Using Activated Carbon in Aquarium Filters

Using Activated Carbon in Aquarium Filters

Activated carbon has been used in home aquariums for decades, and it is still 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 for special needs, and still, others believe that activated carbon should no longer be used at all.

It is important to remember that carbon is exhausted relatively quickly when used in an aquarium filter. 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 made from carbonaceous material that has been heat-treated at very high temperatures to create many tiny pores, greatly increasing its surface area. These tiny pores and massive surface area allow the filter media to trap 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 of material suitable for different uses. In aquariums, the form mostly used is GAC or granular activated carbon. Forms of activated carbon include:

  • BAC, or bead activated carbon
  • EAC, or extruded activated carbon
  • GAC, or granular 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 sizes. Materials such as coal, coconuts, peat, bamboo, and wood are all 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 chloramine and chlorine, tannins (which color the water), and phenols (which cause odors). It will help keep aquarium water from turning yellow over time.

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

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

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 saturated and can no longer remove additional contaminants. Therefore, it must be regularly replaced—once per month is usually sufficient. Longer intervals between replacements will not harm the tank, but the carbon will gradually lose its ability to remove toxins from the water. If you see yellowing of the water or smell an odor in your tank, it's past time to change the activated carbon.

Myth of 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 kitchen oven. It is better to just buy new carbon from the fish store when needing to replace your activated carbon, and be sure to keep unused activated carbon in an airtight container or it may adsorb odors and chemicals from the air.


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

However, the processes used to create some activated carbon can result in the presence of phosphate in the end product. In this case, it is possible for phosphate 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 phosphate and can find no other cause, remove the activated carbon entirely. Perform normal tank maintenance for a couple of months and see if the phosphate remains elevated. If it stays high, the carbon was probably not the reason for your elevated phosphate.

Cautions With Carbon

In general, using activated carbon in your filter is a good thing, but not a necessity. If you are testing your water, doing regular partial water changes, and dechlorinating the replacement tap water, you really don't need to use carbon. It is just an additional expense since the carbon needs to be replaced every month.

The carbon in a recirculating filter system will also act as a home to the beneficial bacteria that turn ammonia into nitrite and then nitrate. When you change the carbon each month, you are throwing away part of the biofilter, and it will take a while for the new carbon to grow beneficial bacteria on it. If the carbon media is a significant percentage of your filtration system, you will be losing your biofilter with each replacement. An ammonia spike could possibly occur after adding the new carbon. To prevent this from happening, use sponges, beads, bioballs, or ceramic beads in sufficient quantity in the filter to act as the main media for bacteria that form the biofilter.

Powder activated carbon, when used in the filter system, has been reported to blow into the aquarium as a fine dust that gets trapped on the gills of the fish. There are cases where the fish have died after using powdered carbon that wasn't properly contained in a media bag in the filter. Necropsy on the dead fish found carbon particles in their gills and fin tissues. Using larger pelleted or granular activated carbon, and rinsing the dust off with distilled water before placing it into your filter chamber will prevent that problem. Use distilled or deionized water for rinsing the new carbon to prevent it from adsorbing chlorine from the tap water before it is even placed into your filter!