Do fish produce enough CO2 for plants?


Everything above the water's surface lives in harmony. As we breathe out, we release carbon dioxide into the atmosphere. Terrestrial plants and trees absorb this. They use this carbon dioxide, along with nutrients and light from the sun, to grow using a process called photosynthesis. The same symbiotic relationship between animals and plants exists below the water surface - but do fish produce enough co2 for plants to thrive? Read on and find out.

What is photosynthesis?

All plant life (or flora), from the great redwood trees down to the smallest algae, uses photosynthesis. This is when they use the sun, water and carbon dioxide energy to create glucose which helps them grow. Photosynthesis is vital to the food web as it produces oxygen - something all living things need! Fish and other aquatic life also rely on plants to provide them with oxygen.

Plants and algae use this natural process to absorb carbon dioxide (CO2) from the environment to survive. When these plants take up CO2, they can convert it into energy to grow. Animals (including us people) also breathe in oxygen when we take a breath and expel carbon dioxide. It is the perfect example of balance between plants and animals.

A planted aquarium, filled with plants and fish, also displays this natural partnership but can aquarium plants rely on the carbon dioxide expelled as a byproduct alone? Let's find out.

Where exactly does CO2 come from in nature?

In a natural habitat such as a river, ocean, or lake, aquatic plants absorb carbon dioxide from the environment and use it to photosynthesize. CO2 is generated by the decomposition of organic matter, fermentation, and respiration of animals. So rivers containing the co2 rich waters flow continuously, providing a steady supply of CO2 to the aquatic plants.

Furthermore, not all plants we use within the aquarium are 100% aquatic plants. Many species, such as Anubias or Bucephelandra, grow along the banks of streams that may only be fully submerged during wet seasons or after a storm. These plants will receive a healthy amount of CO2 directly from the air.

Another example of this is Java Moss. This popular aquarium plant can be found growing on tree trunks, rocks and other items lying along the side of rivers, with only a tiny amount submerged underwater.

Keeping plants in the aquarium would not be as successful without CO2, so it's essential to understand how plants receive this gas.

Do fish produce CO2 at high levels? While fish and other aquatic inhabitants produce CO2 through their respiration processes, in an aquarium, even heavily stocked aquariums, fish respiration alone would not produce co2 levels high enough to support most plant species.

How much CO2 does a fish produce?

Freshwater fish do produce CO2 as a byproduct of breathing. For example, a goldfish swimming in an aquarium produces about 70-170mg of CO2 daily. However, this is less than the amount of CO2 that most aquarium plants need to grow and thrive. Many hobbyists prefer to supplement their aquariums with additional CO2 from pressurized or non-pressurized systems.

discus fish

Is nature enough?

While nature is terrific and can ideally sustain itself, it simply cannot produce enough CO2 in fish tank settings. The CO2 levels in a wild environment are much higher than in an aquarium, and many aquatic plants rely on these high concentrations to survive.
One area of a river will benefit from natural CO2 generated further upstream. Therefore, the continued supply of CO2, the constant cycle of organic matter decaying and the abundance of aquatic life all contribute to a much higher Carbon Dioxide level. More significant than increased co2 in aquarium naturally occurs.

For these reasons, to achieve our planted aquarium goals, the CO2 levels must be increased using a co2 system to provide our plants with the energy needed to grow and thrive.

planted aquarium

Do all plants require lots of CO2?

Not all plants require CO2 fish tank systems. Some aquarium plants can grow in low-CO2 environments. These fish can receive enough or just as much co2 as they need to thrive. However, that does not mean these plants won't benefit from added carbon dioxide! Adding a CO2 system to your tank will help the growth of all aquatic plants and balance out the system. Without sufficient CO2 levels, many aquarium plants can suffer and may even die.

What does a non-CO2 fish tank look like?

Planted aquariums without pressurized CO2 that have many plants growing will also tend to have a lot of algae in the tank. The lack of CO2 makes it harder for plants to take up nutrients, and this results in an increase in the growth of algae. Without sufficient CO2 levels, one should expect heavy algae blooms in their non-CO2 fish tanks. To ensure a healthy balance between aquarium plants and algae, adding a CO2 system to the tank is strongly advised.
Adding more fish than the average planted aquarium can also help control algae growth. Fish such as the Siamese Algae Eater, Otocinclus Catfish, and Flying Fox are great for controlling algae in non-CO2 fish tanks. These species actively clean the tank walls, glass, decorations, and plants of unwanted algae. The more fish species, the more CO2 is produced, resulting in a healthier tank with fewer algae blooms. Lastly, manually removing algae from the tank is also an option. This can be done using an algae scraper or other tools to remove algae from aquariums. Doing this regularly can help maintain a healthy balance between aquarium plants and algae in non-CO2.

Which plant species would grow well in a non-CO2 aquarium?

A few plant species require low carbon dioxide levels, making them suitable for an aquarium with a low carbon dioxide level, also known as a low-tech tank.

  • Amazon Swords
  • Java Fern
  • Cryptocoryne
  • Pennywort
  • Anubias

The above plants demonstrate plant growth in low co2 setups. They are also known as low-light plants. It is advised to provide supplemental nutrients when adding these plants without additional CO2. Place root tablets directly into the substrate for substrate-rooted species like Amazon Swords and Cryptocoryne or liquid fertilizer for all other plants. If you decide the try a low-tech tank, your choice of fish species will depend on your plant choice and the water parameters in which you wish to maintain them. For example, Java Ferns prefer low fish populations and slower-moving water, while Anubias can survive in higher fish concentrations and a higher pH range.

One of the best plants to use in a low-energy aquarium comes from the amazon basin and can be enjoyed in many different aquarium settings. The Amazon Sword plant has a strong root network that will grow in most substrates without problems. The large elongated leaves provide decor and also cover for shy fish. This plant is an excellent choice for low-tech aquariums, and with its hardy nature, it requires minimal maintenance allowing you to focus on other aspects of the tank. There are several variants of Amazon Swords, meaning you will find one with the perfect size and colour pattern to fit your aquarium.

How many plants can be grown in a low-energy aquarium?

When deciding on plants for an aquarium without additional CO2, light, or fertilizers, it is vital to consider the number of plants in relation to the aquarium size. A general rule of thumb is that one plant should be added to every gallon of water in the tank. This ensures that each plant has enough space and access to nutrients. When creating a planted tank, it’s also important to remember that some plants require more light and nutrients than others. As a result, the number of plants may need to be adjusted accordingly. You can also look for hardy species such as Anubias and Java Ferns, which are well-suited for low-tech aquariums with fewer resource demands.


To answer the question of whether fish can produce enough co2 to sustain aquarium plants, we conclude that yes, they can. They can with a limited amount of low-light, slow-growing plants specifically chosen for a low-tech tank. In addition, supplementing with liquid fertilizer or substrate-rooted species like Amazon Swords and Cryptocoryne can increase the chances of success. Finally, matching fish species and water parameters to the plants chosen for your tank is essential to maximize success. You can create a thriving low-tech with careful planning and attention to detail.
Alternatively, if you wish to create an aquascape with any plant species you like, you should explore the world of high-tech tanks. By implementing lights, CO2 and fertilizers, you can open up a new world of creative opportunities.

To learn more about the world of planted aquaria, visit our aquascaping blog. Learn how to grow all kinds of plants, including high-light plant species and how to get faster growth from your aquarium plants.