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. 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”? This is true not only for animals like us, but also for plants. For plants to photosynthesise and produce food, they need carbon. 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. 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. When combined with proper lighting and fertilization, CO2 injection can give plants the absolute best chance at thriving and growing quickly in an aquarium.

Aquarium plants that have a lot of CO2 can photosynthesize and produce enough oxygen in the water to make the leaves appear “pearling” with oxygen bubbles.

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. Plants use light to create oxygen and CO2 for photosynthetic purposes. If there is no sunlight available at night and the plants cannot photosynthesize, then they will consume oxygen and produce CO2. 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 require a liquid reagent in order to measure CO2.

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

A planted tank will thrive if it receives enough light. However, they will need additional nutrients to compensate for the increased light intensity. 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 can thrive in a situation where plants are having trouble surviving and will 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. 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 become saturated with CO2 at spring heads where the water comes up from beneath Earth’s surface. 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. 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 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.

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. 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 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. A densely planted aquarium is more beneficial than one with less 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. In general, low tech tanks are cheaper and easier to maintain, which is important while you are still learning how to keep 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. Peruse our collection of hardy, beginner-friendly species to get started with planted tanks today.