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Revisiting Activated Carbon
by Dr. Timothy A. Hovanec


First, a little vocabulary. Not everything sold in stores for use in aquarium filtration is activated carbon even though it might be black and it might even have the name activated carbon, or a close variation, on the box. Boxes of cheap black shiny bits of material are probably anthracite coal and have little to no adsorption capability.

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While there are standards for activated carbon, they are more informational than legal; companies do not have to meet certain criteria to label their product activated carbon or similar. Activated carbon (or just carbon for this article) can be made from almost any organic based material such as wood, coal, peat, coconut shells, and many other things. Activating the material, which increases its internal surface area and so to adsorption capacity, it is done through special processes which involve heating the material to nearly 2,000 (F in a controlled environment. A substance becomes activated carbon by undergoing the controlled process of increasing its internal surface area, so as to increase its ability to remove certain substances from the water (or air). The process of the carbon removing a substance from the water is adsorption, not absorption.

Activated carbon removes a class of material called dissolved organic compounds (DOC). These compounds are carbon based and are produced by many different processes in the aquarium. The two major categories of organic substances that carbon removes are tannins and phenols. Tannins give the water a yellow-brown color. They are mainly produced by the breakdown of plant material but there are other sources too. Phenols give a fish tank that characteristic "fishy" smell. Carbon can also remove chlorine and some heavy metals through other processes.


Perhaps the biggest difference between 1993 and 1998 is that there are very few coconut-based carbons available now. In the late 1980's and early 1990's, carbon made from coconut shells was introduced to the hobby. It was the subject of much marketing hype and every magazine had advertisements with many claims about its superior ability. Today, the ads have disappeared as have most coconut carbons from the store shelves.

It took awhile, but the drawbacks of using coconut carbon (which were detailed in the 1993 article) in the aquarium finally merged with the markets forces, and sales of this type of carbon dropped. The reason that coconut carbon failed is mainly that coconut carbons have the wrong pore size for filtering the target materials from water. Coconut carbon is mainly for filtering airborne materials.


When a carbon-based material is considered for conversion to activated carbon, test runs are performed to get an idea as to the internal pore structure of the material after activation. The final performance of a carbon is based upon the types and number of internal pore sizes, the internal surface area and percent of ash in the carbon. The most important determining factor is pore structure.

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In the case of coconut-carbon, due to the coconut shell structure when it is activated, coconut based carbons have only very small pores when created. These are called micropores which are classified as pores with a size less than 40 angstroms. The problem is the DOC in the water that the carbon is to remove are much bigger than 40 angstroms. So even though coconut carbon has a large internal surface area, the pollutant can't come in contact with most of it because it does not fit inside the pores. Therefore, the activity of coconut carbon is relatively poor in water.

Carbon made from peat is at the opposite extreme compared to coconut carbons. When processed into activated carbon, peat forms pores that are large, above 5,000 angstroms, and called macropores. The large pore size permits the water pollutants to enter the carbon particle where they are adsorbed. Thus, peat is a better GAC than coconut based materials for water use. However, the large pores cause a large reduction in the internal surface of carbon compared to others which means a loss in adsorption potential. Thus, while peat does work in water, it is not the best carbon available.

The best carbon for use in water filtration to remove the dissolved organics is bituminous coal-based carbon. Carbon from this base material contains a large amount of pores that are between 40 and 5,000 angstroms (called the transitional pores). Bituminous coal based carbons have the right mix of pore size and internal surface area such that they remove more pollutants per gram of carbon than any of the other types of base materials. Generally, carbon from bituminous coal will be slightly more expensive than others but they remove far more material. Thus, they are actually more economical--the carbon of choice in aquaria is bituminous coal based.


However, as always seems to be the case, not all bituminous coal based carbons are the same. Another new development since 1993 is the importation of activated carbon made in China. Previously, most of the sources of carbon were in the United States (domestic carbon) with some imported from Germany and other parts of Europe. However, nowadays China is becoming a major supplier of carbon. There are many sources of coal in China and the prices paid in the United States for activated carbon make it economical for Chinese activated carbon to be shipped here. Chinese carbons vary widely in quality; what makes it attractive to many companies is that it can be much cheaper than domestic carbon. Unfortunately, in this case cheaper generally means inferior in quality so that the end-user is required to use twice as much, or more, to accomplish the filtration of one unit of domestic carbon. Thus, the Chinese carbon is not really cheaper to use. Few, if any companies, label the source of their carbon, so it can be hard to tell domestic bituminous coal based carbons from Chinese ones. Most of the larger aquarium product companies sell domestically produced activated carbon. However, a guide is that the Chinese carbons tend to be much duster than domestic carbons and more shiny. This is not 100% accurate, but it is a tendency.


The use of carbon substitutes has increased since 1993. There are a few products available which remove some of the same compounds (namely DOC) that activated carbon does. The effective ones are ion exchange resins, the ineffective ones are those that look like pebbles or pea gravel. The ion exchange resin substitutes can be good, but they clog rapidly if you don't use good mechanical filtration and they are expensive. Also due to their small size, they can pack tightly together forcing the water to go around them instead of through them. If you use these make sure to stir them up once a week so they remain effective.


Another change from 1993 is the use of GAC in reef tanks. In the earlier years of reefkeeping many of the "gurus" said, and wrote, that carbon should not be used in reef tanks for a number of reasons. Today, most reefkeepers, even the well-known ones, use some form of GAC in their tanks--some even sell their own mix of carbons. What was the thinking behind the concern of using GAC in reef tanks? The answer is trace elements. It is repeatedly stated that activated carbon removes trace elements from water. And, in fact, the statement is true but it is not complete. It is half of a statement and therein lies the problem. The complete sentence should be: activated carbon can remove some trace elements from water depending upon their concentration and, most importantly, the pH of the water. I'll explain.


