oxygen. In the dark, plants also absorb oxygen and release CO2, but only in smaller amounts.

In nature, CO2 is produced in various ways.

In the aquarium, too, CO2 is constantly produced by the respiration of the fish. CO2 is also produced in filters that work aerobically. The bacteria living there consume oxygen and produce CO2. In aquariums, however, it is not always guaranteed that as much CO2 is generated as the plants need for good growth. How much CO2 is required in addition to fertilizing the aquarium plants depends on various factors:

In aquaria with strong plant growth, an additional CO2 supply can be useful. In an almost unplanted Malawi basin, adding CO2 makes no sense. In aquariums sparsely stocked with fish, in which fast-growing plants grow, less CO2 is often produced than the plants need.

Before buying a CO2 system you should First of all, the CO2 content in the aquarium water is measured. Expensive CO2 systems have often been bought and then it was found that there was already enough CO2 available. Bad plant growth is not always caused by too little CO2. In many aquariums, there is magnificent plant growth even without CO2 fertilization.

A CO2 system should only be procured after a corresponding CO2 test has confirmed that there is not enough CO2 in the water.

In dimly and normally lit aquariums, 10 to 20 mg/liter CO2 is sufficient. A value of up to 30 mg/litre may be useful in very brightly lit aquariums with correspondingly strong plant growth. Most plants do not process more CO2, so additional CO2 is wasted. More important than the absolute level of the CO2 value is that CO2 is always available and that it is transported to the leaf surfaces by slight water movement and can then be absorbed.
With good lighting and without water movement, the plants absorb the CO2 faster than it can get back from the free water to the leaf surface by diffusion, because diffusion is a very slow process. The plants can then get too little CO2 although there is still enough CO2 in the open water.
It is just as important that all other conditions, e.g. B. light and nutrient supply are adapted to the CO2 content that is available to the plants. When there is little light, only a small amount of CO2 and other nutrients are required. If there is a lot of light, more CO2 and more fertilizer are required.

The CO2 content can e.g. B. be 20 mg/liter in the morning and 10 mg/liter in the evening. It is ideal if the CO2 supply starts 2 hours before the lights go on and ends 2 hours before the lights go out. When the light goes on, plants start photosynthesizing very quickly. In the morning, the plants then have enough CO2 available that is consumed over the course of the day. In this way, the pH value also changes within limits that the fish can tolerate.

The best way to monitor the CO2 value is with a drop test. The calculation of the CO2 value from the pH value and carbonate hardness is only correct if the carbonate hardness is determined with an accuracy of 0.5° and the pH value is measured electronically and with a well-calibrated electrode.

The aim is not always to have a constant CO2 content in the water. This is practically impossible to do manually anyway. It would be possible with a pH controller and solenoid valve, but this would lead to constant fluctuations in the pH value.
When the solenoid valve is switched off, the CO2 level drops quite quickly. When the solenoid valve is open, a lot of CO2 has to be replenished because the losses when the valve is closed also have to be compensated. The pH value fluctuates accordingly. The solenoid valve must switch constantly.

It is not exactly known at what level the CO2 value is harmful to fish. It can be assumed that values ​​up to 15 mg/liter are harmless. In addition, the harmfulness depends on the species of fish, the level of oxygen in the water and other conditions. American dwarf cichlids and rainbow fish are considered z. B. as sensitive to higher CO2 values. The optimal CO2 content in the aquarium can therefore not be given in general terms.

A failure or switching off of the CO2 supply has direct no negative effects on the fish, as long as the pH value does not rise too quickly. But that would only be the case if the CO2 supply was extremely high.

With strong lighting and strong plant growth, so-called biogenic decalcification can occur if not enough CO2 is available. The plants then cover their need for carbon from bicarbonate in the water. This increases the pH value.

With strong lighting, pH values ​​above 10 are possible. At such high pH values, ammonium converts into ammonia, which is very toxic to fish, and the filter bacteria stop working. Plants can die off even at pH values ​​as low as 9. The resulting organic waste leads to even more ammonia. Shutting off the CO2 supply could even lead to the death of fish.

In order for the plants to remain healthy, the lighting must also be reduced with the CO2 supply in heavily lit aquariums. Too much light would otherwise lead to deficiency symptoms due to a lack of CO2.

The reduced assimilation of the plants means that the plants break down fewer pollutants, e.g. B. consume nitrate. Therefore, the fish stock may have to be reduced or more water changed to remove pollutants.

The CO2 supply mainly changes the pH value. The total hardness GH remains unchanged.
The supplied CO2 largely dissolves in the water. Only a small part reacts with water to form carbonic acid. A fixed percentage of the carbonic acid breaks down into hydrogen ions and hydrogen carbonates. The acid-binding capacity SBV increases slightly due to the hydrogen carbonates. The SBV corresponds to the carbonate hardness KH measured in aquaristics.

The carbonate hardness increases arithmetically slightly. In practice, however, the effect can be neglected.
If the CO2 content in water with a carbonate hardness of 7 and 20 mg/liter CO2 is doubled to 40 mg/liter CO2, 0.0044 mg/liter CO2 react to HCO3-. The carbonate hardness thus increases by 0.00028° dH. The carbonate hardness only changes measurably at pH values ​​below 5.

GH and KH practically do not change at all because the Ca/Mg ions are not directly involved in the reaction. Effects arise only indirectly via the pH value.
If there is precipitated lime in the aquarium, e.g. B. limestone, the CO2 reacts directly with the solid CaCO3 to form Ca++ and HCO3-. In this case, both GH and SBV increase.

Carbon is the main nutrient of all plant life. Aquatic plants get carbon mainly from carbon dioxide CO2, just like land plants.

Plants produce carbohydrates from CO2 and water H2O with energy from light. This is assimilation via photosynthesis. Like all living things, plants convert carbohydrates with nitrogen, sulfur, phosphorus, and other nutrients into the various compounds that the plant needs to live and grow. During photosynthesis there is a fixed relationship between the amount of light and the need for CO2. With very strong lighting, the plants will produce more than with weak lighting.

CO2 is produced naturally in the aquarium. 70-90% of fish food is breathed in by fish and filter bacteria to form CO2.

In a dimly lit aquarium with few plants or with slow-growing plants, the amount of CO2 naturally produced may be sufficient to feed the plants.

In a heavily lit aquarium with many plants, the amount of natural CO2 may not be sufficient to feed the plants. Additional CO2 must then be supplied from outside. Otherwise there is a risk of so-called biogenic decalcification, in which the plants get the carbon they need from the carbonate hardness.
As a result, the pH value can rise well above 9 and furnishings and plants are covered with a kind of layer of lime. In the long run, the plants can die and fish can be killed.

A CO2 system is therefore only required if more CO2 is used in an aquarium by the plants than is newly introduced into the aquarium by the respiration of the fish and filter bacteria.

A CO2 content of 10 to 20 mg/liter of water is sufficient for most plants and harmless to the fish. Few plants need more CO2.

CO2 levels that are too high first lead to breathing problems and ultimately to the death of the fish. With the CO2 values ​​mentioned, there is also a buffer for measurement errors, slightly increased nitrite values, lack of oxygen, etc. Together with other factors, e.g. B. high nitrite values, even seemingly harmless CO2 values ​​can have dramatic consequences.

In many aquariums, enough CO2 is naturally available and CO2 fertilization is unnecessary.