CO2 – Carbonic acid, carbon dioxide, CO2 gas.
Regarding the choice of plants by keeping difficulty, as Takashi Amano says, there are no simple or difficult plants. They only differ in that some need a little more light and CO2, while others need a little clay in the substrate. By providing a normal lighting level, CO2 supply, and all nutrients in the water, absolutely all plants will grow. If a plant description states that a certain species prefers soft water and recommends CO2 (carbon dioxide) fertilization, it is risky to buy this species. In hard water, without additional CO2 supplementation, this plant will die. Plants love CO2 additions – it is their main food! If you don’t have CO2 and powerful lighting, you can still create a beautiful planted aquarium, just with slightly different plants.
Plants, unlike algae, need carbon dioxide for photosynthesis, and in the absence of CO2, plants cannot absorb the nutrients necessary for their development; accordingly, all nutrients begin to be absorbed by algae, which successfully multiply in such an environment. Light also plays a major role, because the more intensive the lighting, the faster the process of photosynthesis takes place in aquarium plants. Therefore, if the lighting for your plants is sufficient, the issue of CO2 and nutrients becomes even more important.
Now the most important thing is to determine the ratio of these three elements, because if there is an excess of one element, the others will naturally be in short supply. For example, if there is a low level of lighting, there will be an excess of carbon dioxide and nutrients in the aquarium, and plants will deteriorate and die. If the nutrient level is too low, a situation will arise where plants have nothing to feed on. In this case, as a reward for their labor, the aquarist will get the development of Blue-Green algae in their aquarium.
In water, carbon dioxide exists in three forms: free, semi-bound (bicarbonates), and bound (carbonates). In the free state, it is dissolved carbon dioxide in water, and in the bound state, it is in the form of calcium and magnesium carbonate salts, the so-called carbonate hardness salts. Carbon dioxide (CO2) dissolves 50 times better than oxygen and forms carbonic acid (H2CO3); its presence causes the pH to shift towards the acidic side. If the concentration of carbonic acid is excessive, it is called free or dissolved carbon dioxide. Oxygen is a universal oxidizer of organic compounds, while carbon dioxide is the foundation for the growth of underwater flora and one of the main regulators of the water’s hydrogen exponent – pH.
The presence of carbonic acid in water is very important. Its compounds with calcium and magnesium form carbonate water. The higher the proportion of calcium bicarbonate in water, the higher the proportion of bound carbonic acid. Carbon, the chains of atoms of which form the basis of organic molecules, comes from nowhere else than carbon dioxide. Plants assimilate carbon very well and quickly from carbon dioxide dissolved in water and poorly from carbonate hardness salts. In most cases, especially if the aquarium water has significant hardness, a deficit of free carbon dioxide is noted in the aquarium, and plants receive an insufficient amount of carbon.
The thing is that, according to the laws of electrical dissociation, for each value of water carbonate hardness to ensure chemical equilibrium in water, a certain amount of free carbon dioxide must correspond. If the CO2 content remains stable for a sufficiently long time, a carbon dioxide-lime equilibrium is established in such water: no new bicarbonate ions are formed. If for any reason there is less carbon dioxide in the aquarium water than is needed for the given carbonate hardness value, calcium and magnesium salts precipitate into a practically insoluble sediment. They deposit on the leaves of plants, and their leaves begin to “turn gray” as they become covered with a crust of calcium carbonate. This continues until the carbonate hardness decreases to a value at which equilibrium is reached with the reserve of unbound carbon present in the water in the form of carbonic acid.
This is accompanied by an increase in water pH to a value greater than 7, which further complicates the situation regarding the possible amount of free carbon for plant nutrition. Even the smallest amount of carbon dioxide affects the pH of any water. Usually, plants can raise the pH of aquarium water to 8.3–8.5. At such an active reaction value of water, there are almost no carbon dioxide molecules in it. As a rule, they cannot raise the pH any higher, because its further growth strongly worsens the functional state of the plants themselves: photosynthesis, and consequently the extraction of CO2 from the system, slows down, and carbon dioxide from the air dissolves in the water and stabilizes the pH.
If the carbonate water hardness is quite high, this process can lead to an increase in the pH of the aquarium water to 9, which is dangerous not only for other plants but also for the vast majority of aquarium fish. In aquarium water with high pH values, it is impossible to grow a whole range of plants, and very many species of aquarium fish definitely do not like alkaline water. Thus, aquarium plants can literally suffocate each other. The winners are those species that better extract carbonic acid from bicarbonates, while those that do not know how to do this, for example, Rotala and Aponogeton species from the Madagascar group, suffer. It is precisely these plants that aquarists consider the most delicate.
CO2 is of paramount importance for the aquarium, for fish and plants, especially for plants. But the problem is that in striving to improve conditions for plants, you can seriously change the water composition, and this will turn out to be very detrimental to fish. With an excess of CO2, plants will not suffer much, but fish will hardly survive such an innovation; a minor change in content in the water, up to 30 milligrams per liter, is safe for fish. Keep in mind that some fish, despite theoretically having a fairly wide range of water composition requirements, can experience small changes in the usual chemical composition of water as their last.
The main danger in using CO2 is that when dissolved in water, it turns into carbonic acid, and this greatly affects the acidity of the water, noticeably lowering the pH level. Increasing hardness slightly neutralizes the drop in acidity, but all this creates rather sharp changes in the chemical composition of the water. Most fish, as a rule, will not survive such sharp drops, and the very next day you may find them dead. If there is a need to change the water composition, do it gradually, not all at once, and don’t be afraid to spend plenty of time on it. If fish begin to swim to the surface and gulp air, it means there is too much CO2 and too little oxygen in the water; if they become lethargic and behave inadequately, it means the process has gotten out of control.
