Activated Carbon

A lot of us have probably already used a corner–box air­driven filter. Usually you will use aquarium moss and charcoal as media for this type of filter. Many hobbyists have managed to keep their first fry tanks successfully using this filtration scheme, which is still among the best choices for such a tank – and not only.

Of course, aquarium moss (a mechanical medium) is not responsible for this success, as it can only trap big particles or waste. On the contrary, what is most important are the invisible pollutants that remain in the water giving it the characteristic yellowish color or an undesirable odor that, in the end, can be transformed into lethal substances. There is only one solution to this: the use of charcoal. Charcoal will effectively remove organic wastes before they convert into pollutants that affect water clarity, smell and safety.


The use of special manufacturing techniques results in highly porous charcoals that have surface areas of 300­2,000 square meters per gram. These so­called active, or activated, charcoals are widely used to adsorb odorous or coloured substances from gases or liquids. Activated charcoal or activated carbon is an amorphous form of carbon. This means that it has no regular atomic structure, unlike the other forms (allotropes) of elemental carbon: diamond, graphite, fullerenes or nanotubes.

How do we ask for it? Charcoal is silken, black, shining, irregularly–shaped fragments of charred wood or bone, with no further elaboration. This type of carbon is of limited use in aquariums. It is good only for fresh water tanks, with a minimum of fish load. It is sold as aquarium carbon or filter carbon. However, charcoal has been replaced by “Activated Carbon” (AC for short) during the last years.

Different kinds of carbon can be produced which can then be used for removing different kinds of impurities. The secret of Activated Carbon power lies in its elaboration. For example, when acids are used during the activation procedure, the result is a carbon with an extremely porous structure. This kind of carbon is mainly used for air filters. Different kinds of carbon can be made if carbon is subjected to high temperatures and pressure.

It always becomes a very porous medium, but the size of its pores depends on the gases used during activation and the inorganic salts (copper, phosphates, sulfates, silicates or zinc salts) that were added before the activation procedure. Thus, we can now produce activated carbon with very specific properties. Anybody can find special Activated Carbon for aquarium use, which has been activated at 2.000º C in vacuum, not washed with acids, etc. It is sold as activated carbon or “super” activated carbon.

Activated Carbon Function

Activated Carbon works in two different ways. It can remove organic substances by absorbing them inside its sponge­like structure and/or by adsorbing them with chemical bonds.

Absorption is based on the vast number of pores. You can imagine it as a material with too many pockets (very tiny pockets, the smaller the better), which can be filled with small things (molecules). It could be referred to as a very fine mechanical filter, which can absorb things the size of a molecule. Thus, it is important to keep those pockets functional for as long as possible. In this aspect, a good mechanical filtration is essential before water reaches the activated carbon part of your filter, otherwise carbon will be clogged very quickly.

The word adsorb is important here. When a material adsorbs something, it means that it attaches to it by chemical attraction. The huge surface area of activated charcoal gives it countless bonding sites. When certain chemicals pass next to the carbon surface they attach to the surface and are trapped.

Activated charcoal is good at trapping other carbon­based impurities (“organic” chemicals), as well as things like chlorine. Many other chemicals are not attracted to carbon at all ­ sodium, nitrates, etc. ­ so they pass right through it. This means that an activated charcoal filter will remove certain impurities while ignoring others. It also means that, once all of the bonding sites are filled, an activated charcoal filter stops working. At that point you must replace the filter. Adsorption relies on the polarity of the various molecules. Polar molecules have two “poles” that differ in their affinity for the water molecules, so one side of the molecule is hydrophilic (likes water) and the other one is hydrophobic (dislikes water – likes lipids). When the polar organic molecule approaches the polar surface of Activated Carbon, it is attached on it by its hydrophobic (lipophilic) side and thus is removed from the water solution that contained it. However, carbon will remove not only organic compounds but also inorganic ones. This is especially important. The following list shows some elements / molecules that will be removed by the use of activated carbon. You can see that it will remove both organic and inorganic substances (data obtained by the Greek Water Board).

