Understanding & Using Activated Carbon in Your Aquarium

Activated carbon filtration has a long history in the aquarium hobby. It’s the most-used filter media in freshwater and marine aquariums world-wide. Just about every hang on the back filter cartridge contains activated carbon. Canister filters also use carbon filtration. Even reef tanks with filter sumps and protein skimmers make use of carbon’s assorptive properties.

activated carbon in the aquarium practical guide

But even after over 40 years of use, activated carbon is still shrouded in a bit of mystery and a lot of misconception when it comes to how it works and what it does for your aquarium.

We’ll explore what activated carbon is and how it’s made. We’ll discuss the different types of activated carbon and their unique properties. You’ll learn how best to use it in your freshwater and saltwater aquarium. Be sure to check out my recommendations too!

Activated carbon confusion

Perhaps the best place to start is the difference between charcoal, coal and activated carbon. Charcoal is what you burn in a barbeque grill. Charred sawdust is compressed into briquettes and used for outdoor grilling. Coal is a hard carbon-rich substance mined from the Earth. Coal is formed when dead plant matter decays and is converted into coal by the heat and pressure.

Charcoal and coal do not make good aquarium filtration materials. You’ll often see activated carbon labeled as “filter coal” and “activated charcoal” even though these terms are incorrect. This just keeps the confusion going in the aquarium hobby. Here’s the facts about activated carbon.

What is activated carbon?

Activated carbon is a highly porous, high surface-area adsorptive material. It is composed of sheets of carbon atoms joined by random cross-links that form a porous internal network. It helps to think of a structure like a honeycomb. The millions of cracks and voids between the carbon layers make activated carbon very effective filtration material. Pollutants are attracted to the vast surface area inside the porous carbon network. Activated carbon is used in industry and at home for adsorbing a wide range of impurities from liquids and gases.

What is activated carbon made of?

All activated carbon starts off as a carbon-rich raw material. Almost any carbon-containing material can be used to produce activated carbon. But the most common materials are coal-based and include anthracite, lignite and bituminous coals.

For example, Marineland Premium Activated Carbon is made from bituminous coal. Coconut shells are another popular material used to manufacture activated carbon for air filtration. EnviroSupply Granular Activated Charcoal is made from coconut shells.

There are also carbon filtration products made from wood, cherry pits and even recycled tires. The raw material used to make the activated carbon has a tremendous influence on the properties of the final product.

As we’ll see, not all activated carbons are equal when it comes to aquarium filtration.

How is activated carbon made?

No matter what the starting material, the activation process is where it all begins. It’s the activation process turns the raw material into a highly porous carbon structure capable of adsorbing pollutants from water. Here’s how activation works.

  1. The raw material is heated to remove moisture. It’s then ground into a powder.

  2. A harmless binder is added to the powder and compressed into a block.

  3. The block is crushed into pieces and sent to a kiln for a two-step baking process.

  4. The pieces are baked at 200-750° F to drive off volatile organics then again up to 1000° F to form charred carbon particles. A porous network is beginning to form.

  5. The carbon is then treated with steam at 1800° F to fully open up the pores.

The activation process opens up a massive internal network of pores inside each carbon particle. This controlled high-temperature reaction converts portions of the solid carbon to volatile gas. This volatilization opens up the pores.

During the activation process about half of the “char” material is removed to create the internal pore structure. Some carbons, depending on the raw material, can contain residual ash. This will leach out and add minerals to the water and even raise pH.

To remove the residue the activated carbon is rinsed with a dilute acid to flush out the ash. You may see “acid-washed” activated carbon when comparing carbon products. Acid washing doesn’t make the carbon acidic or lower pH in your aquarium. It is just a step to remove residul iron, calcium carbonate and silica from the carbon.

Low-grade activated carbon made from sawdust is chemically activated using phosphoric acid. The paste of raw material and phosphoric acid is dried and then heated in a kiln. This type of carbon will release some residual phosphate into the water.

How does activated carbon work?

To understand adsorption, we’ll have to enter the world of physics. I’ll keep it simple! Pollutants are adsorbed onto the surface of the carbon. In our case organic aquarium pollutants are attracted to and held onto the carbon surface by “Van der Waals attractive forces”. These are the same forces that make water droplets stick together.

Van der Waals attractive forces are relatively weak and allow the molecules to move around inside the carbon’s pores.

