Aquarium CO2 System | When to Use It | Advantages | Benefits | Nighttime Shut-Off | Instructions
Buy a CO2 System for Your Aquarium - Advantages & Benefits, Including Instructions for Building Your Own
- 1.What is a CO2 system?
- 1.1CO2 is a nutrient for fish and plants
- 1.2Components of a CO2 System
- 2.3 Benefits of a CO2 System
- 2.1Boost for Plant Growth
- 2.2Preventing Algae Growth
- 2.3Good for the Fish
- 3.Setting Up a CO2 System in Your Aquarium
- 3.1Which size for which aquarium?
- 3.2The Optimal CO2 Concentration in Your Aquarium
- 3.3Determining the CO2 Level in the Aquarium
- 3.4How to Adjust Your Aquarium’s CO2 System
- 3.5CO2 Fertilization for Aquascaping
- 4.Buying a CO₂ system: What to look for
- 4.1Bio CO2 System or Pressurized Gas CO2 System?
- 4.2Pressurized Gas Cylinder: Disposable or Reusable
- 4.3The Right Diffuser
- 4.4Properly Adjusting Your CO2 System
- 4.5Buying a CO2 System as a Complete Set
- 5.Build Your Own Affordable CO2 System
- 5.1Build Your Own Biogas CO2 System: Instructions
- 5.2Make Your Own CO2 Inexpensively
- 5.3Building a CO2 System with a Sodastream Gas Cylinder: Instructions
- 6.Summary
Do you want to transform your aquarium into a lush, plant-filled wonderland while also preventing pesky algae? Then you need a CO2 system. It works very well in freshwater aquariums with lots of plants and in aquascaping. In this guide, we’ll explain exactly how CO2 fertilization works and what you need to keep in mind when using a CO2 system.
What is a CO2 system?
In short, a CO2 system supplies your aquarium with additional CO2, or carbon dioxide. The tiny bubbles dissolve in the water, lower its pH level, and serve as food for the plants. CO2 systems for aquariums come in a wide variety of forms and operate in different ways. Before we look at the differences, however, let’s explain why carbon dioxide is important for keeping your aquarium inhabitants healthy and in a species-appropriate environment.
CO2 is a nutrient for fish and plants
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The explanation of why CO2 is important in an aquarium starts with the term “photosynthesis,” which you’re surely familiar with from biology class. During the day, all green plants use sunlight to convert carbon dioxide from the surrounding air into oxygen, while producing energy in the form of sugar. Carbon dioxide dissolved in the water also enables your aquatic plants to grow. Meanwhile, the oxygen produced as a byproduct of this process benefits the fish and other aquatic life. They also thrive in the slightly acidic pH of the water when the carbon dioxide content is high.
Only a few species of fish and shrimp prefer a pH level above 7. If you keep these species, you should exercise caution when using a CO2 system.
Without added CO₂, the plants in the aquarium rely on the carbon dioxide exhaled by the fish and shrimp in your tank. This allows for normal growth at best. More demanding plants, which generally also require more light, depend on supplemental CO2 fertilization.
By the way, providing your plants with good nutrition not only promotes their growth but also helps prevent the arch-enemy of aquarists: algae blooms. These are significantly less common in tanks with ample CO2 because the well-nourished aquatic plants simply leave no habitat for the algae.
Components of a CO2 System
Many aquariums are sold as complete sets. This saves you a lot of work and research, especially when you’re just getting started in the hobby. However, a CO2 system is usually not included in these complete sets. You’ll need to purchase it separately.
A classic, pressurized gas-powered CO2 system consists of several parts:
- Pressurized gas cylinder: It contains carbon dioxide (similar to a SodaStream cylinder) and is either a disposable product or a refillable cylinder (more on this below).
- Pressure regulator with pressure gauge: This allows you to manually adjust the amount of CO2 and check the cylinder’s fill level.
- Bubble counter: It counts the CO2 bubbles released, thereby controlling the amount of CO2 dispensed.
- Backflow preventer: It protects the system from backflowing water.
- CO2 reactor/diffuser: It dissolves the gaseous CO2 into the water.
- CO2 hose: This prevents carbon dioxide from being released into the air around the aquarium.
