Use of Activated Carbon in Freshwater, Marine Aquariums and Ponds

Updated 10/12/09

Carbon is primarily an adsorbent which is a very popular chemical filter media that is often misunderstood as to use in established aquariums and ponds as well.
A healthy established aquarium (fresh or salt) with regular water changes generally needs little carbon (although more carbon is generally needed in marine reef aquariums and less in low pH freshwater aquariums). Carbon will NOT remove or absorb ammonia, nitrites, or nitrates. Carbon is very useful in removing medications after treatment or even between doses.


Please read the entire article for a good understanding of what activated carbon can or cannot do for your aquarium/pond; including the table of what carbon can and cannot remove as well as the referenced resources
How Activated Carbon Works

Activated carbon has an extremely large surface area per unit weight, which makes AC an extremely efficient absorptive AND adsorptive material. The activation of carbon and its manufacture creates many pores within the particles, and it is the vast areas of the walls within these pores that accounts for most of the total surface area of the carbon. In water, activated carbon has a preference for large organic molecules and for substances which are non-polar in nature. The forces of attraction between the carbon and the absorbed molecules are greater the closer the molecules are in size to the pores. The best absorption takes place when the pores are just large enough to admit the molecules.
Activated carbon, when contacted with water containing organic material, will remove these compounds selectively by a combination of adsorption of the less polar molecules, absorption (filtration) of the larger particles, and partial deposition of colloidal material on the exterior surface of the activated carbon.


As well as absorption, activated carbon uses a process called Adsorption, in fact adsorption is the primary method of molecule removal by carbon, not absorption.
When a material adsorbs something, it means it attaches it by chemical attraction.
The extent of removal of soluble organics by absorption depends on the diffusion of the particle to the external surface of the carbon and diffusion within the porous adsorbent. For colloidal particles, internal diffusion is relatively unimportant because of particle size.
Organic substances that pass through the column consist of hydrophilic organic molecules (substances that are attracted to, and dissolve well within, water) and hydrophobic molecules (repulsed by water). If the molecule is “polar” (having both a hydrophobic and hydrophilic attributes) which many organic molecules are, the hydrophobic side will be attracted (attached) to the activated carbon.
Adsorption is partially the result of forces of attraction at the surface of a particle that cause soluble organic materials to adhere to the activated carbon. The limited water solubility of many organic substances will affect AC adsorption of these molecules.

Put more simply (I hope): Polar, hydrophobic and hydrophilic interactions are important interactions necessary to understand how activated carbon adsorb the certain molecules. A Methylene Blue dye molecule is hydrophobic and has a large affinity to the hydrophobic carbon rings of the activated carbon. The dye prefers to interact with the carbon rather than water. Where as non chelated metals (such as copper ions) are positively charged (hydrophilic), and the carbon is neutral and hydrophobic. Therefore, the positively charged metal ions prefer to interact with the water, which is hydrophilic. “Like dissolves Like”.
This applies to most metals from the periodic table, including calcium, magnesium, etc., and for this reason most essential minerals are not removed by activated carbon unless chelated or if they loose their positive charge due to oxidative processes of the Redox Potential (which is another reason to replenish these positive mineral ions, please see this article for further information: The Aquarium Redox Potential)
This positive/negative ionization is why DOC (organics) will also negatively affect the Aquarium/Pond Redox Balance


Common Uses
As already noted healthy aquariums often do not need much activated carbon, however this is a rather ambiguous generalization. An aquarium keeper need to consider DOC (dissolved organic compounds/carbon), pH, desired compounds that can or cannot be removed, bio load of the aquarium or pond, desired environment (an Amazon River aquarium will generally need less or no carbon while a heavily fed reef tank will need more).
I have noted performed a controlled study of how much carbon is best however I have made many observations over the years in freshwater and marine aquariums of different environments, as well as ponds. I would use tests of pH/KH (which will tend towards lower pH/KH without carbon use), nitrates (although carbon cannot remove nitrates, it can remove DOC that will eventually end up as nitrates), even TOC or Redox tests if available (Methylene Blue can be used a Redox indicator, please see this article for much more about Redox: The Aquarium/Pond Redox).
Simply gauging carbon use by tank water color, can be a simple method to base carbon use by (admittedly very unscientific though).
The use of carbon if only for a day or even hours after or between medication treatments is another important aspect of carbon use.

In Reef aquariums, the use of carbon (even though less effective in higher pH water) is important in my experience/opinion as part of a regimen that where carbon is but one piece of the aquarium maintenance puzzle that often includes Protein Skimmers (which also remove many similar DOC, but not as quickly after production), water changes, micron filtration, de-nitrification (with deep sand beds or products such as Matrix), and use of chemical absorbents such as Purigen.

