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 …

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. 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”? That phrase is not just true for animals like you and me, but it’s also true for plants as well. Plants need carbon to conduct photosynthesis and produce food for themselves. This is the most basic requirement of any aquarium, no matter how much CO2 is injected. 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. Some plants are even able to use carbonate and bicarbonate compounds (KH) within the water as a source of carbon for photosynthesis, but this requires more energy from the plant compared to using CO2 gas. A high-tech aquarium uses supplemental CO2 to increase the plants’ growth and provide them with plenty of carbon food. When combined with proper lighting and fertilization, CO2 injection can give plants the absolute best chance at thriving and growing quickly in an aquarium.

When aquarium plants have plenty of CO2 to photosynthesize, they can produce so much oxygen in the water that the leaves begin visibly “pearling” with bubbles of oxygen.

CO2 Lowers pH

When dissolving CO2 into water (H2O), a small amount of carbonic acid (H2CO3) is formed. This mild acid can lower the pH 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 phenomenon is one of the reasons why it is important to use a timer in order to run CO2 injection when lights are on, but 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. 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 types of tests use a liquid reagent to determine CO2 levels.

The color of this CO2 dropchecker reacts with the pH of the aquarium water. This allows you to track the CO2 level.

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. When the lighting, nutrient, and CO2 levels in the aquarium are not matched up, the tank is not “balanced” and plant health may be adversely affected. Algae will thrive on plants that aren’t able to survive and may grow out of control. If your aquarium is limited by CO2, adding CO


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 gets into 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. Spring water can become saturated with CO2 at spring heads where the water comes up from beneath Earth’s surface. The type of groundwater is highly saturated in CO2 because it has been exposed to a continuous release of organic compounds combined with 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. 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. Limestone, on the other hand, is mostly composed of calcite or aragonite and is high in carbonates. Limestone is a great buffer because it neutralizes carbonic acid and raises the pH. CO2 is less highly concentrated in these bodies of water, so different species of plants have developed to grow in these conditions.

Some plants found in the trade are from partially terrestrial areas, which allows them to access unlimited CO2 in the atmosphere. 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 plant, like other red and carpeting plants thrive in high-tech tanks with strong lighting and high fertilizer dosing.

Injecting CO2 into plants can speed up their growth, provide high-quality lighting for plants, and transform them from submerged to fully 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. 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 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. Take a look at our selection of sturdy, beginner-friendly species today to get started with your planted tank.