CO2 for aquariums: what it is, how to use it and alternatives

CO2 for aquariums is a complex topic and only recommended for the most demanding aquarists , since adding CO2 to our aquarium can affect not only our plants (for better or for worse) but also the fish and the overall balance of the ecosystem.

In this article, we'll discuss in detail the uses of CO2 in aquariums , the different types of kits available, how to calculate the amount of CO2 you need, how to measure it manually and automatically, its relationship with algae and oxygen levels, and what alternatives exist to gaseous CO2. Additionally, if you'd like to delve deeper into the topic, we also recommend this article on DIY CO2 systems for aquariums.

What is CO2 for in aquariums

CO2 is one of the most essential elements for planted aquariums , as without it your plants would die or, at the very least, become diseased or stunted. It is a crucial element used in photosynthesis, during which CO2 combines with water and light to allow the plant to produce carbohydrates and energy. As a byproduct, the plant releases oxygen, another essential element for ensuring the survival and health of the aquarium.

In nature, CO2 reaches the water from the atmosphere, from CO2 dissolved in the soil, from the decomposition of organic matter and from respiration itself de pecesbacteria and plants. However, in an artificial environment such as an aquariumThis contribution is usually limited, especially when using powerful lights and seeking dense growth of demanding plants.

In an artificial environment like an aquarium, we have to provide our plants with the nutrients they need , or they won't develop properly. Therefore, CO2, which in nature plants normally obtain from the soil and other decomposing plants, is not naturally abundant in aquariums with high-nutrient-demand plants.

When CO2 is added in a controlled manner, plants can photosynthesize at a much higher rate. This results in faster growth , larger leaves, more intense colors (especially in red plants), and a greater ability to compete against algae by utilizing available nutrients more quickly.

Furthermore, CO2 is directly related to the pH balance in the aquarium. Some of the dissolved CO2 reacts with the water to form carbonic acid, which can slightly lower the pH. A stable CO2 level helps maintain a more consistent pH, which benefits both plants and fish, provided we work within safe ranges.

Another key aspect is their role in the aquarium's carbon and nutrient cycle : plants take in CO2 and, as they grow, incorporate that carbon into their tissues. When you prune or remove old leaves, you're removing carbon and nutrients from the system, helping to keep the water clean.

How do we know if our aquarium will need CO2? As we'll see below, it largely depends on the amount of light the aquarium receives : the more light, the greater the need for CO2 for your plants. In aquariums with moderate light and few plants, natural CO2 can sometimes be sufficient, while in densely planted aquariums with intense lighting, pressurized CO2 becomes almost essential.

Benefits of adding CO2 to a planted aquarium

Beyond being a simple nutrient, CO2 provides a number of direct advantages when properly controlled:

• Accelerated plant growthWith sufficient CO2, plants produce more carbohydrates and energy, resulting in noticeably faster and more vigorous growth, with strong roots and healthy leaves.
• Denser and more aesthetically pleasing plantsThe continuous supply of CO2 promotes denser foliage, tighter carpets and more compact stem masses, allowing for the creation of highly elaborate aquatic landscapes.
• Better coloringMany plants, especially red or orange ones, show much more intense colors when CO2 and light are abundant and nutrients are well balanced.
• Effective competition against algaeBy growing quickly and consuming nutrients and light, plants leave fewer resources available for algae, making it harder for them to take over the aquarium.
• Improving water qualityHealthy plants consume nitrates, phosphates, and other compounds that, in excess, can be harmful, helping to maintain cleaner and more stable water.
• Greater oxygenationDuring photosynthesis, plants release oxygen into the water. An aquarium with good CO2 and light usually shows bubbling in the leaves (pearling), a sign of high oxygen saturation that benefits fish.
• A more natural and stable ecosystemA well-planted aquarium with CO2 provides shelters, spawning areas, and hiding places for fish and invertebrates, creating a more comfortable and less stressful environment for them.

How are CO2 aquarium kits

There are several ways to introduce CO2 into your aquarium water. While there are a couple of simple methods, which we'll discuss later, the most efficient is to use a kit that adds carbon to the water regularly and in a controlled manner.

A well-set-up CO2 system allows you to precisely adjust the amount of gas entering the aquarium, synchronize it with the photoperiod of the lights, and maintain stable levels throughout the day, something your plants and fish greatly appreciate.

