CO2 – How to provide aquarium plants with proper nutrition

Many people think that light and fertilizer are enough for beautiful aquarium plants. But often it is carbon dioxide, or CO2, that is the missing piece of the puzzle. Without it, even under ideal conditions, plants cannot reach their full potential.

Imagine that CO2 is to plants what oxygen is to us. They absorb it from water and, with the help of light, convert it into energy for growth. There is plenty of it in nature, but in a closed aquarium, plants quickly use up all the available carbon dioxide and begin to starve.

That is why adding CO2 is not just a whim, but a basic requirement for a lush underwater garden. It allows plants to grow quickly, be healthy and vibrant, and also helps fight algae by leaving them less food.

In this article, we will discuss how to easily and safely provide your green pets with this vital nutrient so that your aquarium truly comes to life.

Carbon dioxide is essential for the development of aquatic plants

Life on Earth is carbon-based—carbon compounds are the main component of most organic substances. While animals consume everything they need to build body tissue from food, most plants synthesize their own “building materials.”

The process by which plants obtain the simplest carbohydrate (glucose) is called photosynthesis. Its main reaction is:

6CO₂ + 6H₂O = C₆H₁₂O₆ + 6O₂

Synthesis occurs with energy absorption; the formation of each glucose molecule requires about 674 calories. Plants obtain it by absorbing chlorophyll molecules and other pigments (chromophylls).

Subsequently, under the action of enzymes, simple carbohydrates are converted into;)

• more complex sugars;
• amino acids, proteins, and fats, the synthesis of which requires macro- (nitrogen, phosphorus, potassium) and microelements.

This ensures plant growth, root system development, leaf mass accumulation, etc.

Some plants can use other compounds as a source of carbon. For example, some aquatic plants (Elodea, Vallisneria, Hornwort, Anubias, Echinodorus, and some others) have adapted to consume hydrocarbonate ions (HCO₃^— and carbonates (CO₃^2-) , the presence of which is due to a non-zero concentration of hardness salts.

Thus, several factors are necessary for plant development:

• The presence of water (irrelevant for aquatic plants, since water is their natural habitat).
• For most, the source of carbon is CO₂.

• Energy (read: lighting).
• Macro- and microelements.

Aquarium plants have virtually no problems with light energy and nutrients. For most of them, natural daylight and nutrients stored in the soil are sufficient. For more demanding plants, it is easy to arrange artificial lighting and fertilization. However, there are certain problems with the main building block (carbon).

The reason for the lack of carbon (carbon dioxide)

The deficiency of carbon compounds in artificial home water bodies is quite simple to explain.

In natural conditions, the sources of carbon in water bodies are:

• Carbon dioxide from the atmosphere. CO2 dissolves in water much better than oxygen, but is also easily released. At the same time, due to the large contact area between the water surface and the air, the concentration of CO_2, obtained from this source can be quite large.

• Salts (carbonates) of various elements. Along with poorly soluble substances, sources also bring easily soluble substances (e.g., sodium salts) into water bodies.
• Carbon dioxide released during respiration by aquatic fauna. This source also provides an intensive supply of CO₂.

As a result, the concentration of carbon dioxide in natural water bodies can range from 3 to 10 mg/l (in flowing water) and up to 30 mg/l in stagnant water. This is sufficient for the growth of aquatic plants.

In home aquariums, even those with a significant volume, the picture is different:

• Aquarists, as a rule, strive to reduce the carbonate hardness index to the recommended values.
• The surface area is insufficient to enrich the volume of water with carbon dioxide.
• Those who specialize in plant breeding are very reluctant to introduce fish (especially large ones) and invertebrates.

Accordingly, without an additional source of carbon, underwater flora will be doomed to carbon deficiency and developmental problems. CO₂ into the aquarium.

The optimal concentration of carbon dioxide in aquarium water

The best way to achieve growth and harmonious development of aquarium plants is considered to be ensuring a concentration of carbon dioxide that corresponds to their natural habitat.