A second stronger adsorption process (chemisorption) also occurs. Chemical adsorption is a reaction between the adsorbed molecule and the activated carbon surface. This stronger chemical bond is formed between the molecule and carbon surface, locking the adsorbed pollutant inside the carbon’s pores.

As aquarium water comes in contact with activated carbon, both types of adsorption take place.

Pore size matters

The activation process opens up the pores and creates adsorption sites within the porous network. You’ve often heard about the huge surface area within each granule of activated carbon.

A spoonful of carbon has the surface area of a soccer field! That sounds impressive but there’s more to the story. Surface area alone is not the indicator of effectiveness. Pore size also plays a key role. The internal pore network can be compared to a roadway system.

  • Macropores are multilane highways.
  • Mesopores are two-lane highways.
  • Micropores are small one-lane country roads.

It’s possible for an activated carbon to have an enormous surface area created by tiny pores too small to capture organic molecules. If the organic pollutants can’t enter the pore network, they won’t be adsorbed.

Coconut-based carbon has a high surface area and tiny micropores. It works great for air purification where the pollutants are in the form of small molecules. Coconut carbon is also used to remove (neutralize) chlorine from drinking water.

Coal-based activated carbons have more macro and mesopores, making it easier to trap larger organic molecules that cause discolored aquarium water.

Types of activated carbon

The most common form of carbon is granular activated carbon or GAC for short.

GAC can be milled and sieved to a variety of sizes ranging from 0.2 to 5 mm. Granular activated carbon is used mainly for water purification. The granules allow water to flow through a bed of carbon and have adequate contact time to remove impurities.

Pelletized activated carbon is manufactured through an extrusion process. The 1-5 mm pellets are very hard and when packed into a filter create a lot of void space in between the pellets. Void space allows air to flow through a bed of pelletized carbon without reducing the flow rate, which is important for air or gas purification systems. Another reason pelletized coconut shell carbon is used for air purification is its small pore size.

Powdered activated carbon, made from coal or coconut shell can be glued to a synthetic filter pad, creating an activated carbon filter pad. Carbon filter pads for aquariums act as mechanical and adsorption filters all in one pad.

What does activated carbon remove in an aquarium?

You’ve probably heard the saying “Dilution is the solution to pollution.”

Our aquariums, although relying on many natural processes, are not truly self-sustaining. Pollution builds up faster than they can be recycled. Aquariums are really closed aquatic systems. There’s no place for many pollutants to go.

The truth is you’ll never find so many fish and invertebrates packed in so little water in nature. Aquariums tend to accumulate dissolved organic compounds. These biological byproducts come from fish, inverts, plants, algae and the foods we add to the aquarium.

Research has correlated poor health and increased disease problems when aquariums are high in organics. One of the easiest ways to remove these dissolved organics is with activated carbon.

Carbon will remove a variety of organics that cause yellowish water, unpleasant odors and an overall decline in water quality. If you’ve got a reef tank with corals you’re also concerned about light penetration. The build-up of organics in saltwater adsorb the blue portion of the light spectrum. This means your corals aren’t getting a complete PAR profile. Activated carbon combined with protein skimming will dramatically improve light penetration in your reef aquarium.

How to use activated carbon in freshwater aquariums

If you’re using a hang on the back power filter, the cartridges come pre-filled with activated carbon. All you need to do is give the cartridge a quick rinse under the faucet. This will flush away dust that may have formed during shipping.

With canister filters there are several options. Your model may provide pre-bagged carbon designed to fit into a filter basket. Some canisters allow you to layer in your own filter media. Flow-through filter bags make it easy to use loose activated carbon. Just pour in the amount you want and tie the bag.

Aquatic Experts activated carbon is made from bituminous coal. It provides very good removal of organics, colors and odors. It even includes a filter bag! Ideally, the order of filter media in a canister filter is mechanical media first to remove particulates followed by activated carbon. Place bio media on top on the carbon so it gets the cleanest water flow.

How to use activated carbon in saltwater aquariums

There is no special way to use activated carbon in a saltwater aquarium. It’s basically the same as a freshwater aquarium.

If you have an all-in-one reef aquarium with a built-in filter, you’ll simply use a filter bag filled with the carbon of your choice. Just be sure that water flow is not restricted by the placement of the bag within the filter system.

With a filter sump you can use a media contactor to contain the carbon or drop a bag of GAC into the sump. Since the aquarium water continuously recirculates through the sump, a bag of carbon positioned in the water flow will work quite efficiently.