One important component isn’t automatically included with every CO2 system: the nighttime shut-off. It ensures that your aquarium plants have less carbon dioxide available at night. The reason: Without sunlight, even aquatic plants adjust their metabolism and no longer convert carbon dioxide into oxygen—but rather the other way around. As a result, the CO2 level in the water could become significantly too high at night, posing a real danger to your fish.
Instead of using a pressurized gas cylinder, you can also generate CO2 for your aquarium “biologically.” Read below to learn how these bio-CO2 systems work.
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3 Benefits of a CO2 System
A CO2 system in your aquarium isn’t a must, but if you want really beautiful, lush plants and your fish prefer a lower pH level, then CO2 fertilization can give your tank a real boost.
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There are three main benefits that make a CO2 system a worthwhile investment for your aquarium—and this investment doesn’t even have to be particularly expensive, as we’ll show you.
Boost for Plant Growth
The difference is impossible to miss and cannot be argued away: With a CO2 system, the plants in your aquarium grow significantly better. The additional carbon dioxide provides a real boost—not only to the speed of growth but also to the overall appearance of the plants. Just a few weeks after setting up the CO2 system, you’ll notice brighter colors, stronger leaves, and denser shoots. In addition, adding CO2 also helps rid your aquatic plants of a very annoying problem: algae.
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Preventing Algae Growth
A CO2 system is an effective and elegant way to prevent algae infestations. These pesky invaders tend to appear when plants lack sufficient nutrients. Insufficient nutrients lead to stunted growth, which in turn allows more light to penetrate the deeper areas of the tank. For algae, this is an opportunity they seize immediately.
Caution: A CO2 system is, of course, not a cure-all for algae. If other parameters in your aquarium are out of balance or if the algae infestation is already severe, CO2 fertilization alone won’t be enough to solve the problem.
Prevention is better than fighting algae—and a CO2 system is a smart preventive measure.
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Good for the Fish
It’s not just your aquatic plants that benefit from more carbon dioxide in the water. Fish and other aquatic life also benefit from the pH level remaining slightly acidic as a result. The vast majority of ornamental fish species thrive at a pH of around 6.5.
Their risk of disease also decreases. The low pH level makes it harder for pathogens (bacteria) to multiply.
However, you need to be careful not to introduce too much CO2 into the water. It’s always advisable to turn off the system at night so that the CO2 level doesn’t rise too high overnight and the fish accidentally suffocate.
Setting Up a CO2 System in Your Aquarium
CO2 systems come in a wide variety of sizes. Whether you use such a system is, in principle, independent of the aquarium’s size—it’s worthwhile for both nano-aquariums and 400-liter tanks.
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In the following sections, we’ll explain how to set up your CO2 system, what the optimal carbon dioxide level is, and how you can actually measure it.
Are you wondering where plants get enough carbon dioxide in nature? In nature, CO2 is usually stored in the soil layer, which contains an abundance of bacteria and microorganisms. An aquarium substrate can’t provide that, so you’ll need a CO2 fertilizer.
Which size for which aquarium?
For smaller aquariums up to about 80 l, we recommend an affordable biogas CO2 system or a small hang-on system. These produce enough CO2 for little money and effort. If your tank is larger, you should opt for a pressurized gas system to generate enough carbon dioxide for the tank’s inhabitants.
Important: Don’t choose a CO2 system that’s too small for your tank. Otherwise, you’ll have to replace or refill the gas cylinder very frequently, which leads to higher costs. It’s less of a problem if the CO2 diffuser is slightly too large for the tank volume. With a 2-liter gas cylinder in a 100-liter tank, you can adjust the CO2 level down accordingly.
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This table clearly shows how large your CO2 system should be in relation to the size of your aquarium.
Table: Which aquarium size for which CO2 system?
| Aquarium Size | CO2 System |
| Nano aquariums (30–50 L) | Biogas |
| Small tanks (50–80 L) | Biogas/hang-on with pressurized gas cylinder |
| Medium tanks (80–100 L) | Pressurized gas system with a 500-ml cylinder (also available as a complete set) |
| Large tanks (120–180 L) | Pressurized gas system with a 2-liter cylinder |
| Very large tanks (200–350 L) | Pressurized gas system with a 2-liter reusable cylinder |
| and nighttime shut-off |
Of course, there are even larger aquariums that hold 400, 500, or even over 700 liters of water. With such large volumes of water, a standard 2-liter CO2 cylinder reaches its limits or has to be replaced so often that it becomes extremely expensive for you.
