Selection and Placement of Activated Carbon in Marine Aquaria

By Adam Jenkins


Activated carbon -- when used properly -- is one the most efficient filters available to the home and commercial aquarist. But as with most things in saltwater fishkeeping, there are different and sometimes contradictory opinions on its selection, placement, and even its use in general. Nonetheless, with proper use activated carbon can be a very helpful tool in the quest for exceptional water quality and healthiest possible aquarium inhabitants.

Activated carbon is either made from plant materials such as coconut shells and wood, or plant-based fuels like charcoal and lignite (brown coal). There are two steps to the manufacturing process, carbonization and activation.



Carbon filtration can be a useful way to optimise water quality in marine aquaria. © Adam Jenkins





Carbonization is the process of heating the carbon in an anaerobic (oxygen-free) environment to break down the complex organic chemicals from which the material is made from, driving off almost everything but carbon and ash.

After carbonization the resulting carbon is then activated by being subjected to exposure to heated gas, most commonly steam, as well as a series of chemical washes. Activation improves the quality of the carbon as an adsorbent by increasing the sizes of the pores on its surface, maximizing its surface area. Activation also washes away any remaining residues left behind by the carbonization process.

Activated carbon is sold in powder, granular and pellet forms. Granular is one most commonly used in the aquarium trade and is the one that we will discuss.

When shopping for activated carbon you will probably gravitate towards one of the more colorful, bright containers that are labeled as research grade, or aquarium grade etc…Fight the urge!

Grade is nothing more than a marketing tool designed to siphon those hard-earned dollars from your pocket. Grade does not equal quality, and nor does price. Quality is determined by variations in the material used to make the carbon and the type of activation process used by the manufacturer. These variations mean that not all activated carbons are the same.

Although the carbon base and activation method determine quality, the specific measurements the shopper should be aware of are the iodine number, molasses number, and ash content.
The iodine number (or iodine value) is an indicator of micro-porosity, in other words, the activated carbon's ability to adsorb small molecules. Iodine is notable for being one of the few inorganic chemicals that is readily adsorbed by activated carbon, and the iodine number is the mass of iodine (in milligrams) adsorbed by one gram of activated carbon. For marine aquarium applications, an iodine number around 1000 mg/g is ideal.




A package of activated carbon. © Adam Jenkins




The molasses number (or molasses efficiency) refers to the ability the activated carbon to adsorb larger molecules, what is referred to as meso-porosity. Slightly confusingly, there are two ways of quoting the molasses number, a North American system of percentages, and a is around 600 (in the European system) or 185% (in the North American system European system of numbers.

Ash content is simply a measurement of the residual materials left behind from the carbonization and activation process. The more ash, the less efficient the activated carbon will be, so aquarists should look for carbon with the lowest possible ash content.

Although not a scientifically measured indicator of the quality of activated carbon, if a package of activated carbon contains minimal amounts of dust, that's a definite plus. The dust does not pose a specific threat to health, but if enough is present it can be a (likely temporary) irritant to your tank inhabitants, and will also make the water in your aquarium appear cloudy.


Also look for activated carbon in a resealable container. As well as being good at removing impurities from water, it is just as good at removing impurities from the air. So a package of carbon left in the open will adsorb impurities from the surrounding air, and once placed in the aquarium, could release them into the water. That could cause problems, so be sure to always keep carbon in a sealed, airtight container to protect it from contamination.

Aquarium Use
Activated carbon has many uses in the home aquarium, but the most popular, and arguably the most important, is the removal of dissolved organic compounds from the water column.

Dissolved organic compounds( DOCs ) are the result of decomposition of uneaten food, digested food and dead inhabitants among other things. While small quantities of DOCs may be harmless, as a source of nitrogen compounds, at higher levels they can lead to water quality problems. Dissolved organic compounds contribute to the yellowing of water between water changes, reducing light penetration, an important issue when photosynthetic invertebrates and macroalgae are being maintained. Elevated DOC levels can also lead to outbreaks of nuisance algae, and there also appear to be correlations between elevated DOC levels, stress, and certain fish diseases as well.

Another use of activated carbon is the removal of other potentially toxic organic and inorganic chemicals that may enter the aquarium in a variety of ways. Among the substances that activated carbon removes at least moderately well are acetone, alcohols (including isopropyl alcohol), antifreeze, benzene, chloramines, chloroform, chlorine, chlorophyll, citric acid, dyes, herbicides, hydrogen peroxide, insecticides, iodine, ketones, dissolved oils, organic acids, pesticides, phenols (including those that create unpleasant odors), radon, solvents and tar emulsions.

Since at least some of these chemicals are used in fish medications, particularly things like dyes, activates carbon will normally be removed when fish are being medicated. On the other hand, using activated carbon to remove traces of medication after treatment is concluded is a useful application of the material.

Activated carbon does not remove all potential toxins or unwanted chemicals though, including the ammonia produced by livestock, and nor does it substantially affect water hardness. Chemicals that activated carbon has little or no ability to remove include: ammonia, calcium, carbon dioxide, fluoride, lime, magnesium, nitrates, nitrites, phosphates, sodium, and iron. Lead and other heavy metals will only be removed by the use of a very specific type of carbon not normally used by aquarists.


Activated carbon in a media bag for use in a sump. © Adam Jenkins





With the popularity of small tanks and the trend towards mixed reef systems, carbon is a useful addition to the nano aquarist's bag of tricks because of its ability to adsorb, and thereby neutralize, allelopathic chemicals secreted by some invertebrates.