Various sources and aquaculture forums cite different limits for CO₂. Thus, experienced aquarists often talk about concentrations ranging from 7 to 30 mg/l, while manufacturers of aquarium equipment, including CO₂, they prefer to use figures ranging from 15 to 40 mg/l. For example, Dennerle recommends maintaining a concentration of 15-30 mg/l, with an optimal value of 20-25 mg/l.

There is a general estimate of the lower acceptable concentration limit. It should be at least 3 mg/l, otherwise aquatic plants will experience real starvation.

— Arend van den Nieuwenhuizen Aquascaper

It is somewhat more difficult to estimate the upper limit. Usually, the fact that CO₂ in water with carbonate hardness (kH) and acidity (pH). It allows you to calculate the concentration of carbon dioxide based on kH and pH. But the reverse relationship is also true: an increase in carbon dioxide content leads to an increase in hardness and a decrease in pH.

Accordingly, when gas is artificially supplied to the aquarium, the water parameters may change, putting aquatic organisms at risk. With an initial kH value of about 4 degrees and a neutral reaction, CO₂ already 30 mg/l, however, high hardness and low pH will not contribute to the health of aquarium flora.

There is a concept known as the “carbonate buffer.” The essence of this phenomenon is that at high kH levels, a large concentration of carbon dioxide is required to significantly increase the pH. Accordingly, in harder water, the CO2 content can be higher without irreversible consequences for the inhabitants of the aquarium.

The importance of balance

Simply ensuring the optimal concentration of carbon dioxide in the water is clearly not enough to ensure the stable growth and development of aquarium plants. It is also important to remember the other factors mentioned above: lighting and the availability of nutrients.

Proper lighting of the aquarium also affects the CO2 balance in the aquarium.

Experts consider the following parameters to be sufficient for stable, albeit slow, growth of aquarium flora (given for a water column of 0.4-0.5 m):

• Carbon dioxide concentration 5-7 mg/l;

• Lighting – 0.4-0.6 W/l.

In this case, there are sufficient nutrients resulting from the vital activity of nitrifying microorganisms and hydrobionts.

When the CO2 content increases to 15-20 mg/l, it will be necessary to increase the lighting to 0.7-0.8 W/l. It will be necessary to add nutrients, primarily nitrogen. The need for phosphorus and potassium supplementation should be checked using aquarium tests.

Aquarium owners should remember that it is in conditions of balance that higher aquatic plants demonstrate their full effectiveness and gain a competitive advantage. Their rapid development suppresses algae, which find themselves “starved,” and the aquarium looks clean and healthy.

But as soon as the balance is disturbed, simple ancient algae (thread algae, “black beard”), which are more adapted to difficult conditions, gain the upper hand. Their growth provokes a further deterioration of the situation.

Control of CO2 concentration in the aquarium

The most accurate and effective method of controlling CO₂ – measurement of carbonate hardness and pH. It was mentioned above that the concentration of carbon dioxide in water is strongly dependent on these indicators. The values for different pH and kH are summarized in the table. The optimal content of 15-30 mg/l is highlighted in green.

The main advantages of this method are:

• Accuracy of carbon dioxide concentration measurement;

Many aquarists, even those with considerable experience, prefer simpler methods:

• Use of drop checkers

This indicator is a small vessel filled with a special compound (usually water with kH=4 and a chemical pH indicator). The vessel is immersed in the aquarium, the solution comes into contact with the water and changes color depending on the CO2 content.

The advantage of this method is its simplicity, while its disadvantages are its limited operating time (but the indicator liquid can be refilled) and inertia (it takes 0.5 to 2 hours for the drop checker to change color). Many test indicators, which are easy to find in pet stores, work on the same principle.

• Counting the bubbles of gas coming from the system

A bubble counter is built into the line from the system to the aquarium—a transparent vessel filled with water, in which it is convenient to observe the release of gas. The fastest way—allows you to estimate the concentration of CO₂ concentration during delivery, even before it dissolves in water.

It is believed that 1 bubble per minute per 10 liters of aquarium volume corresponds to a CO₂ concentration of 7-19 mg/l. Of course, the accuracy of the control is not high, but it is quite possible to estimate the concentration and prevent it from exceeding the norms.

