How to use liquid Carbon in your planted aquarium

One of the most asked aquascaping questions entered into google these days is 'How to use CO2 in aquarium'. This is because it is widely accepted that Carbon dioxide (CO2) is vital to the success of most plants. Their need to photosynthesize and create energy relies on having a steady carbon source and intense lighting. In the planted aquarium, we must provide Carbon in place of nature. Aquascapers and planted tank enthusiasts install pressurized CO2 gas systems when running high-tech tanks with many aquatic plants. This effectively turbo-boosts plant growth. However, only some people are willing to invest in a high-pressure system, especially if they are on a budget or need more space around the aquarium. Many aquarium companies promote the idea that liquid Carbon is a good substitute for CO2 gas. However, many hobbyists disagree and instead use this method of adding Carbon as a chemical treatment for removing unwanted algae. We hope this article explains the topic and gives you an idea of whether liquid co2 suits your aquarium.

What is Liquid Carbon?

Liquid carbon dioxide, or liquid CO2 as it is often miscalled, is sold by many aquarium companies as a plant fertilizer. While it can promote plant growth, it does this indirectly while primarily focusing on removing algae. When fewer algae compete for nutrients, light, and carbon dioxide, aquatic plants can access all of the above elements and ultimately grow faster and healthier. This is due to a chemical called glutaraldehyde, which is a known algae growth inhibitor. 

Activated carbon is added to aquarium filters and is an effective method of removing pollutants from the aquarium water.

Is activated Carbon the same as liquid Carbon?

Although their names sound similar, their uses within an aquarium are entirely different. The purpose of activated Carbon in planted tank settings primarily involves chemical filtration and removing medicines and pollutants. Activated Carbon is generally used within the filter media and traps medications, tannins, and other contaminants from the water column.

Can we use liquid Carbon instead of pressurized CO2?

Liquid Carbon is sold as a 'source of Carbon'. However, most aquatic plant experts agree it's no match for pressurised co2 injection in a planted tank. Carbon dioxide can only exist in liquid form while stored in high pressure containers such as fire extinguishers. It is common for aquatic plants to be grown terrestrially with Co2 in abundance. In nature, many 'aquatic' plants spend long periods out of the water, only becoming fully submerged during rainy seasons. Therefore, when we add these plants to our planted aquariums, we must provide higher carbon dioxide levels for them to thrive. 

Liquid Carbon cannot provide these levels of co2 in aquarium water. This is why most aquascapers use high-pressure co2 systems to provide the optimum conditions. However, liquid co2 cannot directly deliver the concentrations required to increase plant growth. 

Some plants, such as Vallisneria, cannot rely on only liquid Carbon. There needs to be more than this source of Carbon, especially within high-energy setups. If you want fast growth, there's no better way of adding Carbon than via CO2 injection. For some, the ultimate is to see their tanks full of oxygen bubbles or pearling. This results from saturating the water with oxygen via photosynthesis. For this, few things compare to pressurized CO2 injection.

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Co2 injection is controlled with a regulator. The rate of flow can be increased or decreased using a needle valve on the regulator itself and then broken down into a fine mist using a diffuser. High pressure co2 injection systems are effective on both large and small tanks.

Liquid Carbon does not cause plant growth despite what the label may suggest. The main chemical compound found in these commercial products is Glutaraldehyde which is actually a biocide. This is why it is more effective as a treatment for algae. By actively reducing the algae levels, more nutrients become available for the aquatic plants to access. When this happens, plants show signs of more stable growth, which is why many people consider liquid carbon products helpful in planted tanks. Care must be taken when handling this chemical and we advise using gloves while dosing your aquarium. The chemical Glutaraldehyde is commonly used in small amounts by hospitals to kill germs. It is also a known neurotoxin even in low concentration.

Explore the differences between liquid Carbon and pressurised co2 in the following article Does liquid co2 replace pressurized co2 gas?

How to Use Liquid CO2 in planted aquariums.