At pH 8.3, free carbonic acid is practically non-existent. With an increase in carbonate hardness, the CO2 concentration increases as follows: at KH less than 2° up to 15 mg/l, from 2 to 10° up to 20 mg/l, over 10° up to 30 mg/l. Temperature increases reduce the solubility of carbon dioxide from 0.7 mg/l at 10°C to 0.38 mg/l at 30°C. Supersaturation of water with CO2 causes the Root effect. Its essence lies in the fact that blood hemoglobin gives preference to CO2, displacing oxygen, as a result of which fish suffocate even under outwardly favorable oxygen conditions. Aquarists are often surprised that a fish can die from carbon dioxide poisoning (“suffocate”) even in water saturated with oxygen.
CO2 is not harmful to fish if the oxygen concentration in the body of water is high enough. Prolonged exposure to elevated concentrations of carbon dioxide on aquarium inhabitants can cause acidosis, metabolic disorders, and sudden cardiac arrest. In some fish, an increase in carbonic acid concentration causes an increased appetite, but food is not properly assimilated, and increased feed consumption is accompanied by slow exhaustion of their bodies. For the normal life activity of fish, a dissolved oxygen concentration of 5 mg/l in water is sufficient.
Plants exhibit a phenomenon that aquarists, without delving into its essence, called incompatibility. However, there are practically no incompatible plants in our aquariums, but there are plants with different pH barriers. For example, Cabomba stops photosynthetic activity when the pH rises to 8, Vallisneria continues it up to 10, and Elodea up to 11. It is clear that Cabomba will first stop the growth of apical stems and then shed its leaves. Gradually, the tips of Vallisneria leaves near the surface will also begin to disintegrate; the degree of alkalinization of the upper layers of water by Elodea will prove to be an intolerable daily test for these two species. More complex plants are difficult to keep precisely because their lower and upper pH barriers are close to each other – after all, in the flowing waters of their homeland, they do not experience such pH fluctuations that occur in aquariums with standing water.
pH – KH – CO2 Equilibrium.
The three-way relationship between these values means that once the required value of two quantities is reached, a specific value of the third is established automatically. KH and CO2 are stabilizers, i.e., components of the buffer system in water. The initial hardness of tap water allows you to adjust the pH by normalizing the hydrocarbon CO2 level, but these are extreme measures that are highly undesirable.
The pH value in the aquarium is subject to fluctuations associated with changes in dissolved CO2 and carbonate water hardness KH. Thus, an increase in the amount of CO2 or a decrease in KH makes the water more acidic, while an increase in KH or a decrease in CO2 makes it more alkaline. This means that all aquatic organisms through their respiration contribute to lowering the pH. This decrease is especially noticeable at night when plants do not absorb carbonic acid. During the day, in the light phase of photosynthesis, the intensity of carbonic acid consumption by plants increases significantly. Therefore, with prolonged bright lighting and a large number of plants intensively absorbing carbon dioxide, the pH value can rise to 9. And at night, due to the absence of photosynthesis and the release of CO2 by plants and fish during respiration, it can drop to 6, which is unfavorable for the aquarium population.
In hard water, at a pH around 8, the amount of CO2 is insufficient for plants, and in this case, you either need to reduce water hardness or introduce carbon dioxide, which will shift the pH to a favorable range. The more blue-green algae, the more alkaline the environment. This means that at night the pH in the aquarium shifts towards the acidic side on the scale, and during the day towards the alkaline side. To reduce sharp fluctuations, mandatory nighttime aeration of water with air is recommended.
By monitoring them in a timely manner, one can promptly intervene in the operation of an overpopulated and therefore unstable system, supplying it with missing resources and removing excess waste that the aquarium “biocenosis” itself is unable to utilize. One such necessary resource for an aquarium with live plants is carbon dioxide. A low pH indicates an excessively high concentration of carbonic acid. Carbon dioxide can be used as a means of changing the aquarium pH by constantly adding gas to the water, thereby maintaining the pH level required for the growth of specific plants.
To avoid problems associated with this, it is advisable to test the concentration of carbonic acid in the water. If you have been supplying carbon dioxide to the aquarium for a long time, and there is limestone in the aquarium, then after stopping the CO2 supply and vigorously aerating the water, a sharp jump in pH towards the alkaline side is possible. If this level turns out to be too high, you will need to use buffer solutions, a pH minus product, peat extract, alder cones, sphagnum moss, and other remedies at the right moment.
However, acidity and carbonate hardness are not the only factors determining the carbon dioxide-lime equilibrium of water in an aquarium. This equilibrium depends on a number of factors:
1. Volume (capacity of the aquarium). As a rule, equilibrium processes are more stable in large-volume aquariums.
2. Geometric dimensions of the aquarium (ratio of length, height, and width). An aquarium with a large surface area provides better gas exchange with the air.
3. The number of fish and other aquarium animals that release carbon dioxide through respiration.
4. The number of plants in the aquarium, which, depending on lighting, release or absorb carbon dioxide.
5. Lighting intensity, which affects the vital activity of aquarium plants.
6. The chemical composition of the water poured into the aquarium. The most important factor is carbonate hardness (KH).
7. Fish feeding regime. Decaying excess food becomes a source of carbon dioxide.
8. Water temperature. Affects the solubility of carbonates, carbon dioxide, and the rate of all chemical reactions.
9. Substrate. The hydrochemical composition of the water depends on the carbonate content in the substrate.
10. Water movement due to aquarium filters, micro-compressors, pumps. Affects the saturation of water with carbon dioxide from the air and the solubility of carbonates.
All these factors indicate the complex dependence of equilibrium on aquarium maintenance conditions: due to such a large complex of factors, it is often impossible to predict the direction of the carbon dioxide-lime equilibrium shift and the corresponding change in the hydrochemical composition of the aquarium water.
