CO2 in Planted Aquariums: Pros and Cons To Consider


CO2 in Planted Aquariums: Pros and Cons to Consider

You may have heard of two types in the planted tank hobby: high tech tanks that emit carbon dioxide (CO2) gas, and low tech tanks which do not. Many believe that CO2 gas is the key to allowing plants to grow at an incredible rate and causing algae to vanish without trace. Let’s discuss what CO2 does for aquarium plants and the pros and cons to using it.

Photosynthesis is done with CO2.

Are you familiar with the expression “carbon is life’s backbone”? This is true not only for animals like us, but also for plants. Carbon is essential for plants to produce food and conduct photosynthesis. This is a fundamental requirement regardless of whether CO2 gas is added to the aquarium. In a low tech tank, plants utilize the 2-3 ppm (parts per million) of CO2 that naturally comes from surface gas exchange and animal respiration. While some plants are able to use carbonate or bicarbonate compounds (KH), in water, this is more energy-intensive than using CO2 gas. A high-tech aquarium uses supplemental CO2 to increase the plants’ growth and provide them with plenty of carbon food. Combining proper lighting and fertilization can help plants thrive and grow quickly in an aquarium.

If aquarium plants have enough CO2 to photosynthesise, they can produce so many oxygen-rich water bubbles that leaves start “pearling”.

CO2 Lowers pH

The small amount (H2CO3) of carbonic acid is created when you dissolve CO2 in water (H2O). This mild acid has the effect of lowering the pH of your aquarium water. The pH of your aquarium water will increase if the CO2 pressure is turned off for long enough. This is because the CO2 is forced from the water. This is why it is important that you use a timer to ensure that CO2 injection is only run when lights are on and not when the tank is dark. When the plants receive light, they consume CO2 to photosynthesize and create oxygen. When there isn’t enough light, and the plants can’t photosynthesize at night, they use oxygen to produce CO2 and then release it as part of their respiration process. In a planted tank with fish and invertebrates, the animals also emit CO2 as they breathe. Injecting CO2 at night can be inefficient, and could lead to a drastic drop of pH.

CO2 Can Affect Fish Health

Some fish species (such as those from certain parts of the Amazon basin) prefer more acidic water, so adding CO2 is one way to help lower the pH slightly when needed. However, too much CO2 can be detrimental in the fish keeping hobby. Excessive amounts of CO2 in aquarium water can cause fish to gasp at the surface or ultimately suffocate if the problem is not corrected. If you suspect that your fish tank has an overdose of CO2, increased aeration using an air stone can help alleviate this problem. A CO2 indicator or CO2 test kit can help you measure how much CO2 is in the water and determine if your fish are in danger. Both tests require a liquid reagent in order to measure CO2.

This CO2 drop-checker color reacts to the pH level of your aquarium water. This helps you track the CO2 levels.

CO2 Helps Limit Excessive Algae Growth

The more light you give a planted tank, the more plants have the ability to grow and thrive, but they will also require additional nutrients to match the intensity of the light. The aquarium may not be balanced if the light, nutrients and CO2 levels aren’t in line. This can lead to poor plant health. If plants are struggling to survive, algae is likely to take advantage of the situation and grow out of control. If the CO2 in your aquarium is low, you can add CO2.


injection can improve plant health and growth tremendously when combined with appropriate lighting and good fertilizer. Plants that thrive with all the elements they require are more likely to be able to compete with algae for nutrients and light.

How CO2 Enters Water in Nature

While it may seem unnatural to use equipment to inject CO2 gas into aquarium water, many of the aquatic plants in the trade originate from places where water is quite rich in CO2 naturally. Spring water can be saturated with CO2 when it rises from below Earth’s crust. This groundwater type is extremely saturated in CO2 due to its exposure to organic compounds and no surface agitation.

Certain water bodies with a low pH or KH buffer allow CO2 to enter at high rates. For instance, this can happen in water with a subsurface of sandstone that is made of silicates because no carbonates are present to neutralize the carbonic acid from CO2. The pH stays low, and the CO2 is concentrated, which allows plants to grow freely. On the other hand, limestone is mostly made of calcite and aragonite, which is high in carbonates (KH). Limestone buffers water by increasing pH and neutralizing carbonic acid. CO2 is less highly concentrated in these bodies of water, so different species of plants have developed to grow in these conditions.

Other plants in the trade originate from areas where their natural environment is partially terrestrial, allowing the plants to gain access to unlimited CO2 in the air. While these may not be true aquatic species because they grow above the water line, many can survive submerged in CO2-rich environments so we can enjoy them in our aquariums.

Certain plants, like many red plants and carpeting plants, thrive the best in high tech tanks with strong lighting, high fertilizer dosing, and CO2 injection.

The use of CO2 injection can be used to speed up plant growth, keep plants that need high lighting and convert plants from submerged to emersed. It also makes more sense to add to a densely planted aquarium than a sparsely planted one that doesn’t use as much carbon. Be prepared to spend the extra effort and cost to maintain a high-tech planted aquarium.

For beginners, we highly recommend that you start with a low tech planted aquarium without CO2 injection. Low tech tanks are generally cheaper and easier to maintain. This is especially important for beginners who are learning how to keep aquatic plants alive underwater. Aquarium Co-Op sells a majority of the aquatic plants that do not require injected CO2 as we want them to be accessible to as many people possible. Peruse our collection of hardy, beginner-friendly species to get started with planted tanks today.