Kit contents

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Without a doubt, the most recommended option by aquarists is pressurized CO2 kits , which release this gas regularly. This allows for very precise control over the amount of CO2 entering the aquarium, something your plants and fish will appreciate. These systems typically consist of:

• CO2 bottleIt's precisely that: a bottle containing compressed gas. The larger it is, the longer it will last. When it runs out, it needs to be refilled, for example, with a food-grade CO2 canister. Some stores also offer this refill service.
• Regulator or pressure reducerThe regulator, as its name suggests, is used to regulate the pressure of the CO2 bottle, that is, to lower it so that it is more manageable and you can safely adjust the flow rate.
• Needle valveMany regulators already incorporate it. It allows for very precise adjustment of the bubble flow, something key to fine-tuning the CO2 dose in the aquarium.
• DiffuserThe diffuser breaks down the CO2 bubbles just before they enter the aquarium, creating a very fine mist that distributes them more evenly throughout the water. It's highly recommended that you place this diffuser near the filter's clean water outlet, which will disperse the CO2 throughout the aquarium.
• CO2 resistant tubeThis tube connects the regulator to the diffuser. Although it may not seem important, it actually is, and you can't just use any tube, as you have to make sure that It is resistant to pressure and CO2preventing leaks and material degradation.
• Non-return valveIt is placed in the tube line to prevent aquarium water from reaching the regulator or the bottle in case of pressure loss, thus protecting the entire system.
• SolenoidBesides having a really cool name that shares a title with a novel by Mircea Cartarescu, solenoids are extremely useful little devices, as they are responsible for Close the valve that allows CO2 to flow when there are no more hours of daylight. (Plants don't need CO2 at night because they don't photosynthesize.) They need a timer to operate. Sometimes the solenoids (or the timers for them) aren't included in aquarium CO2 kits, so it's highly recommended that you make sure they are included if you're interested in having one.
• Bubble counterWhile not essential, it allows for much more effective control of the amount of CO2 entering the aquarium, as it counts the bubbles. It's usually filled with water or a specific liquid and provides a visual reference (for example, 1-2 bubbles per second to start with in medium-sized aquariums).
• Drop checkerThis type of bottle, also not included in some kits, checks and indicates the amount of CO2 in your aquarium. Most contain a liquid that changes color depending on whether the concentration is low, correct, or high. Keep in mind that The color change is delayed about 2-3 hours with respect to the actual CO2 level, so it is always advisable to wait before making drastic adjustments.

In addition to these basic components, some aquarists use external CO2 reactors instead of diffusers. These are closed devices through which water and the gas circulate, achieving very efficient dissolution, which is especially useful in large aquariums.

How long does a CO2 bottle for aquariums last?

The truth is, it's difficult to say for sure how long a CO2 bottle lasts , as it depends on the amount you add to the aquarium, as well as the frequency, operating pressure, and tank size. However, a two-liter bottle is generally considered to last between two and five months in a medium-sized aquarium with reasonable consumption.

To get a more detailed idea, you can consider these factors:

• Aquarium volumeLarge aquariums require more CO2 to maintain the same concentration as a small aquarium.
• Vegetable doughThe more stems, ground cover and fast-growing plants you have, the greater the demand for CO2.
• Surface movementA highly agitated surface facilitates gas exchange and causes CO2 to be lost more quickly into the atmosphere.
• Flow rate adjustmentA poorly adjusted system, with too many bubbles per second, can deplete the bottle much sooner than necessary.
• System leaksLoose connections or old tubing can cause constant CO2 leaks that empty the bottle without it reaching the aquarium.

A very useful practice is to note the date you install or refill the bottle and the approximate flow rate you use. This way, over time you can more accurately predict when you'll need to refill it and whether it's worth using a larger capacity bottle.

How to measure the amount of CO2 in the aquarium

The truth is, calculating the percentage of CO2 your aquarium needs isn't easy , as it depends on multiple factors such as pH, water hardness, light, plant density, and surface agitation. Luckily, science and technology are there to save the day once again. Even so, it's helpful to understand the basic methods.

In planted aquariums, ideal concentrations are usually between 20 and 30 ppm (parts per million). In many cases, a target of 25-30 ppm is recommended for high-nutrient aquariums, always monitoring the fish's response. Remember that in water, 1 ppm is approximately equivalent to 1 mg/L.

