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Can CO2 Alter pH Levels in an Aquarium?

How CO2 Affects pH in Fish Tanks San Diego, CA

The potential of hydrogen in any compound is known as pH. In the case of the water that sustains aquarium ecosystems, pH measurements indicate the levels of acidity or alkalinity, which fish keepers should strive to keep balanced according to the aquatic species they care for. The pH scale goes from 0 to 14. The lower numbers veer toward acid levels while the higher numbers denote alkaline levels. The ionization of water molecules affects the pH. In essence, the higher number of hydroxide ions results in alkaline water while the higher number of hydrogen ions causes more acidic water. Carbon dioxide has an effect on the pH levels of aquariums, and this is important to know for all aquarists, particularly those who care for aquatic plants.

Carbonic Acid Lowers Aquarium pH Levels

The more CO2 generated or injected into aquarium water, the lower its pH level will be. This is caused by the interaction between CO2 and H2O, which results in a release of carbonic acid. Overly acidic water isn’t conducive to marine biology, which is why too much CO2 can be deadly to aquarium species. The same can be said about a complete absence of CO2, which stimulates the chemical generation of carbonate hardness, thus swinging the pH level in the opposite direction.

Carbon Dioxide and pH Levels in Natural Aquatic Habitats

Natural bodies of water go through CO2 and pH level changes on a daily basis. During the daylight hours, plants take advantage of sunlight to go through the photosynthesis process, which takes up dissolved CO2 molecules from the water as it increases the levels of dissolved oxygen. It’s interesting to test your pH (with a titration test kit) in the morning before the lights go on, then again in the evening right before the lights go off. You’ll undoubtedly experience a large swing in pH. This is one of the biochemical advantages of keeping live plants in fish tanks. At night, CO2 levels increase because of the organic process of fish waste decomposition. During these shifts of CO2 levels, pH levels can rise and fall by a full point.

Ideal pH Levels in an Aquarium

The pH level of your aquarium water will reflect the level of care you provide. It doesn’t have to be pH neutral, but it’s best to be within 0.5 of pH 7. For a freshwater aquarium, strive for pH levels between 6.8 and 7.6. In a saltwater tank, pH should be somewhere in the range of 7.8 and 8.4, but a reef tank will have to be closer to 8.0 and no higher than 8.5. The hardiest species are able to withstand pH swings and will thrive in slightly acidic water.

Injecting CO2 in Planted Aquariums

High-tech planted aquascapes are those equipped with a tank that injects CO2. The goal of injecting CO2 is to fill the water with nutrients plants will absorb to maximize growth potential. However, this should be done carefully when the tank is stocked with fish or invertebrates. For example, shrimp and neon tetras are highly resilient and won’t be affected by the lower pH conditions CO2 injections create.

If you have any questions about how to properly maintain your aquarium or need any supplies for you tank, reach out to Aquatic Warehouse, a leading provider of saltwater and freshwater aquarium supplies. We’re located in Kearny Mesa, or you can order directly through our website and have your supplies shipped to you. If you have any questions, don’t hesitate to call one of our friendly and knowledgeable representatives at 858-467-9297.


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    San Diego, CA 92123
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