• Intensity of bubbling.

The intensity of oxygen release by plants also directly depends on the concentration of carbon dioxide. Many experienced aquarium owners can estimate this indicator quite accurately by eye, based on the “bubbling” of plants and the chemical composition of the water.

Plants in an aquarium, like plants on land, need carbon dioxide (CO2) to grow. Without it, even with good light and soil, they will grow slowly, fade, and may become covered with algae. There are various ways to add CO2 to the water, from simple homemade systems to ready-made cylinders, and it is important to maintain the correct concentration so that the plants feel good and the water remains clean and alive.

CO2 supply to the aquarium

In its simplest form, the system for supplying carbon dioxide to an aquarium consists of:

• Generator – a device that is a source of CO₂.
• Reactor – a submersible device located in the aquarium, in which the gas is dissolved.

• Gas line – tubes connecting the generator and the reactor.

Generator options

Fermentation (“mash”)

The simplest method, in which carbon dioxide is obtained by the breakdown of sugars by yeast cultures. In this case, the generator is any container filled with a nutrient solution and a yeast colony.

Such a device is easy to make with your own hands, and the components for it are readily available and inexpensive. The disadvantages of this method include its limited duration, difficulty in adjustment, and uneven gas formation. Despite this, some manufacturers offer mass-produced “eco” versions that use this method.

Cylinder

In such a system, the gas source is a cylinder of liquefied CO₂. The most complex option requires a fitting assembly with a pressure reducer and pressure gauges to monitor the pressure in the cylinder and the main line. However, in terms of adjustment and control, it surpasses any other method—an electromagnetic and controlled needle valve easily solves all problems.

Chemical

The method is based on the release of carbon dioxide during the reaction of salts (carbonates, such as soda or limestone) with acids. It requires more expensive components for the reaction, but can provide sufficiently long-term generation (if the problem of component dosing is solved). It also has problems with regulation and uneven gas release.

Exotic options

There are many ways to obtain carbon dioxide:

• Chemical compounds that release gas when in contact with water are available in the form of tablets for aquariums.
• Electrolytic, very interesting in terms of application, but has a serious problem with hydrogen release.

• Use of “dry ice” (problem with dosing and placement of the gas source in the aquarium).
• Sparkling water, etc.

Most exotic methods have not yet found industrial application, but work quite successfully in various homemade devices.

Reactors

The reactor must ensure effective dissolution of gas in aquarium water. In practice, two types are used.

Passive

Dissolution occurs naturally when gas comes into contact with water.

1-Diffuser. 2-Bell, 3-Ladder

Variants of such reactors:

• Diffuser. A device with micro-holes through which gas is forced into water. The small size of the bubbles ensures effective dissolution. It can use both industrial materials (diaphragms) with micropores and natural materials (for example, wood from rowan, linden, and some other species).

• Bell. It is an upside-down vessel immersed in water. Gas collects in it and dissolves at the water interface.
• “Ladder”, “Labyrinth.” A submersible reactor in which the path of the gas bubble to the surface is artificially increased due to its complex configuration. As a result, CO₂ has time to dissolve naturally.

Active reactors (pumps)

In such devices, a counterflow of water and gas is created in the working space, which leads to active dissolution of the bubbles. This is the most complex system in terms of technical implementation, but it is the best option for large aquariums.

Additional devices

In addition to the generator and reactor, the system may require:

• A shut-off solenoid valve. Used to supply gas on a timed basis and regulate concentration.
• Check valve. Prevents water from being sucked into the system from the aquarium, which can lead to component failure.

• Bubble counterto monitor system performance and CO concentration₂.
• Pre-filter, is relevant for a “moonshine still” or chemical generator, especially one assembled by hand.

Mass-produced systems for CO2 supply

Aquarium equipment manufacturers offer a variety of CO₂. Brief information about some of these products is provided below.

Dennerle

The company offers CO₂ various levels, using different technologies for aquariums of any volume.

Thus, the Einweg 300 Space cylinder system for a 300-liter aquarium includes:

• Replaceable cylinder with 500 g of carbon dioxide.
• Reducer with built-in solenoid valve.
• Check valve.