Optimal usage
Although activated carbon filtering abilities may be thought of as being common knowledge, in fact recommendations for optimal amounts and placement are heavily opinionated. For example, the amounts of activated carbon needed for marine aquarium applications range from as high as 2 cups per 55 US gallons of water, down to just 3 tablespoons per US 50 gallons.

In fact there probably isn't one ideal amount of activated carbon that works equally well in all marine aquaria. A sensible approach is to start using a small quantity, e.g., 3 tablespoons per 50 US gallons, and then only increase that amount if water clarity and stocking levels demand it.

Before using carbon, it should be cleaned first. This is done by placing the carbon in one or more media bags, as required, and then placing those bags in a container of reverse-osmosis water for at least 24 hours. This soaking gets rid of some of the ash and phosphate residues.

Yes, I said the dreaded P-word, phosphates! Before you get all bent out of shape and call a priest to exorcise your bag of black stuff, remember, activated carbon is produced from plant materials of some sort or another, and all plant materials contain at least some phosphorus. So there's a good chance there will still be some phosphate left behind by the carbonization and activation processes, though the amount varies between brands. By soaking and rinsing the activated carbon, you will hopefully wash away some of the phosphate residues along with the ash.

Placement is one the most import aspects. The ability of activated carbon to adsorb impurities depends on it being placed in just the right part of the filtration system with areas of high flow being optimal to increase the amount of water exposed.

The most common placement of activated carbon is in media filter bags in the overflow box or sump, below the displays overflow system, using gravity to move the aquariums water through the activated carbon. Placing activated carbon in or below an overflow is called “poor through filtration”. This process is slow and can handle only small amounts of water volume to maintain proper filtration.



Activated carbon in an external filter. © Adam Jenkins



A much more efficient placement for activated carbon is inside some type of powered filter (such as a media reactor or a canister filter) where water is pumped through the activated carbon, thereby maximizing the amount of water exposed to the carbon at any given moment. The amount of turnover associated with the use of powered filters increases exposure time by increasing the actual amount of times the water is exposed.

Activated carbon also needs to be positioned somewhere it is exposed to clear, silt-free water, otherwise the carbon will end up being used as a particle filter. Those particles of silt block up the pores on the activated carbon, and stop it from performing its proper function. Therefore a pre-filter of some sort is important to trap silt before it reaches the carbon. This is especially critical when keeping activated carbon in media bags. The particles can actually stop up the bag preventing contact between the activated carbon and water. If a pre-filter can't be installed, a regular rinsing with clean water will remove the particles from the bag and thereby restore proper function.

Because of the porous nature of activated carbon, it is readily colonized by substantial populations of the bacteria that perform biological filtration. It is possible that the bacteria living on the carbon end up playing a useful role in water quality management. Although the colonization of the activated carbon by bacteria decreases its effectiveness to filter, the sudden removal of portions of the natural biological filtration could cause a sudden decrease in water quality. That being the case, replace no more than half the activated carbon at any one time, and allow two weeks before replacing anymore.

The five major factors that effect adsorption are:

  • 1)         Physical properties (pore size, surface area, etc.)
  • 2)         The chemical nature of the activated carbon
  • 3)         Composition and concentration of the particular contaminant
  • 4)         Water pH and temperature
  • 5)         Exposure time of the contaminant to the activated carbon

  • Limitations of Activated Carbon Use
    Although the use of activated carbon provides useful benefits, there are also some drawbacks. Along with the removing of unwanted organics and inorganic chemicals, activated carbon also removes essential trace elements vital to the health of marine livestock. The amounts removed are of unknown quantity and significance, but nonetheless a precautionary approach towards offsetting them by performing regular water changes is recommended.

    Another problem associated with its use is what might be termed ‘shock’. In reef aquariums where there has been no prior use of activated carbon, the sudden removal of large amounts of DOCs can actually have negative effects on the system. The best way to prevent this problem from occurring is simply to phase in activated carbon use by employing only small amounts of activated carbon at first, and in stages across several weeks raising the amount until the desired amount and effects are attained.


    Activated carbon is a tool that provides several benefits to the marine aquarist, but it is in fact just one of many tools, and its functions and limitations should be understood. Activated carbon is an adjunct to proper maintenance of an aquarium, and not a reason to skip water changes.

    Greenfield, Richard. Activated Carbon/Charcoals - The Rundown., 2000
    Straughan, Bob. Activated Carbon: Filter Media Extraordinary., 2000
    Layton, James R. An Overview of Activated Carbon in the Marine Aquarium
    Sprung, Julian and Delbeek, Charles. The Reef Aquarium Volume 2, Ricordea Publishing 1994
    Goldstein, Robert. Marine Reef Aquarium Handbook, Barron's 1997
    Fenner, Robert . The Conscientious Marine Aquarist, T.F.H. Publications, Inc 2001
    Seelig, Bruce; Bergsrud, Fred; and Derickson, Russell Activated Carbon Filtration North Dakota State University Agriculture and University Extension


    SW Carbon Filtration on WWM

    Related FAQs: Carbon 1, & FAQs on Carbon: Rationale/Use, Types/Qualities/Selection, Placement, Renewal, Negative Reactions, Sources/Brands, & Marine Chemical Filtrants

    Related Articles: Phosphates in Carbon; An analysis of the phosphate content of activated Carbon by Steven Pro, Marine Chemical FiltrantsZeolite Filters: A Discussion of What Zeolites Are and How They Function by Jens Kallmeyer,


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