CO2 in Planted Aquariums: Pros and Cons To Consider


CO2 in Planted Aquariums: Pros and Cons to Consider

In the planted tank hobby, you may have heard of two types of aquariums: high tech tanks that inject carbon dioxide (CO2) gas and low tech tanks that 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 examine what CO2 does to aquarium plants, and some of its pros and cons.


Photosynthesis is done with CO2.

Ever heard the phrase “carbon is the backbone for life”? That phrase is not just true for animals like you and me, but it’s also true for plants as well. For plants to photosynthesise and produce food, they need carbon. This is a fundamental requirement regardless of whether CO2 gas is added to the aquarium. The plants use 2-3 ppm (parts-per-million) of CO2 in a low tech tank. This comes from both surface gas exchange or animal respiration. Some plants are able to use carbonate, bicarbonate compounds (KH), within the water as a source for carbon for photosynthesis. However, this requires more energy than using CO2 gases. 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. The best way to ensure that plants grow fast in an aquarium is by using CO2 injection.

If aquarium plants have enough CO2 to photosynthesise, they can produce so many oxygen-rich water bubbles that leaves start “pearling”.

CO2 Lowers pH

Water (H2O) is dissolved into CO2. A small amount carbonic acid (H2CO3) results. This mild acid has the ability to lower the pH in 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 is important that you use a timer to ensure that CO2 injection is only run when the tank is lit. Light is what plants need to grow. They use CO2 to produce oxygen and photosynthesize. If there is no sunlight available at night and 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. 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 require the use of a liquid to determine CO2 levels.

This CO2 drop detector color adjusts to the pH in the aquarium water. This aids in tracking the CO2 levels.

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 can thrive in a situation where plants are having trouble surviving and will grow out of control. Your aquarium’s CO2 level is too low.


With the right lighting and fertilizer, injection can dramatically improve plant health and growth. When plants thrive and have all the required elements, there is little chance that algae will be able to outcompete them for nutrients or light.

How CO2 Enters Water in Nature

Although it might seem strange that equipment is used to inject CO2 gas in aquarium water, many of these aquatic plants are native to areas where CO2 is abundant naturally. 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. This can occur in water that has a silicate subsurface. The carbonates cannot neutralize the carbonic acids from CO2. The pH remains low and the CO2 stays concentrated, allowing plants to grow abundantly. On the contrary, limestone is mostly made from calcite and aragonite which are high in carbonates. Limestone helps to buffer the water by neutralizing the carbonic acid, and raising the pH. CO2 is less highly concentrated in these bodies of water, so different species of plants have developed to grow in these conditions.

Other plants in the trade were grown in areas that are partially terrestrial. This allows them to have unlimited CO2 in their 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 plant, like other red and carpeting plants thrive in high-tech tanks with strong lighting and high fertilizer dosing.

For speeding up plant growth and keeping plants that need high light, CO2 injection is a useful tool. It can also convert submerged growth 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. 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 usually cheaper and more manageable, which is crucial while you’re learning how to keep plants healthy 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.