If you have a tank this large, you should switch to larger cylinders. These are not available as accessories for ready-made CO2 systems. So you’ll need to assemble a CO2 system yourself and install a 6-kg or 10-kg pressurized gas cylinder.
The Optimal CO2 Concentration in Your Aquarium
First off: Normally, a CO2 level of 20 to 25 mg/l is sufficient for healthy plants and fish. However, this guideline alone doesn’t tell you how much CO2 is already dissolved in your aquarium water and how much more you need to add. That depends on various factors:
- The shape of your aquarium
- Water movement (stronger currents break down CO2 more quickly)
- Plant species and number of plants
- Types of aquatic life
- Lighting intensity
Depending on the type and setup of your aquarium, a higher or lower CO2 level may be appropriate.
So, the next step is to determine the carbon dioxide level in your aquarium.
Determining the CO2 Level in the Aquarium
The best way to find out how much carbon dioxide is in the aquarium water is to use a calculation trick. To do this, we assume that the CO2 level is directly proportional to the carbonate hardness and the pH of the water. First, measure the water hardness using an appropriate test, then measure the water’s pH level. The table will then show you the range within which your aquarium’s CO2 level should fall based on these measurements.
The areas marked in red indicate that the CO2 level is too high; numbers marked in blue are too low. The wide green areas allow you to specifically adjust the CO2 level so that the pH value is optimally suited to the needs of the fish.
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How to Adjust Your Aquarium’s CO2 System
Now you know what value to set your CO2 system to. We’ll explain how to do this step by step:
- Assemble the unit.
- Open the valve on the neck of the gas cylinder (pressure regulator/gauge).
- Adjust the valve and the fine-tuning needle valve (modern units only have one valve) and set the pressure to 1 to 2 bar.
To achieve the optimal CO2 level in the tank, use the number of bubbles per minute as a guide—which you can read on the bubble counter. If your aquarium’s CO2 requirement is low, 20 to 30 bubbles per minute are sufficient. If the CO2 requirement is high, it should be up to 60 bubbles per second.
Important: To prevent the CO2 level in the tank from rising too high, you must monitor it continuously. A level of 20 mg CO2/l should not be exceeded over the long term.
The pressure gauges display not only the operating pressure of your CO2 system (1–2 bar) but also the pressure in your gas cylinder. When the cylinder is full, the pressure is approximately 60 bar.
CO2 Fertilization for Aquascaping
Aquascaping tanks contain only plants. Despite this—or perhaps precisely because of it—these elegant, decorative tanks are extremely popular. A beautiful aquascaping tank with densely growing, vibrant aquatic plants practically cannot do without CO2 fertilization. So-called “natural aquascaping” traces its origins to Takeshi Amano, who transforms ordinary fish tanks into imaginative and deceptively realistic underwater landscapes. Whether you want to recreate a gentle hilly landscape, a jungle, or a fantasy landscape in your aquascaping tank is entirely up to you.
In this video, for example, you’ll see an aquascaping tank that recreates a landscape from the movie *Avatar*—complete with a waterfall made of sand.
AVATAR Landscaping aquarium 阿凡達造景水族箱
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However, all aquascaping tanks have one thing in common: they only thrive with a plentiful supply of carbon dioxide. And you don’t have to spend a lot of money to achieve this: a small aquarium of about 50 liters can be perfectly supplied with a biogas system that you can build yourself and operate at virtually no cost. (Read more about this below.)
Without fish in the aquarium (and their waste), the ecosystem lacks important nutrients. In that case, you’ll need to add not only carbon dioxide but also plant fertilizer, such as JBL Ferropol.
