An Overview of Activated Carbon in the Marine Tank

By James R. Layton


The use of activated carbon in Marine aquariums has always sparked debate. In the past opinions ranged from “never use it” to “never be without it.” Today most aquarists consider activated carbon a beneficial and necessary component of their filtration system. Although many marine aquarists use activated carbon few know what is being removed or how carbon is beneficial to the marine aquarium. Then the manufacturing processes will be described and its effects on the finished carbon product. The filtration or sorption mechanism will be discussed in addition to factors affecting activated carbon performance. Lastly techniques for the use of carbon products is provided as well as a “consumer checklist” for evaluation and selection of activated carbon products for use in marine aquaria.

Organic Pollutants in the Marine Aquarium.

The world’s reefs and oceans maintain a perfect balance of metabolic “waste” materials and nutrients via a series of recycling systems. The marine aquarium however, is quite different than nature relying on synthetic sea salt, artificial lighting, frozen foods, and an extremely high specimen to water ratio. While inorganic ammonia and nitrite are easily managed with “biological filtration” many organic compounds tend to accumulate resisting microbial degradation. These natural metabolic compounds remain largely unidentified but include organic acids, phenolics, proteins, carbohydrates, hormones, and antibiotic compounds. Few of these organics are directly toxic to marine specimens but can stimulate the growth of heterotrophic bacteria thus increasing demand for oxygen in the aquarium, producing carbon dioxide, lower pH, and lowering redox potential. Excess organics tax ozonation and foam fractionating systems. Certain organics that tint aquarium water yellow also reduce light penetration especially Actinic (420nm) type lighting. It is difficult to determine exactly which organic compounds are present in the aquarium and what specific effects they might have on different organisms. It had been observed that “organic laden” aquariums experience more disease problems and reduced fish growth while invertebrates close or cease reproduction. Some researchers believe that there is a direct relationship between high levels of organics and dense populations of disease organisms in aquaria. Reduction of naturally occurring organic compounds ultimately leads to improved water quality and healthier specimens. Activated carbon filtration is one of the most effective and easiest methods of removing organics from aquarium systems.

Manufacturing Processes of Activated Carbon.

Many natural substances of base materials are used to make activated carbon. The most common of these are wood, coal, lignite, and coconut shell. The base material is first subjected to a heating process called carbonization. This initial treatment forms a fixed carbon mass full of tiny pores. The carbonized base material is then activated by a second heat - steam treatment (200-1600 C) while regulating oxygen and carbon dioxide levels. Activation creates a fast internal pore network and imparts certain surface chemistries (functional groups) inside each particle. Thus activation gives carbon its unique filtering characteristics. The carbon product may be supplied as granular activated carbon (GAC), powdered activated carbon (PAC), or in pelleted form (compressed PAC). Some carbons are activated or washed with phosphoric acid, zinc chloride, or potassium hydroxide. These chemically treated activated carbons are unsuitable for use in the aquarium. These products could leach phosphate (an algae promoter), heavy metals, or alter pH.

The Sorption Process: How Activated Carbon Works.

Activated carbon removes organic compounds from aquaria by adsorption and absorption principles. Both processes involve the transfer of the adsorbate (pollutant) from the liquid phase (water) to the solid phase (carbon). Adsorption is the primary sorption mode relying on electrostatic Van der Walls forces. This attractive “force” forms relatively weak bonds between the carbon and adsorbate. In theory activated carbon could release or desorb what it removed at some point. But practical experience with aquarium filtration and laboratory experiments show desorption rarely occurs or causes any type of “toxic release”. Bacteria readily colonize the outer surface of the activated carbon and consume some of the sorbed organics. The bacterial action reactivates a small portion of the carbon and perhaps prevents desorption.

Absorption refers to the diffusion of a gas or compound into the porous network where a chemical reaction or physical entrapment take place. Ozone for example is absorbed into activated carbon where it oxidizes a portion of the carbon’s surface. Ozone (O3) is reduced to oxygen (O2) thus “detoxified” and made safe for the aquarium. Ozone does not accumulate or build-up in the carbon structure. A third process called chemisorption forms an irreversible chemical bond between the carbon surface and the adsorbate. Pollutants are tightly bound to the sorbent.

