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 talk about what CO2 actually does for aquarium plants, as well as some of the pros and cons of using it.
CO2 is Used for Photosynthesis
Are you familiar with the expression “carbon is life’s backbone”? 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. 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. Combining proper lighting and fertilization can help plants thrive and grow 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 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. Plants use light to create oxygen and CO2 for photosynthetic purposes. When there isn’t enough light, and the plants can’t photosynthesize at night, they use oxygen to produce CO2 and then release it as part of their 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
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 the CO2 in your aquarium is low, you can add CO2.
When combined with good fertilizer and lighting, injections can greatly improve plant growth and health. When plants are thriving with all the necessary elements they need, algae stands little chance of outcompeting the plants 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. 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.
Certain water bodies with a low pH or KH buffer allow CO2 to enter at high rates. 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. 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 greatly buffers the water by neutralizing carbonic acid and raising the pH. These bodies of water have lower concentrations of CO2, so many species of plants have evolved to thrive in them.
Some plants found in the trade are from partially terrestrial areas, which allows them to access unlimited CO2 in the atmosphere. While these may not be true aquatic species because they grow above the water line, many can survive submerged in CO2-rich environments so we can enjoy them in our 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.
The use of CO2 injection can be used to speed up plant growth, keep plants that need high lighting and convert plants from submerged to emersed. It is also more sensible to have a densely-planted aquarium than one that uses less carbon. Just be prepared to invest the extra cost and effort it takes to maintain a high tech planted aquarium.
For beginners, we highly recommend that you start with a low tech planted aquarium without CO2 injection. 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. Take a look at our selection of sturdy, beginner-friendly species today to get started with your planted tank.