Buying a CO₂ system: What to look for
There are numerous CO2 systems for aquariums available on the market. In principle, they all work very similarly and introduce carbon dioxide into the water. However, they differ significantly in their design, the method of carbon dioxide production, and, not least, their ease of use. Last but not least, the cost of a CO2 system varies greatly depending on the type.
Bio CO2 System or Pressurized Gas CO2 System?
The most obvious difference among CO2 systems is the source or method of production of the injected carbon dioxide. There are two options here:
- In biogas systems, carbon dioxide is produced as a result of a chemical reaction, typically through the growth of yeast. The yeast is “activated” with a nutrient solution, begins to multiply, and produces CO2 in the process.
- In pressurized gas systems, the carbon dioxide flows from a gas cartridge or a pressurized gas cylinder, into which it was previously filled and compressed at very high pressure.
These two very different methods each have their own advantages and disadvantages and are suited to different needs. We’ll briefly compare them here so you can quickly decide for yourself which type of system you’d like to use.
| Biogas CO2 System | Pressurized Gas CO2 System | |
| Advantages | Very inexpensive (approx. 30 euros) | Ready for use quickly |
| Can be built entirely by yourself | CO2 output can be precisely controlled | |
| Sufficient for tanks with low CO2 requirements | Also suitable for very large tanks or for use in multiple tanks simultaneously | |
| Nighttime shutdown possible | ||
| Disadvantages | Sufficient only for smaller tanks | Significantly higher initial cost (starting at approx. 100 euros) |
| Limited lifespan; the filter media must be replaced about once a month | High operating costs, especially with disposable cartridges | |
| CO2 output cannot be precisely controlled because the pressure is not constant | Refills must be done externally (pet stores, hardware stores, gas suppliers) | |
| For reusable cylinders, regular TÜV inspections are required | ||
| Larger bottles require | ||
| a lot of space |
You can easily run an aquarium smaller than 150 l with a biogas CO2 system, at least in the initial phase. If your needs and expertise grow, or if your tank is significantly larger, a pressurized gas system is recommended.
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Pressurized Gas Cylinder: Disposable or Reusable
Pressurized gas cylinders or cartridges containing carbon dioxide come in a wide variety of sizes, which you should choose based on the size of your aquarium. One of the most important differences is whether the CO2 cylinder for your system can be refilled or whether you have to throw it away once it’s empty.
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We generally recommend using disposable cylinders. These cylinders are available in very affordable three-packs, and you’ll save yourself the hassle of running around to refill them (a convenient alternative would be to use a standard SodaStream cylinder, which we’ll discuss in more detail below).
Only if you have a very large aquarium or are supplying multiple tanks with a single gas cylinder is it worth the effort to refill a reusable cylinder. In this case, you should use larger cylinders right away; the cost per refill will then decrease proportionally.
For a disposable gas cartridge, you’ll need a different connection for the pressure regulator! Keep this in mind if you’re not using a complete set or if you want to switch from disposable to reusable cartridges later on.
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The Right Diffuser
The diffuser in the CO2 system performs the important task of dissolving the gaseous CO2 that escapes from the bottle into the water. This is the only way the plants can absorb it. The visible bubbles streaming from the CO2 bottle must therefore be made as small as possible.
Depending on the device, this is achieved through several methods:
- Splitting: A mini CO2 diffuser manually breaks down the gas bubbles into as many tiny bubbles as possible by passing the gas through many small ceramic pores or through linden wood.
- Redirection: A “flipper” or high-diffusion reactor extends the path of the gas bubbles, which would otherwise rise directly to the top. A spiral or staircase are the most common designs for this extension.
- Paffrath bowl: Here, the CO2 is held underwater in a large bubble until it gradually dissolves. This bowl is particularly suitable for biogas plants with more irregular output. However, it is less reliable to install and, above all, cannot be installed out of sight.
- Active Reaction: In the active reactor, water is directed downward by a turbine, while the CO2 rises from the bottom to the top, thereby remaining suspended in the center until it has dissolved.
The major advantage of an active reactor is its precise adjustability; furthermore, it can be operated very discreetly even outside the tank. On the other hand, it is another power-consuming device and can be annoyingly loud under certain circumstances. Modern active reactors are built directly into the filter; for example, the JBL CO2 Taifun Inline Diffuser, which is integrated directly into the water return hose of the external filter. This makes the diffuser completely invisible in the aquarium.
