How to supply carbon dioxide into the aquarium.
Pet store chains offer a large selection of CO2 devices that have proven themselves in practical use. The devices are designed for various aquarium volumes. As a rule, CO2 is stored in high-pressure cylinders, supplied in measured doses through a controlled valve or pressure reducer, and enters the aquarium water through a diffusion device.
There are four types of CO2 supply systems for aquariums: – Cylinder system with refilling of the cylinder with compressed carbon dioxide. One refill is enough for a year or more; – Fermentation method from a bottle with yeast. Requires constant attention, and it is unknown how long one refill will last; – Electrolysis method using a Carbo-Plus device; – Chemical methods – various reactions producing CO2.
The cylinder equipment includes a pressure gauge and a pressure-reducing valve. Anyone who wants to adjust the CO2 supply with special precision can do this using a long-term controller: it constantly regulates and automatically sets the pH level through precise CO2 dosing. Another component of this system (used in combination with continuous control) can be a solenoid valve, which can be used (via a timer) to suspend the CO2 supply at night when plant assimilation processes stop.
Gas from the cylinder exits through a pressure reducer equipped with a needle valve, allowing precise control of its consumption. Then, through tubes, it enters the diffuser, where it comes into contact with water. 
Some diffusers are a plastic spiral or labyrinth, moving along which a small gas bubble travels a multi-meter path and dissolves. Others are made in the form of a bell, with gas in the upper part. There are also internal filters containing built-in diffusers. If there is a current in the aquarium generated by the filter, this will help evenly distribute the carbon dioxide-enriched water.
In an attempt to create a practical yet inexpensive device, alternative sources of carbon dioxide were developed. In these, gas is released when a carbon plate is oxidized underwater. Such devices look very attractive; unlike bulky cylinders, they are compact and consist of a plastic housing containing a transformer, a carbon plate, and electrodes connected to it. The cost of this device is also low, but unfortunately, it does not justify itself in operation. Within a few weeks, the carbon and electrodes become covered with a layer of insoluble salts, and the gas release process stops. If you replace the carbon plates with new ones, ultimately, the operating costs of this inexpensive device will exceed the cost of a setup with cylinders and regulators.
An installation for fertilizing aquarium plants consists of a generator and a reactor. CO2 is produced in the generator, and the reactor is intended specifically for saturating aquarium water with it. A more advanced reactor is in the form of a flat box placed in the water with its open side down. Carbon dioxide is fed through a hose either directly into the box or through an atomizer into the water beneath it. In the latter case, the box accumulates that portion of CO2 that did not have time to dissolve while rising to the surface. The carbon dioxide dissolution process can be intensified by organizing an additional water current along the open part of the box.
If fish begin to show signs of oxygen starvation by morning, the system can be turned off at night manually or automatically, or a regular air pump can be turned on at night instead.
If you are unsure whether you need a carbon dioxide cylinder or not, you can practice with a device from Tetra called CO2-Optimat. This is 
a very simple device, but it is quite effective and has a number of advantages. CO2-Optimat consists of an aerosol can with carbon dioxide, a hose, and a reactor in the form of an inverted cup with a semi-permeable membrane instead of a bottom. The cup is attached with suction cups inside the aquarium, somewhere near the filter outlet. One cylinder contains 50 portions of CO2, and the entire device is designed for use in a 100-liter aquarium. In reality, in Moscow medium-hardness water, if you do not get carried away with elevated water temperatures, one cylinder handles a 100-180 liter aquarium for a couple of months. Every morning you need to press the button on the cylinder and completely fill the cup with carbon dioxide.
CO2 dissolution from the cup occurs in two ways: from above, through the membrane, and from below, by diffusion. After some time, the membrane clogs and becomes overgrown with mucus and algae. It has to be regularly cleaned and rinsed, and replaced with a new one after a while. If you position the cup so that a strong current of water from the filter passes near its lower part, dissolution by diffusion from below will be much more efficient.
In addition, you can fertilize plants with CO2 using special tablets placed in the aquarium in a special device. They gradually release carbon dioxide into the water. A very interesting 
system was recently introduced by Sera – the CO2-start kit. CO2 is released from a special tablet. The tablet is dropped into a special slot, gets into the right compartment, begins to bubble actively, and as a result releases about 100 cm3 of CO2. The trick is also that the tablet, in addition to gas, contains micronutrients necessary for plants, so in one fell swoop you not only saturate the water with carbon dioxide, but also provide micronutrient fertilization for plants.