Manual method

First, we'll show you the manual method for calculating how much CO2 your aquarium needs. Remember that, as we've mentioned, the necessary proportion will depend on several factors , such as the aquarium's capacity, the number of plants you have, the amount of water being processed, and the carbonate hardness.

This method involves calculating CO2 from pH and KH (carbonate hardness) . Tables and formulas exist that, by cross-referencing these two values, indicate the approximate concentration of dissolved CO2.

First, you'll need to measure the pH and carbonate hardness (KH) of the water to determine the percentage of CO2 present in your aquarium. This will tell you the specific CO2 concentration in your aquarium. You can find tests to calculate these values ​​at specialized stores.

Once you have the pH and KH, you can use a specific table or calculator to obtain the approximate ppm. Remember that it is recommended that the CO2 concentration be between 20 and 25 mg/l as a minimum , and can be increased to about 30 mg/l in demanding aquariums as long as the fish are comfortable.

Next, you'll need to adjust the CO2 level as needed in the aquarium water (if applicable, of course). As a general guideline, aim for about 10 CO2 bubbles per minute per 100 liters of water to start with in moderate systems, and gradually adjust based on the plants' response, the color of the drop checker, and the fish's behavior.

Automatic method

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This is undoubtedly the most convenient method for calculating whether the amount of CO2 present in our aquarium is correct or not. For this, we will need a CO2 tester, also called a drop checker , a type of glass bottle (which attaches with a suction cup and is shaped like a bell or bubble) containing a liquid that uses different colors to indicate the amount of CO2 present in the water.

The colors used to indicate the level are usually the same: blue for low , yellow for high , and green for ideal . Green typically corresponds to a concentration of around 20-30 ppm, which is the recommended range for most planted aquariums.

Some of these tests will require you to mix aquarium water into the solution , while others won't and use a fixed KH reference solution. In any case, always follow the manufacturer's instructions to avoid surprises and inaccurate readings.

Keep in mind that the liquid in the drop checker takes approximately 2 to 3 hours to react to a change in the CO2 concentration in the water. Therefore, when adjusting the CO2 flow rate, wait that long before making any further changes, and always make adjustments gradually.

CO2, oxygen and algae control in the aquarium

In a planted aquarium, it is essential to maintain a balance between CO2 and oxygen (O2)Both gases are related and directly affect health. de peces and plants.

• CO2 It is the main fuel for photosynthesis in aquatic plants and is needed above all during the photoperiod, when the lights are on.
• The Oxigen It is essential for breathing de peces, invertebrates, bacteria and from the plants themselves, especially overnight, when there is no more photosynthesis.

When there is a lot of surface agitation, it promotes the entry of oxygen from the atmosphere, but it also accelerates the loss of dissolved CO2. Therefore, in planted aquariums with CO2 injection, it is generally recommended to maintain a slight surface ripple , avoiding excessive turbulence that would increase gas consumption.

During the day, plants consume CO2 and release oxygen. If CO2 levels are adequate and lighting is good, the water can become saturated with oxygen, and you'll see small bubbles rising from the leaves . This is a sign that the system is working properly.

At night, the situation reverses: there is no photosynthesis , the plants stop consuming CO2 and continue respiring, consuming oxygen. This is one of the reasons why the CO2 levels are reduced when the lights are turned off , since the plants don't use it and an excess of CO2 with little oxygen could stress the fish.

Regarding algae, CO2 plays a key role. An adequate supply of CO2, combined with balanced fertilization and well-regulated lighting , promotes the growth of higher plants, which compete with algae for nutrients and light. This makes it difficult for algae to proliferate excessively.

However, CO2 alone is not a magic solution against algae . You also need to consider other factors:

• Control the lighting: avoid excessively long photoperiods or very high intensities without adequate CO2 and nutrient support.
• Maintain a good water change schedule: to remove excess dissolved nutrients and algae spores.
• Pruning and maintaining plants: removing damaged leaves or those with algae so that the plant can concentrate its energy on healthy tissue.
• Manual removal of visible algae: crystals, rocks and decorations, so as not to give them an advantage.