We recommend the following diffusers to our customers:
- for small tanks (40–300 L): JBL ProFlora CO2 Taifun Glass Midi
- for external filters: JBL ProFlora CO2 Taifun Inline
- for larger tanks (150 l and up): JBL ProFlora CO2 Taifun Spiral
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Properly Adjusting Your CO2 System
For beginners, it’s not that easy to set the CO2 level correctly. If you don’t want to constantly worry about the number of bubbles and the pressure regulator, a continuous CO2 test is a good idea.
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These tests are also called “Drop Checkers.” They contain a test solution that indicates the carbon dioxide level in the water. At an optimal CO2 level, the test solution turns bright green, and you can see at a glance that everything is fine. Depending on the brand, you may need to replace the liquid from time to time, but this is rarely necessary.
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Even more convenient, though slightly more expensive, are pH controllers. These use a pH electrode to continuously measure the pH level of the water. Depending on the pH value you’ve set on the pH controller (e.g., 7), the solenoid valve for the nighttime shut-off on the CO2 system will open or close until the desired value—for example, 6.5—is reached again.
A pH controller is a technical aid for very demanding aquariums or for very cautious owners. However, a continuous CO2 test is sufficient for the vast majority of needs.
Buying a CO2 System as a Complete Set
Phew, that’s all so complicated! Fortunately, CO2 systems are also available as convenient complete sets. This is a particularly appealing option for beginners. After all, you have all the parts in one place and can’t go wrong. Or
Most complete sets include everything you need to operate the CO2 system, whether it’s a biogas system or a pressurized gas system with a gas cartridge. There are still a few things you should keep in mind when buying a complete set. Often, the very inexpensive complete sets are quite basic, and you’ll have to do without some adjustment options. You should be aware of this so you don’t end up regretting it later.
- Pressure regulator: Ideally, you should be able to adjust the operating pressure yourself. With inexpensive complete sets, this isn’t always possible, but it isn’t necessary at first.
- Pressure gauge: It should display both the working pressure and the residual pressure in the gas cylinder. This is the only way to tell when the CO2 in the cylinder is running low.
- Backflow preventer: It’s important to ensure that no water flows back from the tank into the CO2 system. Therefore, it’s best to use two check valves: one on the tank and one just before the gas cylinder.
- Diffuser or reactor: The various methods for dissolving CO2 in the water differ significantly in terms of effectiveness and space requirements. You’ll need to determine what works best for you and your tank.
- Night Shut-Off: This not only prevents your fish from dying from a CO2 overdose at night (which is quite rare), but more importantly, it reduces CO2 consumption and thus saves you money. A night shut-off (a simple solenoid valve) is included in only a few complete kits.
In general, a complete set is a good idea, especially for beginners. However, as soon as your needs grow or you need to supply CO2 to a very large tank, you’ll have no choice but to build your own setup—if only because of the larger gas cylinder.
This video describes the complete sets from JBL, which you can also purchase in our store.
JBL TV 31: BASIC. ADVANCED. PROFESSIONAL. Die ideale CO2 Düngung für Pflanzen im Aquarium.
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Build Your Own Affordable CO2 System
A CO2 system can be an expensive purchase, and even during ongoing operation, unexpected costs for new gas cylinders can quickly add up. That’s why there are several tips circulating in aquarium enthusiast circles on how to build your own CO2 system on a budget. It’s actually very easy to do and will save you a lot of money.
We’ll show you two ways to build a DIY CO2 system for your aquarium:
- Operating a pressurized gas system with a SodaStream bottle
- Build your own biogas CO2 system
A third, fairly inexpensive alternative is CO2 tablets: Simply drop them into the water as needed and wait until they’ve dissolved. Always make sure the dosage instructions match the size of your aquarium as closely as possible.
Build Your Own Biogas CO2 System: Instructions
A standard biogas CO2 system consists of a bottle into which a yeast tablet and the nutrient solution are added to start the reaction. The other parts are the tubing, through which the escaping carbon dioxide flows to the diffuser, and the diffuser, which dissolves the CO2 into the water. You can easily buy these parts separately and assemble them yourself, or even make them from items you have at home.