Contaminants *MCL/ mg/L
Inorganic Contaminants
Organic Arsenic Complexes 0.05
Organic Chromium Complexes 0.05

Mercury (Hg+2) Inorganic

Organic Mercury Complexes 0.002
Organic Contaminants
 Benzene 0.005
Endrin 0.0002
Lindane 0.004
Methoxychlor 0.1
1,2-dichloroethane 0.005
1,1-dichloroethylene 0.007
1,1,1-trichloroethane 0.200
Total Trihalomethanes (TTHMs) 0.10
Toxaphene 0.005
Trichloroethylene 0.005
2,4-D 0.1
2,4,5-TP (Silvex) 0.01
Para-dichlorobenzene 0.075
Contaminant **SMCL
Colour 15 colour units
Foaming Agents (MBAS) 0.5 mg/L

Placement ­ Utilities

Once we decide which kind of carbon is suitable for our tank and our fish, we have to decide about the place of the medium in our filtration system. There are many options. We can put it inside the chamber of an internal or a canister filter, or in a chamber in the external power filter. We may also have a specially constructed carbon chamber in our internal, self–made, filter. We can even place it in the carbon chamber that we have designed in our self–made sump. Finally we can place a layer of carbon in our trickle filter. We can also construct a contact chamber and place it in – line with our system. We may put hose fittings at both sides of a PVC section and place Activated Carbon filled bags in it. We can locate this pipe on the return of the tank so all returning water is forced to pass through the carbon before it returns in the tank.

Alternative utilities

Forcing water to pass through a contact chamber containing bags of carbon is not the best way to use it, as it is easily clogged from particles, the same way as a mechanical filter does. So, much of the carbon becomes “isolated” and some water ends up traveling between the bags. At the same time water flow slows down. We may avoid this effect if we do not use media bags (especially the ones with fine mesh size) or by using grate screens between the bags. The whole idea is to use bags for carbon (because handling it becomes easier), but to place the bags in such a way that water is not forced directly through them. It must be flowing freely around them, allowing enough contact time between water and the carbon’s porous surface in order to remove the waste compounds from it.

We may simply put Activated Carbon bags in the sump and the organic compounds will be removed, as water will be flowing over Activated Carbon (through diffusion). This way, the water will be kept clear and trace elements will not be depleted as quickly as when we force water to pass through Activated Carbon.

We have to be careful and make sure that the bags cannot move freely in the sump and be drawn up against the pump’s intake. We may use plastic baskets, panels or egg crate diffusers to keep the Activated Carbon bags in place (away from the pump intake).

We may even place the bags inside the tank, near the maximum water flow or behind the rockwork.

In a tank with an under gravel filter, we may bury bags with Activated Carbon under the gravel next to the airlifts. This method becomes very messy when the time for replenishing Activated Carbon comes.

Quantity – Frequency of replacement

After finishing with the installation issue we have to deal with the quantity we must use in our system as well as the frequency of changes and the amount of Activated Carbon we have to replace in each change. The answers to those questions are not easy since there are no recipes.

Different tanks have different needs since there are many parameters (different sizes, different animals, different stocks, different foods and different ways of care) that define the need for Activated Carbon changes and the overall quantity of Activated Carbon for a system. All the parameters mentioned earlier influence the amount of the Dissolved Organic Carbon, which will be present in our system (fatty acids, amino acids, organic acids, proteins, carbohydrates, plant hormones, carotenoids, phenols, vitamins, etc). DOC is the total of all chemical compounds that include the carbon element in their structure. DOC affects growth negatively while it induces metabolic stress and reduces disease resistance.

During the 80’s and 90’s many studies were worked out and published on this issue (Thiel, Moe, deGraaf, Hovanec, Spotte). Most of them suggest 36 ounces of Activated Carbon for each 50 gallons of water, or 500 g of Activated Carbon for every 100 liters. You have to start by adding 20 g of Activated Carbon for every 100 liters of water on a monthly basis till you reach the 500 gr. /100 liters mark.

This is more or less a good ratio. This dose is suggested for reef aquaria but it is tested (and works fine) in several kinds of aquariums and different species. Of course, this figure is an average and should be used as such. An overstocked tank will need more Activated Carbon for the same volume.