Tips for using CO2 safely in your aquarium

The topic of CO2 in aquariums is quite complex, requiring patience, a good kit, and even a lot of observation . That's why we've prepared a list of tips to keep in mind when venturing into this world:

• Never add a lot of CO2 at once.It's much better to start slowly and gradually increase carbon levels until you reach the desired concentration. A sudden increase can stress or even suffocate the fish.
• Note that, The more the water moves (due to the filter, pumps, or aerators), the more CO2 you will need., since it will dissipate faster than the aquarium water.
• Surely you will have to do several tests with the water in your aquarium until you find the ideal CO2 ratio For this reason, it is highly recommended that you carry out these tests without any fish present, so you avoid putting them in danger and can make adjustments calmly.
• Synchronize the CO2 with the lightsIdeally, the CO2 system should be turned on between half an hour and an hour before the lights come on, so that the water already has enough dissolved CO2 at the start of the photoperiod. You can turn the system off approximately one hour before the lights go out.
• Finally, if you want to save a little CO2, turn off the system an hour before the lights go out or after dark, there will be enough left for your plants and you will not waste it.
• Always keep an eye on the behavior of your fishIf you see them gasping at the surface, breathing rapidly, or very lethargic, it could be a sign of excess CO2 or insufficient oxygen. In that case, reduce the CO2, increase surface agitation, and, if necessary, perform a water change.
• Check for possible leaks in the system From time to time, check connections, tubes, and joints, as a leak can quickly empty the bottle and leave the plants without CO2 overnight.

Is there a substitute for CO2 in aquariums?

As we mentioned earlier, pressurized CO2 kits are the best option for aquarium plants when you're looking for strong and controlled growth. However, because they are somewhat expensive and require some learning, they aren't always the best choice for everyone. Liquids and tablets , as well as DIY CO2 systems, can be used as alternatives or complements.

Liquids (sources of liquid carbon)

The easiest way to add usable carbon to your aquarium is in liquid form . Bottles of this type of product contain carbon-rich organic compounds that plants can use as a carbon source, although they are not exactly gaseous CO2 . They are usually dosed daily or every few days, typically by measuring with the bottle cap or a syringe.

The main advantage of these products is that you don't need to install a CO2 cylinder system, regulators, or diffusers. They can be a good option for small aquariums or those with low to medium CO2 demand, or as a supplement to a gas system.

However, this method isn't entirely safe, as the carbon concentration, although it dissolves in water, sometimes doesn't disperse evenly without good water movement, and some compounds can be harmful to certain fish and invertebrates . Furthermore, some people report that excessive use has been detrimental to their fish or shrimp, so it's always advisable to follow the manufacturer's recommended dosage and start with smaller amounts.

Pills

CO2 or carbon release tablets may also require separate equipment, as they tend to dissolve very quickly if added directly to the aquarium instead of gradually. This releases a large portion of their contents suddenly near the tablet, where plants cannot readily absorb it , and leaves sediment that can remain for days on the bottom or in the filter.

However, there are simpler options where the product is simply added to the water without the need for special enclosures, although it's still possible that it won't dissolve uniformly. Generally, these types of solutions tend to be less efficient and less controllable than a pressurized CO2 system.

If you decide to use them, it is important not to combine several carbon sources without having done prior tests (for example, tablets plus gaseous and liquid CO2 at the same time) to avoid a possible overdosing that affects fish and plants.

Homemade CO2 by fermentation

A very popular and economical alternative to a pressurized system is homemade CO2 , which is based on the fermentation of a mixture of water, sugar, and yeast in a tightly sealed bottle. As the yeast consumes the sugar, it releases CO2, which is then piped to the aquarium.

This method is interesting for small aquariums or for those who want to experiment with CO2 without a large initial investment, but it has some important limitations:

• Unstable productionInitially, the system generates a lot of CO2, and over time, less and less, causing fluctuations that can stress the plants.
• Difficult flow controlIt is not easy to precisely adjust how many bubbles are produced per second.
• Need to renew it frequentlyEvery so often, a new mixture needs to be prepared.

Even so, it is a valid option if it is understood that it does not offer the same degree of control and consistency as a commercial pressurized system.

CO2 for aquariums is a complex topic that requires kits and even some math to find the ideal ratio for healthy plant growth. Tell us, do you have a planted aquarium? What do you do in these cases? Are you more of a fan of homemade CO2 generators, or do you prefer liquid or tablets?

Sources: AquariumGardens , Dennerle.

The use of CO2 in planted aquariums completely transforms how plants grow, compete with algae, and create a stable environment for fish. Understanding its purpose, dosage, equipment needs, and available alternatives will allow you to design a much healthier, more spectacular, and easier-to-maintain aquarium in the long run, always adapting your strategy to your lighting, plants, and desired setup.