Shopping list for your DIY biogas CO2 system:
- a 1-liter PET bottle (preferably a sturdy, reusable bottle)
- a slightly smaller PET bottle to serve as a gas scrubber and bubble counter
- a CO2 check valve
- a CO2 hose 1 to 2 m long
- Silicone (sealant) or pre-made bottle caps with hose connectors
- a diffuser specifically designed for biogas systems (it must be rated for lower pressure)
Our sketch clearly shows how to connect the two bottles using the hose (which you’ll need to cut to the appropriate lengths).
With DIY biogas systems, you have to be very careful that the fermentation mixture doesn’t get into the water! This is where the check valve and the separate gas-scrubbing bottle—which also serves as a bubble counter—come in.
Make Your Own CO2 Inexpensively
The main component of the DIY biogas system is the contents of your CO2 bottle. There are pre-packaged organic components available, but in principle, you have all the necessary ingredients for producing carbon dioxide right at home.
You can use this recipe to make a delicious nutrient solution for your yeast:
Ingredients:
- 200–300 g sugar
- one packet of cake glaze
- a pinch of dry yeast
- a few drops of cooking oil
Preparation:
- Bring the cake glaze to a boil according to the package directions
- Stir the sugar into the cake glaze while it’s still hot until dissolved
- Pour the viscous mixture into a sturdy bottle (only a few centimeters high!)
- Let it set in the refrigerator
- Fill the bottle with lukewarm water to about 3 cm below the rim
- Add the dry yeast
- Add cooking oil to the surface of the water to prevent the fermentation mixture from foaming over
Fermentation should begin within 24 hours at the latest. Depending on the temperature, this fermentation mixture will produce enough carbon dioxide to last 2 to 4 weeks in a 120-liter aquarium.
It’s important that the bottle is sturdy enough for fermentation, as the yeast fermentation process generates heat and pressure. Additionally, you should always pay close attention to cleanliness when preparing the bottles to avoid hindering yeast growth.
Building a CO2 System with a Sodastream Gas Cylinder: Instructions
Yes, a Sodastream bottle for your home carbonator contains the same carbon dioxide as a pressurized CO2 cylinder for your aquarium. The reasons for using a Sodastream bottle to power a CO2 system for your aquarium are obvious:
- You can have Sodastream bottles refilled anywhere.
- Refilling 435-g Sodastream cylinders costs less than refilling 500-g gas cylinders at specialty stores.
- You aren’t dependent on pet stores (which aren’t available in every small town).
- They’re a bit easier to use, since Sodastream bottles have an automatic valve.
Here’s some more good news: In most pressurized CO2 systems, you can easily replace the included cartridge with a standard Sodastream bottle. There are even complete sets available now that are specifically designed to work with a Sodastream bottle.
You can also find adapters for the pressure regulator in stores, which allow you to connect your Sodastream bottle to a CO2 system without any problems. When making your purchase, be sure to consider whether you want to connect the Sodastream bottle to a refillable gas cylinder or a disposable cartridge. Instead of a pressure regulator adapter, you can also buy complete pressure regulators for sparkling water systems, which are available with either one or two pressure gauges.
Not sure which adapter or pressure regulator is right for your CO2 system? We’re happy to advise you on technical questions regarding aquarium setups. You can reach our expert advisors at 03641/376432. Or just stop by our store!
Summary
A CO2 system may not be absolutely necessary for your aquarium. But once you’ve seen how well your aquatic plants thrive with the extra boost of carbon dioxide, you won’t want to do without CO2 fertilization. While pressurized gas systems for large tanks do require a fairly high initial investment, with reusable cylinders and a nighttime shut-off, ongoing operation is quite affordable—and the return on investment is high.
If you’re just starting out as an aquarist or only have a small tank to maintain, an inexpensive biogas CO2 system will fully meet your needs. We’ve explained in our guide which CO2 level is right for your aquarium, how to determine it, and how to keep it as constant as possible with your CO2 system. If you have any further questions about CO2 systems, feel free to reach out to us—we’re always here to support our customers with our extensive expertise.