Carbon can also be used in mixed products such as Ammo Carb, but zeolites are only for freshwater use. The use of zeolite/carbon mixes is especially useful in areas where municipal tap water contains chloramines (instead of the usual chlorine). I have also achieved good results with carbon/zeolite blends in ponds which are generally exposed to even more contaminants than aquarium.

A good starting point for carbon use (please note that this is a generalization) is one to three teaspoons per ten gallons of water. One teaspoon of activated carbon is equal to approximately 6 grams in weight measurement. This amount will vary greatly depending upon many other factors that the aquarium or pond keeper can determine. As well sometimes an aquarium keeper may choose to only use this amount of carbon as part of a clean up procedure (in particular freshwater aquariums) and then discontinue use after a day or two. If you have an Aquarium Cleaning Machine, I and others will only use carbon in this machine for freshwater aquariums and not during normal tank operation.

Lignite Activated Carbon

The best Premium Activated Carbon is produced from a soft, brownish-black coal (Lignite) in which the alteration of vegetable matter which has proceeded further than in peat but not as far as in bituminous coal. Lignite based carbon is the best choice for use in aquarium & ponds to remove organic molecules, pesticides and for color removal, due to its large pore size. The large pore size is important, because the organics in a aquarium or pond environment will clog and render ineffective, the smaller-pored, coconut shell and Testing has shown other Activated Carbons; such Large pelletized, to be almost useless for aquarium or pond water, but excellent for Chlorine and Heavy metal removal in clear water applications such as swimming pools or drinking water.
6 grams (.21 ounces) of Lignite Activated Carbon has the surface area of a football field, so a little goes a long ways in aquarium use in particular.

Here are a few aspects that impact the effectiveness of activated carbon

Chemical Properties;
The carbon surface may actually interact chemically with organic molecules. As well 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 formed from different activation processes will have chemical properties that make them more or less attractive to various contaminants.

Contaminant Properties:
Large dissolved organic compounds/carbon (DOC) are most effectively adsorbed by activated carbon. A general rule of thumb is that similar materials tend to associate. DOC 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 (such as large complex amino acids or fatty acids) are most strongly adsorbed. Often the smaller organic molecules (such as sugars) are held the tightest, because they fit into the smaller pores.
It is also noteworthy that although larger complex organic molecules (often nitrogen based) are more readily absorbed, these molecules are also not held as tightly and re-leased under certain conditions which is why carbon should not be relied on for the sole form of organic contaminant removal. Other methods such as Purigen, de-nitrifying filters, water changes, Protein Skimmers (marine aquariums), micron filters, UV Sterilizers, etc. should be employed as part of the mix.

Concentration of organic contaminants can affect the adsorption process. A given activated carbon may be more effective than another type of activated carbon material at low contaminant concentrations, but may be less effective than the other carbon material at high concentrations.

Water Temperature and pH;
Adsorption 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 (this is noteworthy for marine/saltwater use and why Protein Skimmers are also important as these devices will remove DOC as well, although not immediately as carbon can)

Exposure Time;
The process of adsorption is also influenced by the length of time that the 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.

There is controversy in what essential minerals carbon will absorb or what activated carbon will or will not absorb in general. I will state based on my own experience and scientific evidence that carbon has many uses in aquariums/ponds but is also over used or incorrectly recommended. Although I use little carbon in my established healthy aquariums and ponds, I disagree with those that state it should not or rarely be used (based on some false assumptions of what carbon removes or adds to water). On the flip side I also disagree with those that make carbon the answer for water quality issues such as nitrates for which carbon does not remove.
Activated Carbon is very useful for removing most medications after or between treatments (this is where I strongly recommend its use), although even here, carbon does not remove most copper formulations effectively.


Possible Concerns with Carbon Use

*Activated carbon can foster the growth of bacteria by concentrating other organics (such as DOC) on its surface.
Although I have not performed controlled tests to confirm this, I have made many observations over the years that increased use of carbon has coincided with increased incidence of bacterial infections such as Aeromonas, especially in lower pH Amazon River & SE Asia water aquarium environments since Redox is also generally less favorable and the removal of tannins from products such as Indian Almond Leaf Extracts allows these bacteria to thrive.
The fact that activated carbon removes oxygen, further increases the risk of an opportunistic Aeromonas Bacterial outbreak since these bacterium are anaerobic.

*Use in planted freshwater aquariums:
This is an area of some controversy of which some information is based on facts, some information is not, with some reasonable questions in between.

The main controversy I will address for now as to carbon use in planted freshwater aquariums is the removal of trace minerals. I read some experiments at “the Krib”, as well I have made observations and tests (as well as research) over the years myself.
The main testable point is that most metals such as Iron (which is important for plants) are NOT absorbed carbon with an important and noteworthy exception; and that is the use of chelation. EDTA (which is an organic molecule) is used to chelate many metals such as iron to make it more readily available for fertilizers or other uses, and since activated carbon is especially effective in removing organic carbon based molecules, these chelated metals are then removed. Any aquatic plant fertilizers that contain chelated metals will be bound to the carbon pores, and as result their concentration into the water column will get lower with the use of activated carbon. If the carbon is left in the aquarium for a period of time, the chelated compounds in aquariums slowly decay and release their metals.

