Activated carbon is by far the biggest selling maintenance product in the aquarium industry and is one of the most effective absorbents currently used.
However, using activated carbon in fresh or saltwater aquarium is a controversial subject and opinions ranged from “never use it” to “can’t live without it”.
What is Activated Carbon?
When carbon is treated by either chemical (with phosphoric acid or zinc) or thermal (glassification or carbonization) reaction, all impurities are released which open up millions of tiny pores between the carbon atoms. This process gives the carbon a large surface area in contact with water. It can have anywhere from 300 to 2,000 square meters of surface area per gram. This forms the substance we call activated carbon which acts like a highly adsorbant sponge that is highly effective against organic chemicals. The types of activated carbon available today include granular, palletized, and powdered. Granular activated carbon is the one we usually see in the aquarium while pellets and powdered are used in other applications such as air filtration.
How does it work?
When certain chemicals in your aquarium pass next to the carbon surface, activated carbon attracts negatively charged chemical particles with its own positive charge. Negatively charged particles get trap in the porous structure of the activated carbon which inhibits them from re-dissolving back into the water. When the activated carbon is removed, the chemicals are removed with it.
What can be removed and what can’t?
It is important to note that there are certain substances that carbon will adsorb easily while others will not absorb in a significant amount.
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 (DOC), Dyes (such as Methylene Blue), Ethyl Acetate, Ethyl Acrylate, Foaming, 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, Calcium Hypochlorite, Chlorophyll, Citric Acid, 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, Tar Emulsion, Tartaric Acid, Xanthophyll
Poor or no absorption:
Ammonia, Calcium, Carbon Dioxide, Fluoride, Formaldehyde, Lime, Magnesium, Manganese, Microbes, Molybdenum, Nitrates, nitrites, Phosphates, Selenium, Sodium, Iron and other heavy metals
Note: Alkalinity and water hardness will not be affected by the use of activated carbon.
How much should I use, and for how long?
Because every aquarium is different and the quality of the activated carbon is different from a brand to another, it is almost impossible to recommend a specific amount of activated carbon.
Some activated carbon products give recommendations while others give no indication at all. For ongoing maintenance, I use 1 cup of activated carbon per 50 gallons and change it monthly. When I use a high grade activated carbons such as Tri Base Pelletized Carbon, I use half a cup per 50 gallons and change it every 3 months. Independent research has shown that “more is better” when using activated carbon. The greater quantity of carbon will work faster and longer than a lesser amount.
For Activated Carbon to work effectively, you should filter the water mechanically before it reaches the carbon.
Activated Carbon de-adsorption
We are often told that exhausted activated carbon will leach adsorbed substance back into the aquarium and cause problems. To avoid this, we are told to replace old activated carbon monthly. This assumption is actually wrong. De-adsroption can only be done by switching from one pH extreme (very acidic or basic) to the other pH extreme. These extreme pH values are way outside the normal range of aquarium so don’t worry about de-adsorption.
If you leave activated carbon too long without changing it, it will no longer be effective as an adsorbent and will instead become part of the biological filtration.
Medication And Activated Carbon
Activated carbon effectively remove medications from the water. So whenever you use medication, first remove any carbon in your filtration system. Once you are done with the treatment, put some carbon back in your filter to remove the medication.
Activated Carbon And Trace Element
A big problem with Activated Carbon is that it also removes some of the good things such as trace elements. Activated carbon actually has a greater affinity for organics than trace metals. Actually, the uptake of substances like iron, manganese, molybdenum, cadmium and zinc is extremely limited and is insignificant if used only occasionally. Saltwater aquarists should also keep in mind that protein skimming will remove much more trace elements than activated carbon.
Phosphate In Activated Carbon
The other problem is that carbon carbons can 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. If you have problems with phosphate, the best thing to do is to switch for another brand of carbon. One way to tell is to look at the product label. If the label talks about the carbon process of using oxygen, carbon dioxide or steam, then it is truly phosphate-free. If the activated carbon is simply marked “phosphate-free”, you can assume it was steam activated. If the label does not mention about the activation process and does not mention “phosphate-free”, ask the vendor or switch to the other brand.