The kit includes 20 tablets, which is enough for 2 months in a 60-80 liter aquarium; one tablet is enough for 3-4 days. With a larger aquarium volume, tablets must be thrown in more often, while the maximum size is limited to 150-170 liters. This is because tablets would need to be added too frequently in a larger aquarium, which causes an excess of micronutrients.
Of course, a high-pressure cylinder with a pressure-reducing valve, a carbon dioxide dissolution system via special reactors, and an electronic pH meter controlling the CO2 supply valve is the most radical solution.
However, such a system, despite all its advantages, has a number of serious drawbacks in everyday use. Let’s note the advantages and disadvantages of such devices:
Advantages: • One cylinder refill lasts for a year or more; • The amount of CO2 can be easily controlled, for example, by a pH controller; • You can adjust the CO2 supply or turn it off at night.
Disadvantages: • Bulkiness of equipment and connections. When using a single cylinder with multiple aquariums, you have to lay quite a few hoses, install tees, adapters, 
a separate pH meter and shut-off valve for each aquarium, etc.; • Danger when used in a living space. Depurssurization of the cylinder can lead to the saddest consequences; • If, due to a valve or pressure reducer malfunction, CO2 begins to be released uncontrollably from the vessel, this can lead to disastrous consequences. The cylinder may be in good working order, but during transportation, if the car collides with an obstacle, it may be damaged. If this happens in a confined space, such as a car interior, the carbon dioxide will instantly fill the interior, the windows will be covered with “snow”, which can cause a person to die;
• In addition, the carbon dioxide valve works well with gas, but not with liquid. Yet there is liquid carbon dioxide inside the vessel. Therefore, both during transportation and during use, the cylinder must stand vertically so that the liquid does not touch the outlet opening. But most importantly: when filling the vessel, demand its integrity test; • Do not turn or tilt the cylinder, as liquid CO2 will enter the pressure reducer in this case; • High cost. Setting up such a system at home for 2-3 medium aquariums is impractical; • Need for periodic recharging or replacement of the cylinder.
Empty cylinders are filled in pet stores or exchanged for new ones. At filling stations, carbon dioxide is poured by weight. Approximately halfway. The pressure gauge on the reducer cannot tell you anything about how full the cylinder is, because the pressure will remain constant as long as there is at least a drop of liquid phase in the cylinder, and then the pressure will begin to drop sharply. However, you should pay attention to the quality of carbon dioxide. Often it contains impurities of oxygen and nitrogen, which dissolve much worse in water and create huge bubbles in diffusers, thereby reducing their efficiency.
Some rules on how to correctly and, most importantly, safely refill a cylinder. Immediately before filling (at the filling station), you need to weigh the empty cylinder. Immediately after filling (right there at the station), weigh the filled cylinder. The weight should increase by 0.7 kg for each liter of volume.
That is: • one-liter +0.7 kg; • two-liter +1.4 kg; • three-liter +2.1 kg; • five-liter +3.5 kg; • eight-liter +5.6 kg; • ten-liter +7.0 kg.
If the weight of the cylinder has increased by a larger number, you need to urgently bleed the gas down to the required weight. If the weight of the cylinder has increased by a smaller number, you were shortchanged on carbon dioxide.
To be safe, you can take the pressure reducer with you. Immediately after filling, you need to screw the reducer onto the cylinder, close the valve on the reducer (usually by unscrewing it), and smoothly and fully open the valve on the cylinder. We see the pressure on the gauge. It should be 55-65 kg/cm² at room temperature. If more, we bleed the gas. You can transport the cylinder in any position. If you took the reducer with you, you can monitor the pressure. If the pressure rises (which can happen when heated from a car heater, for example), it is advisable to bleed it. These rules are especially relevant when dangerous situations can arise due to temperature drops. For example, in summer, you filled a cylinder in a cool room – you put the cylinder in a sun-baked trunk. Or you filled the cylinder in winter at a temperature close to zero, brought it home to a warm room, and also placed it near a radiator.
Let’s also note the advantages and disadvantages of the yeast fermentation device:
Advantages: • It is inexpensive; • A fairly simple CO2 supply option, accessible even to a beginner;
Disadvantages: • Short period between “refills”, one refill lasts about 1 to 4 weeks; • Unstable fermentation. At first, you can easily get an excess of CO2, and then production drops sharply; • 24/7 operation and difficulty in controlling the situation; • This method requires constant attention, because plants get used to CO2 fertilization quite quickly, and when the system runs out, plants weaken and algae starts to grow.