However not all trace elements are chelated, for instance SeaChem Flourish uses water soluble non-chelated iron, as well mineral blocks such as Wonder Shells are non chelated and any possible absorbed trace minerals are rapidly replaced by the Wonder Shell (which although mineral depletion by activated carbon is low, the use of Wonder Shells in aquariums/ponds utilizing activated carbon insures adequate minerals)

The use of Activated Carbon with Marine Protein Skimmers:
Although conclusive tests are forth coming, there is evidence that the use of activated carbon can limit the amount of foam refraction generated by a marine protein skimmer. This is likely due to the adsorption of Foaming Agents (MBAS) by activated carbon.
This presents a problem for many reef keepers since both carbon and protein skimmers are useful aspects of a complete marine filtration system.
My suggestion is to limit carbon use in cleaning filters run during certain times of the day (or week), especially after heavy feeding.

*A final concern with activated carbon is the possible release of contaminants after they have been initially adsorbed. This action is known as desorption or dumping. This could occur if other ambient water quality characteristics change.
Although at the time of writing this article, I have not discovered the exact mechanism for causing this, but I do know that a tank that is not stable in its general chemistry, whether pH or Redox is a candidate for this possible problem of carbon use.


Here is a list of compounds carbon can or cannot absorb.

Please note that some compounds may be desirable to remove depending upon your tank requirements while the same compound may not be desirable in another aquarium environment. A good example would be tannins (which carbon removes reasonably well); in a soft water environment an aquarist would likely want to restrict the use of Activated Carbon, while someone keeping “dirty” goldfish may find the use of carbon on a regular basis (in higher quantities) a necessity.

WHAT CARBON CAN ABSORB:
Excellent Absorption:	Fair/Good Absorption	WHAT CARBON CANNOT ABSORB (or absorption is poor)
*Amyl Acetate *Amyl Alcohol *Benzene *Bleach *Butyl Alcohol *Butyl Acetate *Calcium Hypochlorite *ORGANIC carbon *Chloral *Chloroform *Chlorine *Chlorobenzene *Chlorophenol *Cresol *Defoliants *Diesel Fuel *Dissolved Organic Compounds *Dyes (such as Methylene Blue) *Ethyl Acetate *Ethyl Acrylate *Foaming Agents (MBAS) *Gasoline *Glycols *Herbicides *Hydrogen Peroxide *Hypochlorous Acid *Insecticides *Iodine *Isopropyl Acetate *Isopropyl Alcohol *Ketones *Methyl Bromide *Methyl Ethyl Ketone *Naptha *Nitrobenzene *Nitroluene *Odors (general) *Oil - dissolved *Organic Esters *Oxalic Acid *Oxygen *PCB's *Pesticides *Phenol *Sodium Hypochlorite *Toluidine *Trichlorethylene *Turpentine *Xylene	*Acetaldehde *Acetone *Alcohols *Antifreeze *Chloramine (only the Chlorine, zeolite needed for remaining ammonia) *Calcium Hypochlorite *Chlorophyll *Citric Acid *EDTA (an organic chelator of metals such as iron) *Ethyl Alcohol *Ethyl Amine *Ethyl Chloride *Etyl Ether *Lactic Acid *Mercaptans *Methyl Acetate *Methyl Alcohol *Methyl Chloride *Organic Acids *Organic Salts *Ozone *Potassium Permanganate *Propioc Acid *Propyl Acetate *Propyl Alcohol *Propyl Chloride *Radon *Solvents *Sulphonated Oils *Tannins (such as Indian Almond Leaf extract) *Tar Emulsion *Tartaric Acid *Xanthophyll	*Alkalinity *Calcium *Carbon Dioxide *Fluoride, *Formaldehyde *Hardness. *Lime *Magnesium *Manganese *Microbes, *Molybdenum *Nitrates, nitrites, ammonia *Phosphates *Selenium *Sodium, *Lead, Iron and other heavy metals are removed only by adding a chelation process using EDTA, an organic carbon molecule (then these metals can be readily removed)

References

*http://www.ci.epping.nh.us/MTBE%20Water%20Supply%20Engineering%20Fact%20Sheet.htm
*Activated Carbon For Pure Water
*http://www.ag.ndsu.edu/pubs/h2oqual/watsys/ae1029w.htm
*The Krib; Activated Carbon
*http://www2.hawaii.edu/~delbeek/reefaq3.html
*http://cnsi.ctrl.ucla.edu/nanoscience/pages/waterFilt-teach
* http://infotrek.er.usgs.gov/