All three sorption processes occur simultaneously in the aquarium. The sorption process takes place in three stages:

1) Organic laden water contacts the activated carbon particle.

2) The adsorbate diffuses into the porous network.

3) Sorption onto the carbon occurs.

The sorption process has been described as the activity observed in a parking lot. Vehicles (organics) are moving freely on the main highway (aquarium water). Gradually vehicles enter the lot (pore) in search of a parking space (sorption site). As the parking lot becomes filled the sorption rate slows down. Sorption of large organic compounds takes longer than smaller compounds. The sorption rate is also influenced by water temperature, pH, and salinity, but these factors will not be discussed since they are “constants” in the marine aquarium.

Factors Affecting Activated Carbon Performance.

Over 100 activated carbon products are available. The activated carbon must match the application to obtain the highest water quality. Carbon products are classified primarily as air phase or liquid phase materials. Air phase carbons are used to purify air or gas streams such as air conditions systems and gas masks. Air pollutants are typically smaller, low molecular weight compounds, therefore a microporous activated carbon is the most efficient sorbent. But an air phase carbon has a low efficiency in the marine aquarium. The tiny pores (15 Angstrom) are too small to permit sorption of high molecular weight organics. A macroporous liquid phase activated carbon has a pore structure large enough (30 Angstrom) to remove typical aquarium pollutants. The base material greatly influences the pore diameter of the finished carbon product. Coconut shell typically creates a microporous activated carbon product. Coconut shell carbons are commonly used for the neutralization of chlorine in tap water. Chlorine is absorbed into the carbon where it oxidizes the surface and is neutralized. The disinfection processes such as chlorination create a variety of organic by-products or “surrogates.” These “smaller” organics are efficiently removed by wood and coconut shell carbon. Lignite, a form of coal, is used to produce macroporous activated carbon. Wood based activated carbon is an intermediate material having slightly large pores (25 Angstrom).

Other parameters are often used to market carbon products. Total Surface Area (TSA) is a measure of the surface area (M2/g) available for sorption. It is important to realize that a carbon product may have a relatively high TSA and yet be “locked up” in pores too small for sorption to occur. A measure of microporosity is the Iodine Adsorption Number. An iodine number of 1000 or higher indicates a highly microporous activated carbon. A microporous activated carbon will also have a relatively high TSA. The Molasses Number is a measure of macroporosity. A Molasses Number of 400 or higher indicates a macroporous carbon. Some high performance carbon products have a Molasses Number of 1000. As pore size increases, TSA decreases. Both factors must be considered when examining carbon specification sheets.

On occasion Hardness and Abrasion numbers are given. These figures are useful only if the carbon is going to be handled repeatedly as in reactivation processes. Large scale water treatment plants sometimes reactivate their exhausted carbon. In this case a tougher activated carbon is desirable to resist break down. The marine aquarist cannot reactivate carbon at home. Reactivation requires very high heat in a controlled environment to restore the sorption sites.

Particle size can be important in some filtration systems. Small carbon pieces pack together and reduce flow rates through canister filters. But this reduced flow if not too restrictive will increase contact time for sorption processes. Most aquarium carbon products range in size from 1.4mm to 4.75mm in particle size. Crushing a spoonful of carbon into a powder will not create more surface area. It will expose more of the surfaces already contained in the carbon. This will increase the rate of sorption but not the amount of pollution removed. Thus a poor activated carbon cannot be improved by crushing into smaller particles.

Ash is the trace inorganic material left behind after the activation process. The chemical nature of the ash varies with the type of base material and fluctuates even with the same type of carbon. Iron and calcium oxide are common ash constituents. Water soluble ash will be leached out of the carbon by rinsing or soaking the carbon in water. Quality activated carbons are acid washed to remove most of these inorganic “leftovers.” Unfortunately many carbon products are washed with phosphoric acid.

