Do Fish Emit CO2?

CO2, or carbon dioxide, is an odorless gas, a waste product of breathing. It comes into the body as oxygen and is converted inside after it is used to create energy. Every animal emits carbon dioxide, but the production is balanced out by flora, which work in the opposite way -- plants take in carbon dioxide, converting it and releasing it as oxygen.

  1. Respiration

    • Respiration is the process of taking in oxygen and emitting carbon dioxide. In fish, the process is much like that in other animals, except they take in oxygen from the water instead of the air. Oxygen enters the blood stream through the gills, feeds muscles and organs, and converts to carbon dioxide. This carbon dioxide is released back into the water as part of the natural respiration process.

    Levels in Water

    • The amount of carbon dioxide emitted by fish is minimal and doesn't affect water quality unless the tank has no water movement. Surface movement, usually created by the tank's filter system, brings enough oxygen into the water that carbon dioxide is not a concern. Regular water testing typically does not include monitoring carbon dioxide levels. Too much of this gas in the water, however, such as in stagnant water, can cause the pH to rise. It can also be fatal to the fish since too much carbon dioxide hinders the fish's ability to absorb oxygen. If you suspect carbon dioxide levels are too high, you can check them by using a commercial CO2 drop checker kit or by measuring both the pH and kH, or carbonate hardness, and referencing the levels on a carbon dioxide chart.

    Plants

    • Just like all other plants, aquatic plants placed in an aquarium use carbon dioxide for photosynthesis, or energy, and release oxygen back into the water. Fish emitting carbon dioxide can help meet a plant's needs, while the plant helps keep oxygen levels high for the fish. This partnership is not equally balanced, however, since fish don't always release enough carbon dioxide to keep plants healthy. For this reason, many hobbyists focused on growing aquatic plants take pains to monitor carbon dioxide levels; they inject CO2 as needed into the aquarium to meet the needs of the plants. Adding fish may help balance out the problem in lightly planted aquariums, but the number of fish required to support numerous plants would overcrowd an aquarium.

    Other Sources

    • Fish aren't the only organisms to release carbon dioxide into a tank. Any other animals, such as snails, crayfish, frogs, shrimp or turtles, also emit carbon dioxide, as well as decaying matter like dead plants or fish or leftover food. Even some decorations, like driftwood, can emit carbon dioxide. Like the fish, unless there is an overabundance of these materials or the water is stagnant and not taking in oxygen, the amount of carbon dioxide is usually not enough to negatively affect the health of the fish.