Parameters affecting Activated Carbon performance

The adsorption process depends on the following factors:

  1. Physical properties of the Activated Carbon, such as pore size distribution and surface area the amount and distribution of pores play key roles in determining how well contaminants are filtered. The best filtration occurs when pores are barely large enough to admit the contaminant molecule,
  2. The chemical nature of the carbon source, or the amount of oxygen and hydrogen associated with it. The filter surface may actually interact chemically with organic molecules. Also electrical forces between the Activated Carbon surface and some contaminants may result in adsorption or ion exchange. Adsorption, then, is also affected by the chemical nature of the adsorbing surface. The chemical properties of the adsorbing surface are determined to a large extent by the activation process. Activated Carbon materials formed from different activation processes will have chemical properties that make them more, or less, attractive to various contaminants. For example, chloroform is adsorbed best by Activated Carbon that has the least amount of oxygen associated with the pore surfaces,
  3. The chemical composition and concentration of the contaminant Large organic molecules are most effectively adsorbed by Activated Carbon. A general rule of thumb is that similar materials tend to associate. Organic molecules and activated carbon are similar materials; therefore, there is a stronger tendency for most organic chemicals to associate with the activated carbon in the filter rather than staying dissolved in a dissimilar material like water. Generally, the least soluble organic molecules are most strongly adsorbed. Often, the smaller organic molecules are held the tightest, because they fit into the smaller pores. Concentration of organic contaminants can affect the adsorption process. A given AC filter may be more effective than another type of AC filter at low contaminant concentrations, but may be less effective than the other filter at high concentrations,
  4. The temperature and pH of the waterAdsorption usually increases as pH and temperature decrease. Chemical reactions and forms of chemicals are closely related to pH and temperature. When pH and temperature are lowered, many organic chemicals are in a more absorbable form, and
  5. The flow rate or time exposure of water to Activated Carbon The process of adsorption is also influenced by the length of time that the Activated Carbon is in contact with the contaminant in the water. Increasing contact time allows greater amounts of contaminant to be removed from the water. Contact is improved by increasing the amount of Activated Carbon in the filter and reducing the flow rate of water through the filter.

Characteristics: What to buy and what to consider

Usually good quality Activated Carbon remains active for a period of about six months. After this period organic compounds start to accumulate in the system as Activated Carbon is losing its activity. So we have to replace about 30% of it and at the same time wash the rest with aquarium water. The reason is that Activated Carbon works as a biological medium as well as a chemical one. If we replace the whole amount, the biological substratum of the system will be disturbed and the denitrification potential of the tank will be seriously affected (specially if we use a large quantity of Activated Carbon). We should add the fresh Activated Carbon in front of the old material, so nitrifying and denitrifying bacteria will colonize it quickly and the colonies that already exist in the old Activated Carbon will remain preserved. Rinsing 70% of the old Activated Carbon with aquarium water reduces clogging, while the biological substrate remains stable enough to remove excess of organics for several months.

A good and clever trick to determine when we have to replace the Activated Carbon in our system is to install in one side of the tank a piece of white plastic, half coloured with a yellowish, non – water soluble paint. When we cannot distinguish the faint yellow part from the white one when we view it from the other side of the tank it is time to replace the carbon in the system.

As indicated there are many ways to manufacture Activated Carbon, therefore all Activated Carbon are not made the same. We shall choose Activated Carbon for aquarium use only, with small grains, dull black and as dust ­free as possible. We have to rinse Activated Carbon with water to remove the excess of dust before using it. Some companies use phosphoric acid to increase the carbon’s surface, because acid creates more holes on charcoal. This Activated Carbon is good for air filtration but is totally unsuitable for aquarium use, even if it is thoroughly rinsed. It will still contain ash and phosphates in high levels. If we use it we will always wonder why we have algal blooms in our tank despite the fact that we replace it sooner than the sales man told us.

We can test our Activated Carbon with a phosphate test kit. In our test vial we add purified water until the marked level and we add the phosphate reagent as is prescribed for doing a phosphate test. Then we put some Activated Carbon grains in the vial (5 or 6 are fine). If the color of our supplement shows a blue tint or, even worse, becomes blue, then our carbon releases phosphates. Needless to say, such carbon is not to be used in a tank.

Activated Carbon has another drawback: it ages. When this happens, some of the substances that were absorbed previously will probably be released back in the water.

Some aquarists are relating Activated Carbon use with some strange, and sometimes unexplained effects like lateral line disease or head erosion and other ailments. Therefore, it is better to choose the carbon we use very carefully until new studies on this topic are published.