It would appear that the selection of activated carbon products can be made by comparing specification figures. Unfortunately this is not the case. In _Seawater Aquariums_ Spotte (1979) sums up activated carbon selection with the following quote: “There are no valid theories that allow selection of the best activated carbon in any single case without experimentation.” Under laboratory conditions we have observed that two brands of activated carbon with similar specifications give vastly different sorption rates and capacities. Some highly priced “marine” carbons are less effective than lower priced “all purpose” products. One of the most used (and abused) evaluation techniques is the removal of a dye from water. All this test indicates is the ability of various carbons to removed a particular dye such as methylene blue. One filter cartridge manufacturer designed a test showing how well their cartridge removed malachite green from a jar of water compared to other brands. We found that their product did remove more malachite green than all other brands. But the removal was caused by the polyester cartridge’s unique ability to bind with malachite green, not the activated carbon in the cartridge. Even if all the carbon was removed from the product it still outperformed all other competitors! But this test worked only for malachite green. This same cartridge tested on any other dye or naturally pollutant gave the poorest sorption rate and capacity of all filter cartridges tested. Several important lessons are illustrated here. Aquarists use activated carbon to remove a variety of natural organic compounds from saltwater aquariums. A carbon product must be evaluated using these same pollutants under aquarium conditions in addition to the lab bench. Removal of a single pollutant is not an adequate test of a carbon product. This often results in invalid product claims and aquarists dissatisfaction with product performance. Scientific studies must include adsorption isotherms: the relationship between the amount of substance sorbed and its residual concentration in the surrounding water. Chemical oxygen demand (COD), biological oxygen demand (BOD), and total organic carbon (TOC) are also invaluable tools for analysis. Lasers may be used in the future to help evaluate water quality and the performance of activated carbon in aquaria. Marine aquarists do not have the laboratory facilities necessary to conduct sorption studies. The aquarists must rely on advertisements and recommendations by aquarium shops and friends. Some aquarium books promote certain brands or types of carbon, mainly because the author(s) sells that type of carbon product rather than impartial scientific study or review of current filtration literature. Aquarists must be wary of the assumption that all expensive carbon products are superior to lesser priced items. To be sure quality activated costs “more” but price alone does not guarantee good performance.

Nine activated carbon products were tested for phosphate contamination. Five of these contained phosphate, including so called “marine” carbons. Another misconception is that “good carbon fizzes” or releases bubbles when placed in water. some of the most active carbons release no bubbles at all. Ability to “fizz” is not considered an indication of quality by waste treatment engineers or the activated carbon industry.

The marine aquarist can evaluate several brands of activated carbon in the home aquarium. Each brand is used in the aquarium until the aquarist “feels” it needs to be changed. Observations should be recorded such as water clarity and color as well as foaming in the foam fractioner. pH levels should remain relatively stable. The same volume of activated carbon must be used in each test. If Brand A was used at one cup/55 gallons then Brand B must be used at the same rate. Activated carbon is used by volume rather than by weight, aquarists simply fill up their carbon bag or canister filter. Weight is not a consideration in aquarium testing. The higher grade carbon products should keep the water cleaner and permit better light transmission by removing the organics. Many reef aquarists can judge their water quality (organic loading) by the appearance of certain invertebrates. The particular species will be different in each aquarium but in general the specimen closes up, shrinks, or looks “poor” when water quality declines. A reduction of organics via fresh carbon, a water change, or new air stones in the foam fractioner usually perks up the “indicator species”. It must be stressed that in all tests the aquarium population and feeding regime must be equal. Do not test Brand A with six fish, then add several pieces of live rock, then proceed to evaluate Brand B. Obviously the organic load is increased, invalidating all subsequent data.