• Flipper diffuser with false gas removal.
• Connecting hose.
• Indicator and tests.

The set for Nano nano-aquariums is practically identical, but the cylinder capacity is only 80 g, and a reducer of the simplest design is used.

For more complex systems, components can be replaced (e.g., adding a pressure gauge assembly, installing a night shut-off valve, timer, etc.).

The manufacturer's range also includes BIO systems (for example, BIO 60), in which a cylinder with gel and a starter capsule is used as a generator to initiate the process, and a bubble counter is installed.

Eheim

A German manufacturer offers cylinder systems for aquariums of virtually any volume.

The standard set includes:

• A cylinder with liquefied carbon dioxide (for some, for example, 2000 g) with the possibility of refilling. Capacity – from 200 to 2000+ g.
• Pressure reducer (for refillable tanks – with a refill fitting).
• Bubble counter.
• Diffuser.
• Hose.
• Drop checker and tests.

If necessary, the system can be supplemented with electromagnetic and check valves, control devices (timers). Both company components and accessories from other manufacturers are suitable (selection required).

Ista

Ista produces high-quality CO₂ professional-grade systems and accessories.

The basic system includes:

• Aluminum cylinder with a capacity of 1 liter.
• Reducer with 2 pressure gauges, built-in solenoid valve.
• Bubble counter with a check valve.
• Compact diffuser.
• Hose.

If necessary, the system can be supplemented with control devices (timer, gas concentration controller), atomizer (external reactor to improve CO₂).

Aqua

A Russian company offers cylinder systems both as kits (Standard and Standard+, Profi) and complete sets.

The buyer can choose systems based on:

• cylinder containers;
• reducer design;
• diffuser option;
• bubble counter;
• drop checker options, etc.

Frequently asked questions

Why is it not recommended to aerate and supply CO2 at the same time?

Carbon dioxide dissolved in water is easily released, especially during intense movement of water masses. Accordingly, during aeration, its concentration can decrease significantly, which will require high consumption.

Why is it recommended to equip CO2 supply systems with timers?

It is better to supply carbon dioxide to the aquarium during the daytime while the plants are photosynthesizing. At night, it is not necessary and can lead to excessive carbon dioxide concentration and a drop in pH. To prevent this, it is recommended to start supplying it 1 hour before the start of daylight hours and stop 1 hour before the end. A timer-solenoid valve combination works well for this task.

Why do factory BIO generators use gel instead of nutrient solution?

The gel helps stabilize the reaction rate and extend the generator's operating time. The same purpose is served in our own fermentation generators by adding thickeners such as gelatin or starch.

Why are pre-filters used?

In fermentation generators (even serial ones) or chemical generators, harmful substances (acids, alcohol, etc.) can be captured by the gas flow. To prevent them from entering the aquarium, a prefilter is used to purify the gas.

Which reactor is better – active or passive?

The choice should be based not on the type of reactor, but on its ability to completely dissolve the amount of CO2 required for the aquarium. If the reactor can handle this task for the volume of water, there is no point in discussing which type is better.

So, the underwater garden in your aquarium truly comes to life when the plants get enough carbon dioxide. It's like a magic key that triggers the process of photosynthesis, giving them the energy to grow, flourish, and stay healthy. Without it, even with good light and fertilizer, plants can wither.

Fortunately, providing plants with CO2 is not as difficult as it seems. You can choose a simple method, such as liquid carbon fertilizers for small aquariums. Or install a gas cylinder supply system, which is considered the most effective for densely planted grasslands. It is better to start small, gradually observing the reaction of the plants.

The main thing is to remember about balance. Carbon dioxide, light, and nutrients from the soil and water must work together as a team. When changing one thing, it is important to pay attention to the rest. And, of course, safety for the fish is very important, so constant monitoring of CO2 levels using special tests is essential.

Don't be afraid to experiment. Observe your plants; their appearance will tell you if you are doing everything right. Over time, you will find the right balance to turn your aquarium into a lush, vibrant, and harmonious underwater world. Good luck with your creative endeavors!