Is it all good? Reasons to consider using Activated Carbon in your tank

Activated Carbon filtration does not remove microbes, sodium, nitrates, fluoride, and hardness. Lead and other heavy metals are removed only by a very specific type of Activated Carbon filter. Unless the manufacturer states that its product will remove heavy metals, the consumer should assume that the Activated Carbon filter is not effective in removing them.

As with all known types of chemical filtration, Activated Carbon removes useful substances from the water (trace elements for instance) at the same time as it removes pollutants. This effect can be prevented with partial water changes and/or with simultaneously adding trace elements (e.g. iodine deficiency in reef aquaria, is critical to the well being of soft corals). However, this leads to a great uncertainty, since you can’t be sure how much of the useful stuff is removed (unless you check for every single substance) or how quickly it is removed. This is especially true for planted tanks (removal of fertilisers, chelated iron etc.) as well as special additions for the well being of the fish (like vitamins). In the end, you might end up adding more and more vitamins, trace elements and fertilisers until you saturate your Activated Carbon with your own stuff.

Bacterial growth: When the water passes through the Activated Carbon, some bacteria (primarily heterotrophic) are trapped in the substrate, too. The problem is that these trapped bacteria multiply prolifically in the warm, moist oxygen­containing environment among the grains of Activated Carbon. Because the Activated Carbon is loose, there is no barrier to keep any bacteria growing inside the filter from coming out with the water pushed through each time the filter is used. There have been extensive tests showing certain filters have thousands more bacteria coming out of them than initially went into them!

Fresh activated carbon will lower dissolved oxygen levels for a short period of time, and may, as a result, lower your redox potential as well.

Activated Carbon should be removed each time you have to medicate your tank (otherwise all the medicine will be absorbed by it). This may not be practical, easy or desirable.

There is a general trend to simplify things. The “Keep it Simple” approach has become the principle many aquarists believe in. In this context, the addition of one more (largely unpredictable) factor in a tank should be avoided.

Final Remarks

Some aquarists use Activated Carbon constantly, while others use it periodically. Some don’t use it at all. As we already discussed, there are pros and cons for using it – definite conclusions or suggestions can’t be drawn by us.

It is evident that in the end, the only person to decide if a particular kind of Activated Carbon is beneficial (and to what extent) to any particular system is definitely the hobbyist himself. Your personal observations concerning the appearance of your tank and the well being of your fish coupled to the knowledge of your system are the true guides for that.

The only worthy indication for the proper use of AC is the appearance of your water and inhabitants. The organisms kept in aquariums are more sensitive in water chemistry mishaps than any electronic gadget, so it is better to spend some time observing your system and the captive life kept in it, rather than playing the chemist with exotic instruments.

There are some so–called hobbyists that do not understand even the basics (not even what pH is) and in which ways they affect water chemistry and aquatic life, pro rata. On the contrary, sometimes their tanks are fully equipped with electronic equipment. Almost all of them believe that a well­equipped tank does not require any care; they do not even perform partial water changes. Take our word for it: the most important equipment for a successful tank is you.

There is no kind of chemical filtration (activated carbon, protein skimming, ozone, phosphate adsorbing media, etc), nor any other kind of filtration that can miraculously replace the need for your personal care for a tank. The more sophisticated the system, the more experienced the aquarist it takes. The more personal our involvement with our tanks, the more experienced we get. The more we inform ourselves, the better we use our experiences ­ on a practical level – for the better care of our tanks and fish.


  • MERCK Index, An Encyclopedia of Chemicals, Drugs and Biologicals, 11th Edition, Merck & Co Edtns, USA, 1989
  • Thiel, The Marine Fish and Invert Aquarium, Aardvark Press 1989
  • Moe, Marine Aquarium Reference: Systems and Invertebrates, Green Turtle 1989
  • deGraaf, The Reef Aquarium vol.1,J. C. Delbeek ­ J. Sprung 1994
  • Hovanec, All about Activated Carbon, Aquarium Fish Magazine 1993
  • Spotte, Seawater Aquariums: The Captive environment, Wiley ­ Interscience 1979

Webmaster’s note: This article appeared in the January 2002 issue of FAMA; reprinted with the editor’s permission.

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