What Is Carbon Media Used for in Aquariums?
Activated carbon is one of the most popular filtration media used in the aquarium hobby industry, ranging from beginner freshwater tanks to advanced reef systems. Even though it is a popular product among hobbyists, many aquarists use carbon with little understanding of its function, limitations and best application.
What Is Activated Carbon?
Activated carbon is not just charcoal but is actually a specially reticulated form of carbon material (usually wood, coconut, or coal based) that has been treated with oxygen. It creates hundreds of billions of microscopic pores between carbon atoms. This process, known as “activation,” creates a surface area so expansive that a mere teaspoon of high-quality aquarium carbon can contain an internal surface area equal to an entire football field.
This enormous surface area allows activated carbon to do what it does best—adsorb. Adsorption (which is different to absorption — where liquids are absorbed like a sponge) is a surface process that has molecules adhered to the outer and inner surfaces of the carbon. This immobilization is a result of weak intermolecular interactions that offer no true activation but essentially "manipulate" and trap some compounds in what would be considered their target sites without actually changing their structure.
Carbon Media Enhances Water Clarity
One of the most remarkable attributes of carbon is its removal of dissolved organic compounds (DOCs) from water that contributes to water discoloration and water cloudiness. These components, known collectively as yellowing agents (or "tannins") leach from driftwood, botanicals, and decomposing organic matter, creating a yellowish or tea colored tint to your aquarium water.
When used properly, high-quality activated carbon can drastically improve water clarity and get back that crystal clear look that many aquarists desire within 24-48 hours of adding it to an aquarium system.
Carbon media can eliminate Odors
Aquarium water that gives off any kind of bad smell is often the result of certain volatile organic compounds (VOCs). This is why tanks generally smell much better 24-48 hours after adding fresh carbon media, since activated carbon is particularly effective at binding many of these odor-producing molecules.
Carbon media can Remove aquarium Medications
If you have recently treated your fish or corals with medications, compounds must be removed from water to prevent chronic exposure. Most aquarium medications, including malachite green, methylene blue, formalin, and many antibiotics are readily absorbed by activated carbon. Therefore, carbon is an important part of post-treatment water conditioning.
Carbon media can reduce water Pollutants
Carbon has the potential to adsorb many of the pollutants that can impact water quality, including:
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Heavy metals
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Chlorine and chloramines
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Phenols
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Pesticide residues
What Carbon Cannot do in aquariums
Knowing the limits of carbon is just as critical:
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Does not remove ammonia, nitrite, or nitrate, so you’ll still need to manage the nitrogen cycle with biological filtration and water changes
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Does not have a large impact on pH or water hardness
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It does not remove beneficial trace elements
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It does not substitute for bad husbandry practices - just because you add carbon media, doesn’t mean it will solve common aquarium mistakes such as overcrowding or lack of cleaning
Types of Aquarium Carbon
Carbon from Bituminous (Coal-Based)
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Features: Broad pore structure, high ash content
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Best for: Larger organic molecule removal, general water clarification
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Disadvantages - not as effective as fine chemical filtration, potential phosphates leaching
Coconut Shell Carbon
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Features: Microporous structure, least ash content, max surface area
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Best for: Reef systems, sensitive aquariums, small molecule, and trace element removal
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Disadvantages: More expensive, slightly less effective for large-molecule water clarification
Wood-Based Carbon
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Features: Medium pore size, moderate performance characteristics
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Best for: General freshwater use, budget-minded hobbyists
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Disadvantages: Not as effective for chemical removal as premium coconut carbon
Carbon Media Dosage Guidelines for Aquariums
For most systems, we recommend using 1/2 cup of carbon media per 40 gallons of aquarium water. However, you may need to adjust the media dosage based on the following:
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Bioload: You need more carbon for a higher fish population or feed levels
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Heavily planted tanks may require less carbon
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After using chemical treatments, you may need a larger volume of carbon
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Carbon quality: High-end products may work better at lower volumes - always check the recommended dosage on the label
Where to place carbon media in aquariums?
It’s important to note that activated carbon media quickly loses effectiveness when there is a lot of debris in the aquarium. For this reason, we recommend that you place the carbon after the mechanical filtration media in the filter. You can also place carbon filter media in a mesh filter media bag close to your aquarium system’s water flow so that it can be easily distributed throughout the aquarium water. If you have an aquarium sump, you can also place the mesh filter bag in there.
How To Replace Carbon Media in Aquariums?
As activated carbon binds to the compounds it removes, over time it cannot remove any more contaminants and needs to be changed. You should change the carbon in your reef aquarium every 2-4 weeks. If the aquarium water is starting to turn yellow or you start to smell an odor coming from the water, it’s gone past the time to change the carbon media.
Conclusion
Carbon media is used as a chemical filter system that helps to remove contaminants that may have escaped the main filter. These include dissolved contaminants such as chloramine, chlorine, tannins (which cause aquarium water to turn cloudy/murky or change color), and phenols (which cause bad odors).
Adding chemical media filtration in an aquarium is super easy, however, if you are unsure which carbon media is best for your aquarium or need help with carbon media dosing, contact the team at Reefco Aquariums.