Using Activated Carbon in the Marine Aquarium

The principle of sorption requires aquarium water to come in contact with the activated carbon. As contact is increased sorption also increases maximizing removal of pollutants. Some water treatment plants add powdered activated carbon to a large agitated vessel of water. Sorption (purification) is rapid because of the small particle size and optimal contact with the adsorbate. Optimal contact is essential when using this “once through” methodology. Water has only one “chance” to be purified as it passes through the treatment process. Tap water filters are another example. Once the water exits the filter it cannot be “put back” and refiltered if some impurities were not removed. Aquarium filtration employs the dilution principle. A small portion of the total aquarium volume is removed, filtered, and replaced on a continuous basis. The idea is to remove pollution faster than it accumulates in the aquarium. This is why filter manufactures make different size filtration systems, they are matched to the volume of the aquarium. If the filter is too small or the flow rate too slaw the pollutants build up faster than the filter can dilute them. As activated carbon becomes exhausted the sorption rate slows down and organics begin to accumulate. Replacing the activated carbon with fresh material continues the purification process.

Ideally aquarium water should be prefiltered before contact with activated carbon. Prefilters reduce the amount of particulate matter captured in between the carbon particles. Canister filters often provide an activated carbon chamber with prefiltration capabilities. Special carbon contractors maximize contact time and allow for floss fiber as a prefilter. Contrary to some authors, bags of activated carbon placed in the water flow work quite well. Laboratory studies have shown that bags of carbon or resins can remove substantial quantities of organic pollutants, medications, water hardness, and heavy metals. Actual performance depends on the flowability of the bag material, sorbent partical size, and amount of sorbent in the bag.

Aquarists often ask how much activated carbon should be used in the aquarium. Some carbon products give recommendations while others give no indication at all. Independent research has shown that “more is better” when using activated carbon. When filtering municipal water or aquarium water a greater quantity of carbon will work faster and longer than a lesser amount. A rough guide would be two U.S. cups (480 c.c.) per 55 gallons (280 L.) of aquarium water. Some aquarists use more or less depending on their filtration system and quality of the carbon product they use. Most carbon products last about six weeks in a marine “fish” aquarium. Reef aquaria produce more organics than a regular aquarium and may require more frequent replacement. Activated carbon cannot be reactivated by boiling in water or heating in an oven, the temperature is too low to destroy the sorbed pollutants and restore sorptive capacity.

Ozone Neutralization

As mentioned earlier, activated carbon is used to neutralize ozone in marine aquariums. Ozone gas oxidizes the surface of the carbon particle reducing the ozone to oxygen. Excess unreacted ozone is released to the atmosphere through the “skimmer cup”. An activated carbon impregnated pad cut to fit on top of the cup instantly neutralizes ozone and prevents its accumulation in the home. The carbon pad turns white as the activated carbon is destroyed in the neutralization process.


Most aquarium medications are readily sorbed by activated carbon. Carbon products must be removed while treating with antibacterial drugs and antiparasitic chemicals (formlin, malachite green, copper sulfate.) Use fresh carbon to remove the medication after treatment is completed.

Trace elements.

Some marine aquarists worry that activated carbon depletes the aquarium of “trace elements”. While carbon has the potential to sorb certain metals considered trace elements in seawater, several factors must be considered. Activated carbon has a much greater affinity for organic compounds than metals. Foam fractioners (protein skimmers) and ozone “remove” substantial quantities of trace elements as does the metabolism of all the specimens in the aquarium. The benefits of activated carbon filtration, protein skimmers, and ozone far outweigh the possibility of trace element removal. There are many trace element additives available that replenish the “Essential elements” removed by algae, fish, and invertebrates as well as the filtration equipment necessary to maintain these specimens in captivity.

Conclusions: Selection of a Carbon Product.

As we have seen activated carbon is an important part of the marine aquarium filtration. Removal of organic “pollutants” increases water quality and promotes the health of marine specimens. Not all activated carbon products are equal in performance. So-called marine grade activated carbons may not be the most efficient or cost effective sorbent for the aquarium. Use this check list as a “starting point” when selecting carbon products for the marine aquarium”

1) No chemical activation or washing with phosphoric acid, zinc or hydroxides.

2) Macroporous structure: large pores of 30 Angstrom or above.

3) Low Iodine Number: below 600

4) High Molasses Number: above 400

The most important factor is product performance in your aquarium. All aquarium carbon products claim to be the “finest available”. Contact the aquarium product manufactures for information then begin testing to see which product is really “the best”.