CO2 in Planted Aquariums: Pros and Cons To Consider

Photo of author


CO2 in Planted Aquariums: Pros and Cons to Consider

Two types of aquariums are common in the plant tank hobby. One is high-tech tanks that inject carbon dioxide gas (CO2) gas. The other is low tech tanks. CO2 gas is often touted as the magic key for causing plants to grow insanely fast and algae to disappear without a 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”? 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 the most basic requirement of any aquarium, no matter how much CO2 is injected. Low tech tanks use 2-3ppm of the CO2 from animal respiration and surface gas exchange. While some plants are able to use carbonate or bicarbonate compounds (KH), in water, this is more energy-intensive than using CO2 gas. To provide a rich supply of carbon “food”, high-tech aquariums use supplemental CO2 to promote faster growth. When combined with proper lighting and fertilization, CO2 injection can give plants the absolute best chance at thriving and growing 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 will lower the pH level of your aquarium water. After the pressure-controlled CO2 has been turned off for a sufficient time, the pH of the aquarium will rise again as excess CO2 is forced away 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. 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 is inefficient as it can cause excessive CO2 levels and a drastic 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.

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. Algae can thrive in a situation where plants are having trouble surviving and will 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

It may seem odd to use equipment for injecting CO2 gas into aquarium waters, but many of the plants used in the trade are from areas that have high levels of CO2 in their water. At spring heads, where water is pumped from below the Earth’s surface, water can become saturated in CO2. This groundwater type is extremely saturated in CO2 due to its exposure to organic compounds and no surface agitation.

In certain bodies of water with naturally low pH and KH buffer, CO2 is able to freely enter the water at a high rate. This is possible in water that has a subsurface made of silicates. The pH stays low, and the CO2 is concentrated, which allows plants to grow freely. Limestone is mainly composed of calcite, aragonite, and high in carbonates (KH). Limestone greatly buffers the water by neutralizing carbonic acid and raising the pH. Because CO2 is less concentrated in these water bodies, different species have been able to thrive there.

Some plants found in the trade are from partially terrestrial areas, which allows them to access unlimited CO2 in the atmosphere. Although they may not be aquatic species as they grow above water, many of these plants can grow underwater in a CO2-rich environment. This allows them to be enjoyed in high-tech aquariums.

Certain plant, like other red and carpeting plants thrive in high-tech tanks with strong lighting and high fertilizer dosing.

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. Just be prepared to invest the extra cost and effort it takes to maintain a high tech planted aquarium.

We recommend starting with a low-tech, planted aquarium. 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. In fact, the majority of aquatic plants we sell at Aquarium Co-Op do not rely in injected CO2 because we want to make them accessible to as many people as possible. Take a look at our selection of sturdy, beginner-friendly species today to get started with your planted tank.