Misconception of Liquid Carbon in Aquarium Products

Misconception of Liquid Carbon in Aquarium Products. Myths. Pros and Cons

Carbon dioxide (CO2) is a vital nutrient for aquatic plants. Nonetheless, many aquarists do not want to bother with CO2 systems. Instead, they use so-called “Liquid carbon” as an alternative.

But can liquid carbon truly be considered a viable substitute? Unfortunately, it cannot!

Most aquarium products contain glutaraldehyde in some form. The CO2 dosage from these products, as per instructions, is at least 15-20 times lower than what’s needed for optimal plant growth. Increasing the dosage of glutaraldehyde is not an option because it is very toxic. Actually, this is the main reason why it is often used to control algae.

In this article, I will take a closer look at Liquid carbon and debunk some common myths about it.

Glutaraldehyde in Aquarium Products

In most cases, the main active ingredient of liquid carbon is glutaraldehyde (abbreviated as glutaral) at the most common concentration of about 1-2.5%

Unfortunately, many manufacturers of such products rarely list glutaraldehyde on the packaging, or they simply refer to it by a different name. Nevertheless, in almost all cases, it is indeed glutaraldehyde.

These products are widely used by aquarists to enhance plant growth and control algae. Let’s take a look at some of the most popular brands on the market.

1. Seachem Flourish Excel

Misconception of Liquid Carbon in Aquarium Products. Myths. Pros and Cons Seachem Flourish ExcelThis is one of the most well-known products containing carbon to planted aquariums. They are claiming to use a trade secret compound called “Polycycloglutaracetal”.  

Seachem has not provided any scientific studies (or references) to support their claims, and the term “Polycycloglutaracetal” does not exist in chemistry.

All green plants rely solely on carbon dioxide as their carbon source, with no direct biological pathway for using organic carbon compounds as food. Glutaraldehyde, essentially, breaks down into carbon dioxide, making it a practical solution in this regard.

2. API CO2 Booster

Misconception of Liquid Carbon in Aquarium Products. Myths. Pros and Cons API CO2 BoosterThis is another product designed to provide liquid carbon to planted aquariums.

Unlike some other manufacturers, this company is transparent about its use of glutaraldehyde as the active ingredient. This information is clearly stated in their safety data sheets.

3. Easy-Life EasyCarbo

Misconception of Liquid Carbon in Aquarium Products. Myths. Pros and Cons Easy-Life EasyCarboA Dutch product containing glutaraldehyde is used to supply carbon in planted aquariums and control algae.

It is marketed as an easy and efficient way to supply carbon to aquatic plants, promoting healthier and more robust growth.

4. Aquarium Co-Op Easy Carbon

Misconception of Liquid Carbon in Aquarium Products. Myths. Pros and Cons Aquarium Co-Op Easy CarbonEasy Carbon is designed to be an algae inhibitor as it contains chemical compounds that are known to reduce algae growth within the planted aquarium. In turn, plants often grow at a faster and healthier rate since there is less competition for nutrients and carbon dioxide against algae.

Easy Carbon Contains: 1.5% glutaraldehyde, 0.5% citric acid

Of course, there are also others, such as:

  • AQUADIP Liquid Carbon Plant Fertiliser CO2

What is Glutaraldehyde?

Now, many of you might say, “So what? What’s the problem with this glutaraldehyde? Why all the fuss?”

To understand this, let’s take a look at what this substance was originally intended for.

Glutaraldehyde (chemical formula: C₅H₈O₂) is a disinfectant used in various industrial applications, including medicine, such as in dental practices.

Its applications include:

  • disinfection of medical equipment,
  • water treatment for industrial use,
  • disinfection of industrial facilities,
  • in cleaning products for surface disinfection,
  • as a preservative,
  • for tanning leather,
  • for fixing tissue samples in microscopy,
  • for cross-linking proteins, aimed at increasing mechanical and thermal stability and reducing self-decomposition, etc.

Glutaraldehyde is Toxic

Glutaraldehyde is covered by index number 605-022-00-X of Regulation (EC) № 1272/2008 in Annex VI, part 3, Table 3 (the list of harmonized classification and labeling of hazardous substances) and it is classified, amongst various other hazards, as a respiratory sensitizer.

Of course, the impact of the substance depends on its concentration. Products on the market typically contain solutions with a glutaraldehyde content ranging from 1-2.5%.

According to the CLP Regulation, glutaraldehyde ceases to be classified as a hazardous substance only when its concentration falls below 0.5%.

However, even at lower concentrations, it must still be listed on the label, which is not done by any manufacturer of “liquid CO2” products.

Therefore, if you are using it in your tanks, handle it with care. Glutaraldehyde is a potent irritant and may cause respiratory issues, skin sensitization, and eye damage if proper protective measures are not taken. In case of accidental contact, keep the bottle accessible when seeking medical assistance.

Does Liquid Carbon Equal to Carbon Dioxide (CO2)?

To make it short – no, it does not.

Carbon is a solid substance under normal environmental conditions (at 68°F (or 20°C) and 1 bar of pressure), while carbon dioxide is a gas that only exists in liquid form under high pressure (50 bar).

Therefore, Carbon dioxide can only be sold in gas cylinders under such pressure. As a result, the products in these advertisements cannot contain carbon dioxide in liquid form.

Does Liquid Carbon Contain Carbon Dioxide?

Glutaraldehyde has a molecular formula of C5H8O2. This shows that carbon and oxygen atoms are bonded with hydrogen atoms.

Breaking this bond requires energy. During this breakdown process, not all compounds fully decompose. From one molecule of glutaraldehyde, only 2 molecules of carbon dioxide are released.

