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. The magic ingredient that can make plants grow exponentially and disappear with no trace is CO2 gas. Let’s talk about what CO2 actually does for aquarium plants, as well as some of the pros and cons of using it.


Photosynthesis is done with CO2.

Did you know that carbon is the backbone to life? This is true not only for animals like us, but also for plants. Plants need carbon to conduct photosynthesis and produce food for themselves. This is a fundamental requirement regardless of whether CO2 gas is added to the aquarium. 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. Although some plants can use the carbonate and bicarbonate compounds in the water to produce carbon for photosynthesis (some plants even have the ability to do so), this takes more energy than 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. 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

A small amount of carbonic acids (H2CO3) forms when CO2 is dissolved into 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. 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. 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 use a liquid to test for CO2 levels.

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

A planted tank will thrive if it receives enough light. However, they will need additional nutrients to compensate for the increased light intensity. If the aquarium’s lighting, nutrients, and CO2 levels are not balanced, it can cause damage to the plants. If plants are struggling to survive, algae is likely to take advantage of the situation and grow out of control. Add CO2 to your aquarium if it is already limited in CO2.


When combined with good fertilizer and lighting, injections can greatly improve plant growth and health. 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. 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.

Some bodies of water have a naturally low pH and KH buffer. CO2 can freely enter water at a high speed in these cases. This is possible in water that has a subsurface made of silicates. The pH remains low and the CO2 stays concentrated, allowing plants to grow abundantly. Limestone, on the other hand, is mostly composed of calcite or aragonite and is high in carbonates. 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.

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. These may not be truly aquatic species since they grow above water, but many are able to grow submerged underwater in a CO2-rich environment so that we can enjoy them in our high tech 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.

CO2 injection is useful for speeding up plant growth, keeping plants that require high lighting, and converting plants from submersed to emersed growth. 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.

A low tech, non-CO2 injected planted aquarium is a good choice for beginners. 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 does not sell any aquatic plants that require injected carbon dioxide. We want to make them as accessible as possible. To get started with planted tanks, browse our collection of easy-to-use species.