Which activated carbon should I use?

Shopping for activated carbon can be a bit overwhelming. All the brands seem to make similar claims. The truth is, none of these carbon products are manufactured by the product sellers. Activated carbon production is a huge international industry.

Many carbon products are made in China. But you’ll also find filter carbon produced in the Netherlands, United States, UK, Indonesia and Thailand.

There are certain activated carbons that work better in aquarium water. We know that some aquarium carbon brands use high quality activated carbon based on the raw materials.

The term “research grade” was first used by Aquarium Pharmaceuticals for their lignite-based activated carbon. Not only was the carbon the most efficient at removing organics, it was also sold by laboratory supply companies for use in research laboratories. Other companies later used the same term even if their carbon was not the same quality.

Bituminous coal-based activated carbon like Marineland Premium Activated Carbon and API Activated Filter Carbon are recommended for freshwater and marine aquariums.

You’ll often see generic pelletized coconut shell activated carbon sold as special “reef approved” or similar marketing. The science behind activated carbon makes it clear that coconut shell carbon works best for air filtration. But some aquarists like that coconut carbon is hard and very clean. You probably won’t need to rinse it before use.

Two Little Fishies activated carbon is made from lignite coal. It is a little dusty and require rinsing before use. But lignite has excellent pore structure for adsorbing dissolved organics from water. Several side-by-side tests demonstrated that lignite-based filter carbon out-performs all other types.

How much activated carbon should I use?

This topic has been debated for over 30 years. There are a lot of opinions, but the fact is, there is no set rule when it comes to aquarium filtration.

Engineers can determine how much activated carbon to use in an industrial process because everything in the process is uniform and unchanging. Pollution levels in the aquarium vary daily. No two aquariums are the same. Imagine two 100-gallon aquariums. One tank has a single large cichlid and the other is mostly live aquatic plants with a school of cardinal tetras.

Both aquariums are the same size but the fish load and feeding rate is quite different. A “per gallon” recommendation is simply a guess. If you’re using filter cartridges, its already been decided. Just slip in the cartridge and you’re done.

For all-in-one systems take a look at the available space in the filter. Select a filter bag that will drop into an area of water flow. Fill the bag with carbon and drop it into the filter. It’s the same with canister filters. Some models provide pre-measured pouches or bags of activated carbon.

If you want to fill your own bag, select one that will spread out across the surface of the canister. Fill it so you get about two inches of activated carbon. This is just my suggestion. Some aquarists fill the canister with activated carbon! Remember that a deep bag of carbon will restrict flow and reduce the water output of the canister.

With a filter sump, just place a filter bag or two into the water flow. More carbon will remove more pollutants but it comes with a cost of reduced flow rate in your filter. And as we’ll see next, loading up your filter with a lot of carbon may be a waste of filter material.

When should I change activated carbon?

Aquariums are literally teeming with millions of microbes like bacteria, crustaceans and worms. As soon as activated carbon is placed in your tank it begins to get coated with tiny aquatic life. This means that activated carbon works best for about a week or two while all the pores are open for adsorption. After that, carbon’s adsorption properties gradually decline as the carbon is used up and coated with debris and microbes.

Most aquarists change their filter cartridges every month. Aquarists with canister filters sometimes think the large filter doesn’t need to be changed as often. But this is a myth. The higher flow rate simply brings more debris into the filter, where it gets captured by the various media. This is the purpose of aquarium filtration, to remove unwanted substances.

Leaving the canister for more than a month usually results in slow flow rates and decomposing organics inside the canister. As the debris breaks down, it release algae-promoting phosphate and nitrate into the water.

The idea is to change the filter often and remove the trapped debris from the aquarium, not have it rot inside the filter. The same advice goes for all-in-ones and filter sumps. Replace the carbon as part of your monthly maintenance plan.

Does activated carbon filtration make a difference?

If you’ve never used activated carbon or use a low-grade type, you’ll be amazed at what a difference high-quality carbon makes. The aquarium water will sparkle with clarity. The tank will appear brighter and more attractive.

If you have live plants, it’s not uncommon to see oxygen pearling on the leaves after using fresh activated carbon. This is because the light-adsorbing organics have been removed, improving PAR. Your corals will also perk up and look more vibrant. But that’s not all. Lowering the organics also reduces the chances of disease problems for fish and inverts like SPS and LPS corals. A clean aquarium is a healthy aquarium.

If you have questions or comments, leave them below!