It is also worth noting that carbon dioxide often enters the water in the form of large bubbles that rise to the surface without ever dissolving. There is practically no use from such CO2, so we face the question of dissolving carbon dioxide in water. Devices through which carbon dioxide enters the water deserve a separate discussion. Installations must be efficient, inconspicuous, and inexpensive. Today you can find three types of such devices:
1. Spiral. A gas bubble is fed into a long glass spiral and, slowly rising along its coils, gradually dissolves in water. This is one of the oldest models of such 
a device; it is best used in installations without automatic control. You should adjust the gas supply empirically so that the bubble does not reach the water surface, but completely dissolves along its path. There are some modifications of this device on sale; sometimes the spiral is flat, but the principle of operation is always the same.
2. Diffuser. It is a bell filled with gas. CO2 is supplied from above, and the lower part of the bell opens into the water; at this boundary, the gas penetrates into the aquarium. You can find installation models whose opening is covered with a semi-permeable membrane or mesh. Experience has shown that such devices have no advantages over open diffusers, because the mesh slows down carbon dioxide absorption and clogs quickly. Such devices are cheap and convenient for use in small water bodies.
3. Reactor. Spraying carbon dioxide bubbles directly into the aquarium would be wasteful, but mixing them with water in a small volume is a good idea. The reactor is a transparent plastic cup, to the upper part of which a water stream from a pump and a hose with carbon dioxide are connected. The water flow rate is selected so that bubbles do not escape from the installation. Initially, the lower part of the reactor was closed and gas was released only through a small opening, then the device was blocked across the entire cross-section with a foam rubber plug. Water passes through the foam freely, and gas bubbles cannot penetrate through it. Reactors easily connect to a pH controller – this is an ideal option for professional devices. Carbon dioxide reactors are installed in large water bodies where a diffuser cannot create the necessary CO2 concentration in the water.
Carbonator – a device for enriching water with carbon dioxide.
The carbonator supplies pure carbon dioxide for your aquarium plants. It is designed to fertilize aquarium plants with carbon dioxide, without which it is impossible to grow 
beautiful and healthy plants. CO2 is needed by plants for glucose synthesis, energy, growth, and development. It can operate in aquariums up to 250 liters in volume, introducing about 1 gram of pure CO2 into the aquarium daily.
Compared to other CO2 systems, it has the following advantages: • It has no wires, cylinders, pressure reducers, or hoses; • It produces as much carbon dioxide as the aquarium water can dissolve; • Easy to maintain and safe; • One carbonator refill is enough for a month of continuous operation in a densely planted aquarium of about 200 liters at 25°C; • The kit includes two sets of consumables right away.
Principle of operation. The carbonator works on the principle of successfully functioning oxidizers. The catalyst displaces citric acid solution from a plastic container. Drops of acid fall into a solution of carbonic acid salt, resulting in the release of pure carbon dioxide. Carbon dioxide collects in a “diving bell” and gradually dissolves in the water, from where aquatic plants absorb it. The cessation of the carbonator’s operation can be determined visually: the plastic container is empty, and there is no carbon dioxide bubble under the “diving bell”.
The carbonator is designed so that only pure carbon dioxide enters the water. The citric acid salt solution resulting from the operation is safe and can be poured down the drain.
A stainless steel ball is placed in the lower part of the cup. Substances will react on its surface. The ball will prevent the carbonator from floating up and protects the reacting substances from reaction products. Aquarium water is poured into the upper plastic box using the pipette included in the kit. The box contains citric acid, a hydroperite tablet, and a ceramic catalyst. Pierce the safety valve and, after adding it to the water, shake the mixture. A plastic membrane with a hole is put on the neck of the box. The contents of one package of soda from the carbonator kit are poured onto the ball. The remaining water from the pipette is poured onto the ball.
We secure the box above the ball with the membrane facing down. We tighten the mushroom cap of the device. We lower the device vertically down and install it in the substrate. After 5-10 minutes, a drop of citric acid falls into the soda solution. A violent reaction takes place on the ball. The resulting carbon dioxide collects under the mushroom cap. Light air is displaced, and only a CO2 bubble will be under the cap. Now carbon dioxide will begin to dissolve in the required amount. An empty citric acid box serves as a signal to replace consumables.
Some aquurists use citric acid and a hydroperite tablet instead of store-bought reagents, and pour regular baking soda into the carbonator flask. Proportion: 50 grams of citric acid, 1 hydroperite tablet, 2 tablespoons of soda. This is enough for two to three weeks of operation.
DIY CO2 supply systems.
