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 look at what CO2 gas does for aquarium plants as well as the pros and con of using it.

Photosynthesis is done with CO2.

Did you know that carbon is the backbone to life? That phrase is not just true for animals like you and me, but it’s also true for plants as well. 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. 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. 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

When dissolving CO2 into water (H2O), a small amount of carbonic acid (H2CO3) is formed. This mild acid will lower the pH level of your aquarium water. If the pressurized CO2 is shut off for a long enough period, the pH will begin to increase again as the excess CO2 is forced out of the water. This is why it’s important to use a timing device to make sure that the CO2 injection is turned on when there is light, and off when there is darkness. 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 types of tests use a liquid reagent to determine CO2 levels.

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. 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.

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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

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. 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. This allows plants to thrive and maintain a low pH. The CO2 remains concentrated so that they can grow well. Limestone is mainly composed of calcite, aragonite, and high in carbonates (KH). Limestone is a great buffer because it neutralizes carbonic acid and raises the pH. 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. 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 flowers, just like red carpeting and red plants, thrive in high tech tanks that have strong lighting, high fertilizer doses, 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. 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. Aquarium Co-Op does not sell any aquatic plants that require injected carbon dioxide. We want to make them as accessible as possible. Peruse our collection of hardy, beginner-friendly species to get started with planted tanks today.