First, what are some the trace elements in seawater? By most definitions trace elements are those elements found in the ocean with a concentration of less than 1 ppb. These are elements such as cesium, chromium, cadmium, selenium, cobalt, silver, lead, tin, helium, lanthanum and cerium. It has never been shown that most of these elements are really needed in the aquarium. In fact adding some to the water would really be poisoning the tank. However, in the aquarium hobby most lists of trace elements contain copper, iodine, iron and molybdenum. These really called minor elements.

In any case, whether a minor or trace element, most can have different chemical forms in water and not every form can be adsorbed by carbon (in fact most cannot). The form depends upon the pH of the water. It turns out that for most elements, the chemical form that carbon can remove is only present when the water pH is either below 3 or above 10. These are hardly pH conditions found in an aquarium. Thus, carbon as used in the aquarium environment will not remove most of the trace elements present in sea salt mixes or additives.


Phosphorous is another element linked to activated carbon. Some carbons leach phosphate into the aquarium water. The phosphate can be a naturally occurring part of the carbon or it can be from phosphoric acid which is used in the activation process. In either case, the phosphate is not toxic, rather it can contribute to eutrophic conditions in the aquarium water and may lead to algae blooms. If you are concerned about phosphates, switch brands of carbon. There are several bituminous coal-based carbons available from reputable companies that are phosphate-free.


De-adsorption is another phenomenon that is over-stated in the rumor mills about activated carbon. Again, it is an incomplete statement that is commonly used to described the process. It goes, in one fashion or another, as: don't use carbon because once its adsorption sites are full it will release, or de-adsorb, all the stuff it has adsorbed releasing a large amount of pollutants back into the aquarium. The implication in this sentence that activated carbon works something like a capacitor such that once at its maximum adsorption capacity, it instantaneously discharges all the bad things it has adsorbed is wrong. Carbon does de-adsorb, in fact, that ability is exploited for recycling precious metals. However, in a controlled industrial process, the quick release of the target substance is accomplished by switching the pH of the water. The basic process is to capture the target substance at one pH extreme (very acidic or basic) and then reclaim the substance by switching to the other pH extreme. As stated earlier in this article, these pH values are outside the normal range of aquaria. De-adsorption is not a process to be worried about.


Carbon has also been implicated as the cause of Hole-in-the-head disease and lateral-line-erosion. One theory has the carbon breaking into microscopic particles which get lodged under the scales of the fish and infects them with a virus or bacteria. There have been no published studying confirming this or any other disease caused by GAC. Further, there are many, many fish in tanks with carbon that don't suffer from these maladies. From personal experience, with both freshwater and saltwater fish, the cause of these diseases seems to be related to diet, as fish with these problems can be ''cured" by the proper diet.


Sometimes putting fresh carbon in your aquarium can raise the pH as the carbon can adsorb carbon dioxide and change other characteristics depending upon the water chemistry of the aquarium water and the type of carbon. Usually the pH change is temporary, but how long depends upon the alkalinity of your water. To minimize this, it is best to rinse the carbon under tap water for a minute before placing it in your tank. This will reduce the magnitude of any changes.


Activated carbon should be part of every aquarium filtration system. The removal of dissolved organic compounds from the water will increase the effectiveness of the biological filter, make the water look cleaner because it will be clearer, and eliminate smells. For most filters, GAC comes conveniently pre-packaged so that it can be easily removed from the filter and replaced. The most efficient method is to get as much of the water to go through the carbon on each pass through the filter. This can be done by spreading the carbon out in a thin layer rather than letting it pile up in a bag.

Common usage questions about carbon are: how much carbon do I need in my tank, how often should I change it, and how come power filters don't have much carbon? Only through trial and error can you accurately determine how much carbon your tank/filter needs and how often you should change it. Every tank is different so guidelines are, at best, guesses. However, for most hobbyists the factor which will determine how much carbon they can use at one time is their filter. The most common filter, the power filter, does not contain a lot of carbon in their chemical filtration section compared to canister filters for a necessary reason. These types of filters work by gravity. Water is pumped up above the tank water line into a box which hangs on the back of the aquarium and flows over a weir in the box back into the tank. The carbon is placed in the flow path of the water using a cartridge or similar device. The more carbon in the cartridge, the greater the resistance to the water flowing back into the aquaria. If too much carbon is used, the water will overflow the top of the filter box and spill onto your floor. The overriding design criteria for today's power filters is convenience. The older style power filters which used a siphon to get water into the filter box could hold a lot more carbon, but it was not easy to change. The best advice for power filter owners is to change the carbon once a month and clean the filter pad every few weeks.

For most aquarists, if your tank is not overcrowded or overfed, then changing the carbon once a month is probably also sufficient. If you have big fish or a lot of fish, then you might have to change the carbon every 2 or 3 weeks. Only by keeping an aquarium logbook and noting when you change the carbon and the conditions of the tank before the change (water color, smells, etc.) can you accurately predict the correct amount of carbon required for your situation.

To keep your carbon working longer, make sure to mechanically filter the water before it reaches the carbon as this will keep the carbon from clogging with debris. A good practice is to rinse the carbon under water for a few seconds each week to get rid of particulate material. This will keep the water paths open and help you get the full benefit of the carbon.

Lastly, carbon is not a substitute for water changes--nothing is. Carbon is a part of a filtration system and maintenance regime that when used regularly, will result in less work for you and healthier, longer-lived fish.

© 1998 Dr. Timothy A. Hovanec
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