Therefore, at the usual dosages used in aquariums, glutaraldehyde is not expected to significantly alter the CO2 content in the water. This can be confirmed through CO2 tests, which demonstrate that any CO2 formed from glutaraldehyde degradation is too minimal to measure and falls below levels necessary for biological plant growth.

In practical terms, solutions containing 1-2.5% glutaraldehyde typically contain 10-25 g/L of glutaraldehyde.

This equates to a CO2 concentration of 35.2-55 mg/L of the original solution. Consequently, a 100 ml bottle could generate between 3.52 and 5.5 mg/L of carbon dioxide. In other words, a 100 ml bottle of such a solution can only produce between 3.52 to 5.5 mg/L of carbon dioxide.

Even more, these numbers must then be divided by the volume of water. So, in practice, this result will be at least 2-3 times lower (1 – 1.8 mg/L)!

Such a low level of CO2 cannot even be detected by a CO2 test.

Note: For comparison, the optimal CO2 concentration in a planted aquarium is 15-30 mg/L. 

Thus, answering the question “Does Liquid Carbon Contain Carbon Dioxide?” I can say – yes, but indirectly and in very small quantities. Liquid CO2 produces so little actual CO2 that it has no real effect.

To optimize plant growth using CO2, you need to maintain a target level of at least 20 mg/L of CO2.

The problem though is that this amount is 10-15 times the dosage that acts as an algicide, 4-6 times the amount that is extremely toxic to invertebrates, and enough to cause the first symptoms of toxic poisoning and suffocation in fish.

Note: At recommended dosages, it will not harm fish.

How does Glutaraldehyde Affect Plants in Aquariums?

Glutaraldehyde is a toxin substance that cross-links (fixates) proteins on the outer surface of the cell membrane, thereby reducing its permeability.

Note: The permeability of the cell membrane is essential for metabolic exchange with the environment and communication between cells, akin to the immune or nervous systems in animals. When this metabolic exchange is blocked, the plant becomes “paralyzed,” and its growth rate decreases. This can range from mild to severe effects, and in some cases, the plant may cease growing altogether, melt, and die.

At lower concentrations, plants may still grow despite their weakened state. However, at a concentration of 5% glutaraldehyde solution and a dose of 1 ml per 50 liters of water per day, plants will exhibit clear signs of damage.

At the same time, by releasing some carbon dioxide and being an effective algae control agent (see below), it also promotes plant growth.

This is a fast-acting agent. Its effectiveness usually reaches its peak within 4-12 hours after application in the aquarium. A total breakdown usually occurs within 48 hours.

Furthermore, it does not accumulate in the bodies of plants, animals, and completely decomposes in water.

Nonetheless, comparative experiments clearly demonstrate that aquarium plants supplied with Liquid carbon develop significantly worse compared to plants provided with CO2 gas under the same conditions.

How does Glutaraldehyde Affect Algae in Aquariums?

Glutaraldehyde affects algae in aquariums primarily by acting as a potent algaecide. It disrupts the cellular processes of algae, leading to their inhibition and eventual death. 

  1. Сell Membrane Disruption. It penetrates the cell membranes of algae. Once inside, it cross-links proteins (similar to how it acts on plants) and disrupts their structure. This interference with cellular proteins leads to cell damage and loss of function.
  2. Inhibition of Photosynthesis. Algae rely on photosynthesis to produce energy. Glutaraldehyde inhibits this process by disrupting chlorophyll synthesis and other essential enzymes involved in photosynthesis. Without sufficient energy production, algae cannot survive.
  3. Algae Growth Suppression. Continuous exposure to glutaraldehyde inhibits the growth and reproduction of algae. It hinders cell division and overall growth, reducing algae populations in the aquarium. 
In aquarium maintenance, glutaraldehyde-based products (Liquid carbon) are commonly used to combat algae outbreaks such as:

What Outweighs – the Pros or the Cons?

Sure, plants may have a nice look after applying liquid carbon. Even in small doses, plants receive additional carbon, which is better than nothing. Undoubtedly, this is true.

Additionally, as an algicide, it initially makes the aquarium look cleaner, adding a few points in its favor.

The problem is that, being a highly aggressive substance, it can also harm plants.

There have also been reports that plants completely stopped forming flower stems after the introduction of this product (liquid CO2). Depending on the concentration, it can notably slow down plants growth or even kill plants with delicate leaves or thin structures, such as:

The plants listed here have shown more frequent and noticeable reactions to the use of the products containing Glutaraldehyde. However, this is not set in stone. It’s important to remember that we all have different conditions and water parameters. We may use varying concentrations and different time intervals.



Algae Contro Toxic
Additional Carbon Source Limited Effectiveness (very low Carbon Source)
Easy to Use Potential Plant Damage
Fast-acting agent  
Does not accumulates  

Does Activated Carbon function in the same way as Liquid Carbon?

No, activated carbon and liquid carbon are not the same things. They serve distinct purposes: activated carbon for water filtration and purification, and liquid carbon as a supplement for aquatic plant growth or as a potent algaecide.

In Conclusion

  1. Glutaraldehyde is toxic and harmful. In Europe, suppliers of any products that do not label hazardous substances are intentionally violating laws.
  2. The CO2 dosage obtained using liquid carbon as instructed is 15-20 times lower than what is necessary for optimal plant growth and development. It is not an effective substitute for CO2 in promoting plant growth and should not be used instead of pressurized CO2.
  3. Liquide carbon (or glutaraldehyde) can help control algae.
  4. Carbon dioxide exists only as a gas; carbon itself does not. The term “Liquid carbon” is absurd. It is merely a marketing trick.

Using Liquide carbon requires careful consideration. Proper dosing and monitoring are key to minimizing potential negative impacts.

Related articles:

  1. CO2 in a Planted Tank Guide
  2. How to Spot Nutrient Deficiencies in Aquatic Plants

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