Activated Carbon in the Aquarium

Activated Carbon. Photo from Amazon

Activated carbon has been used in home aquaria for decades, and is the largest selling filtration media product. As new types of filters and media have become available, the debate has raged over the value of using activated carbon in filters. Some believe it should be used as a standard media for continuous use in most filters. Others feel it should only be used in for special needs, and still others believe that activated carbon should never be used.

It is important to remember that carbon is exhausted relatively quickly. For that reason, if the choice is made to use activated carbon on an ongoing basis, it should be replaced regularly. Otherwise, it is of little benefit.

What is activated carbon?

Activated carbon is carbon that has been treated in such a way that creates a large number of tiny pores, greatly increasing its surface area. This massive surface area allows it to absorb a large volume of material, making it useful for removing pollutants from both air and water. Different methods of creating activated carbon result in different forms, which are geared for a variety of uses. In aquariums, the form used is GAC, or granular activated carbon. Forms of activated carbon include:

  • BAC, or bead activated carbon
  • GAC, or granular activated carbon
  • EAC, or extruded activated carbon
  • PAC, or powdered activated carbon (also available in compressed pellet form)

There are also different sources for the carbon itself, each resulting in different possible pore size. Materials such as coal, coconuts, peat, and wood are all sources of carbon used to create activated carbon. For aquariums, the best source is bituminous coal.

What Activated Carbon Does

Activated carbon adsorbs a number of dissolved contaminants such as; chloramines and chlorine, tannins (which color the water), and phenols (which cause odors).

What Activated Carbon Doesn’t Do

There are several important toxins that activated carbon does not remove. Most notably, it does not move ammonia, nitrite or nitrate. Therefore it does not aide in toxin removal during the initial aquarium setup. Water changes or other methods must be used to address elevated ammonia, nitrite or nitrate.

Heavy metals, such as iron, are also not removed. If your water source has heavy metals, use a water treatment product before putting the water into the aquarium.

Does activated carbon de-adsorb?

You may hear that once activated carbon has reached its capacity, it will start leaching some materials it previously adsorbed back into the water. This is not an accurate claim. Although technically possible, to de-adsorb requires changes in the water chemistry that simply do not occur in the aquarium.

What is true is that some activated carbon is created using processes that result in phosphate being present in the end product. In those cases, it is possible for the phosphate that was already present in the activated carbon to leach into the aquarium water. Some activated carbon products will specifically state if they are phosphate-free.

If you are having difficulties with persistently elevated phosphates and can find no other cause, remove the activated carbon entirely. Perform normal tank maintenance for a couple of months and see if the phosphates remain elevated or are reduced. If they stay high, the carbon is probably not adding to your phosphate problem.

Activated Carbon and Medications

Activated carbon will adsorb many medications used to treat fish disease. Therefore, before treating sick fish with medications, all carbon should be removed from the filter. After the course of treatment is fully completed, it is safe to add activated carbon back to the filter. The carbon will remove any residual medication in the aquarium water.

Placement in Filter

Activated Carbon will lose its effectiveness rather quickly if exposed to lots of debris from the aquarium. Therefore, carbon should be placed after the mechanical filtration media in the filter. Keep in mind that if you do not keep your tank clean, and debris builds up in the filter, the activated carbon will not be effective.

Changing Activated Carbon

Since activated carbon binds with the compounds it removes, it eventually becomes ‘full’ and can no longer remove any other contaminants. Therefore, it must be regularly replaced. Generally replacing it once per month is sufficient. Going for an extended period of time without replacing the carbon will not harm the tank. It will eventually lose its ability to remove toxins from the water.

Recharging Activated Carbon

Stories about recharging activated carbon abound. Some even give step-by-step instructions, which generally involve baking the carbon in your oven. These stories are myths. The temperature and pressure required to recharge exhausted activated carbon cannot be achieved in your oven at home.