CO2 in Planted Aquariums: Pros and Cons To Consider

CO2 in Planted Aquariums: Pros and Cons to Consider

In the planted tank hobby, you may have heard of two types of aquariums: high tech tanks that inject carbon dioxide (CO2) gas and low tech tanks that 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 look at what CO2 gas does for aquarium plants as well as the pros and con of using it.


CO2 is Used for Photosynthesis

Have you ever heard the saying “carbon is the backbone of life”? It’s not only true for us animals, but also plants. Carbon is essential for plants to produce food and conduct photosynthesis. This is the most basic requirement of any aquarium, no matter how much CO2 is injected. Plants use the 2-3 ppm (parts/million) of CO2 found in low-tech tanks. This is a result of surface gas exchange and animal respiratory. While some plants are able to use carbonate or bicarbonate compounds (KH), in water, this is more energy-intensive than using CO2 gas. In a high tech aquarium, supplemental CO2 is diffused into the aquarium to provide an abundance of carbon “food” for plants and encourage faster growth. With proper lighting and fertilization, CO2 injection will give plants the best chance of thriving 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 can lower 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. Plants use light to create oxygen and CO2 for photosynthetic purposes. At night when there is no light available and plants are not able to photosynthesize, they consume oxygen and release CO2 as part of the respiration process. In a planted tank with fish and invertebrates, the animals also emit CO2 as they breathe. Therefore, injecting CO2 at nighttime is inefficient and can potentially lead to excessive levels of CO2 that cause a dramatic drop in 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.

The color in this CO2 drop checker reacts to the pH of the aquarium water, which helps to track the amount of CO2.

CO2 Helps Limit Excessive Algae Growth

Plants will grow and thrive in more light. But they will also need more nutrients to match the intensity of 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. Algae will thrive on plants that aren’t able to survive and may grow out of control. If the CO2 in your aquarium is low, you can add CO2.


When combined with good fertilizer and lighting, injections can greatly improve plant growth and health. Algae can only thrive when plants have all the nutrients they need.

How CO2 Enters Water in Nature

Although it might seem strange to use equipment to inject carbon dioxide gas into aquarium water, many aquatic plants traded in the trade come from areas where the water is very rich in CO2. Spring water can be saturated with CO2 when it rises from below Earth’s crust. Groundwater that has not been subject to any surface agitation is highly saturated with CO2.

In certain bodies of water with naturally low pH and KH buffer, CO2 is able to freely enter the water at a high rate. 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. This allows plants to thrive and maintain a low pH. The CO2 remains concentrated so that they can grow well. 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. These bodies of water have lower concentrations of CO2, so many species of plants have evolved to thrive in them.

Others plants in this trade come from places where their natural environment has been partially terrestrial. This allows the plants unlimited access to CO2 in 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 plant, like other red and carpeting plants thrive in high-tech tanks with strong lighting and high fertilizer dosing.

CO2 injection is useful for speeding up plant growth, keeping plants that require high lighting, and converting plants from submersed to emersed growth. A densely planted aquarium is more beneficial than one with less carbon. You should be ready to put in the effort and extra expense to maintain a high-tech, planted aquarium.

We recommend starting with a low-tech, planted aquarium. Low tech tanks can be more affordable and are easier to maintain. This is particularly important as you learn how to keep your 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. To get started with planted tanks, browse our collection of easy-to-use species.