Liquid Carbon is best used for killing algae. When used in this way, plants can grow well enough to outcompete otherwise challenging issues such as black beard algae. Depending on the size of your aquarium, measure out the required amount into the small measuring cup provided. Liquid carbon can also be used to kill algae by applying directly onto affected areas. We will explain that method a little more later.

How often should liquid Carbon be added to the aquarium? 

Dosing liquid Carbon is easy but we must add the required amount according to the instructions to avoid harming any fish. This is usually every two or three days; occasionally, daily application is needed in densely planted aquariums or fishtanks with strong lighting.

Liquid Carbon can also be used to spot-treat extra stubborn black beard algae. To do this, turn your filter off and make sure there is no water movement. Then, using a miniature pipette, carefully apply the liquid co2 directly to a few affected areas. Again, only test a small area to avoid overdosing on your aquarium. Once applied, power everything back up and observe the reaction within 7 days. The algae will begin to show signs of treatment by changing colour. If this is the case, you can treat small areas weekly until they are removed.

Are there any side effects from using liquid Carbon in the aquarium water?

In some cases, liquid Carbon can cause algae particles to float around in the water column. Suppose you don't have regular water changes and improved filtration techniques for removing these dead cells from the ecosystem in a way that doesn't harm beneficial bacteria. In that case, the result will be cloudy water.

Outside of the aquarium, there are potential risks to the user when handling liquid carbon. Thanks to the chemical compound Glutaraldehyde and its potentially toxic effect, prolonged use of such products do pose health risks. Caution should be taken to avoid contact with skin and eyes etc.

Need help getting the results you hoped for?

There are some types of algae that, despite the best treatment, will prevail. This is generally caused by an unbalanced ecosystem. Lighting and/or nutrients not in the correct levels will cause adverse effects in any aquarium by reducing your lighting period (or completely switching off lights for two weeks in severe cases of algae).

Sometimes aquarium plants lack nutrients which weakens them, allowing algae to gain an advantage. Ensure adequate nutrient sources for your aquarium plants to avoid this.

Can liquid Carbon be dangerous to fishkeepers?

The chemical compound glutaraldehyde is potentially dangerous, and care should be taken not to come into direct contact with it while dosing your aquarium. Be careful not to get any on your hands, and avoid contact with your eyes or mouth. If an accident happens, ensure the bottle is handy when contacting a medical professional. 

Can liquid Carbon damage your plant life?

Sadly, some plants do not like liquid Carbon in aquarium water. Vallisneria is probably the worst for this. When liquid Co2 is in the water, these plants will melt away. If you keep such a plant, consider using half the recommended dose. 

If it is harmful to humans, what about our aquarium livestock?

When used correctly, most liquid Carbon does not present any danger to your fish, shrimp or snails. Always read the label carefully before using any chemical-based product. Ensure you understand the dosing and measure the dose carefully.

How to tell if your aquarium has toxic levels of Carbon.

Relying on your fish to alert you of high levels of CO2 in your aquarium can be very tricky, especially for lesser experienced fish keepers. This is because different species of fish have different CO2 tolerances.

For example, bottom-dwelling fish will usually be the first to display a change in their behaviour. This is generally indicated by frequent visits to the top of the aquarium for gulps of air. This is because excess Co2 creates an oxygen deficit. 

To make things more complicated, some fish become very lethargic, remaining at the bottom of the aquarium for long periods. So understanding all your fish's normal behaviours is absolutely essential when it comes to their health. Luckily there are more straightforward methods for monitoring your co2 levels.

CO2 Drop Checker Kit

CO2 Drop Checker Kit!


Most planted tank setups will already be using a drop checker. An indication that your CO2 levels are increasing would be a change in the colour of the drop checker solution. If the drop checker turns yellow, this is a sure sign that you need to reduce your co2 levels. Drop checkers are trusted by aquascapers across the world.

Learn about drop checkers in our dedicated article 7 Things you Need to Know About Drop Checkers


Using both liquid Carbon and pressurized CO2 has many pros and cons. Everything depends on the hobbyist and the budget. One thing is certain: carbon dioxide is vital for aquatic plants and it's worth supplementing your plants.

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