Activated carbon and chemical aquarium filtration

Acanthurus Sohal Surgeonfish

Carbonization is a complex procedure by which the carbon content of organic material is formed by burning anaerobically. Materials best suited for carbonization are charcoal (bituminous), brown coal (lignite), wood, or coconut shells which naturally contain a large amount of the element carbon.

The carbonized base material is then subjected to steam, which increases the pore size and surface area to such extend that one teaspoon of carbon can be compared to the area of a football field.

The increasing of the surface area and pore size is also known as “activation”.

The quality of activated carbon is determined by the carbon base and the above mentioned process, which often includes the use of chemicals and gases, influenced by temperature, burning time etc. The quality of the activated carbon therefore varies greatly between the different brands.

Good quality activated carbon is measured by the iodine number, molasses number and the ash content.

The iodine number indicates the activity performance level. The higher the iodine number the higher the degree of activation. The iodine number also indicates the micro-porosity (smaller pores equals higher surface).

Iodine numbers vary between 600 – 1200 and mainly adsorb low molecular weight particles, a number in the 1000 range being ideally suited for aquariums.

The molasses number refers to the macro-porosity. Adsorbing more complex (higher molecular weight) particles. A practical number for aquariums would be around 225-250.

Activated carbon contains ash as a residue of the carbonization and activation process. These inorganic leftovers, mostly iron and calcium oxide, can be removed by soaking the carbon in distilled or R/O water before placing it into the aquarium. Ash reduces the activity of the carbon and will increase the pH level over time.

Carbons well suited for aquariums are made of bituminous coal as they consist of many small as well as large pore sizes created by the activation process, allowing both, smaller and larger particles to be adsorbed. Lignite is mostly larger in pore size while wood or nut shell based show increased smaller pore sizes.

Carbon is an organic and all organics contain phosphates. During the manufacturing process, the carbon is often washed using phosphoric, hydrochlorid, or sulfuric acid, to remove inorganic and organic residues.

Acid washing and the removal of organic material will lessen the ash content as well as leachable phosphate. Washing carbon with phosphoric acid does not introduce phosphates.

Soaking the carbon prior to use in the aquarium will cause the phosphates and ash residues to leach out safely, therefore the phosphates will not be introduced to the aquatic system unknowingly.

Because of the ash and phosphate contents, it is advisable to soak the carbon in distilled or R/O water for a few days. It will not diminish the performance, but greatly reduce any possible and avoidable side effects.

Other indicators like hardness, density, abrasion and total surface values can be neglected.

In summary, a good quality carbon is determined by the measure outlined above with the iodine and molasses numbers being especially important. Terms like professional grade, aquarium, research, or other grades, do not reflect quality. These “grades” are used for marketing. Price is also no reflection on quality. Pre-washed or marine carbons need no special mark-up either. All activated carbons can be used in both, marine and freshwater environments.

To determine the best suited carbon for an individual aquarium, is to determine the best balance of micro and macro pores depending on the waste to be adsorbed.

Carbon is considered chemical filtration. The effectiveness of activated carbon is based on a three-step process. The first is adsorption. Static forces will attract particles adsorbing them onto the surface of the carbon. Bacteria that settle on the carbon surface will further consume the waste. Another process is the diffusion of gases. Absorbed into the carbon the gases are detoxified (i.e. Ozone O3 into oxygen O2). The third process is called chemosorption, where particles are irreversibly bound to the carbon.

Activated carbon removes organic pollutants, some of which discolor the water with a yellowish tint, organic acids, proteins, hormones, antibiotic compounds, and organic compounds as well as a variety of chemicals, medications, metals, and minerals.

Inorganic substances removed are, chlorine, chloramine, colors and odor causing substances such as phenols.

Removed organic, particles and substances adsorbed by the carbon will remain biologically active. Trapped waste particles will still decay and remain part of the aquatic system. Activated carbon is therefore best used on a short term basis and replaced regularly.

The quantity of carbon needed for an aquarium is about 3 tablespoons of carbon for each 50 Gallons of water. Carbon is quickly used up and can be removed after just 24-48 hours. If left in the system longer, it will no longer serve its purpose.

Once the activated carbon has been used up the re-release of the adsorbed substances back into the aquarium is minimal.

Activated carbon will not remove trace elements and is safe to use in reef systems. The uptake of substances like iron, manganese, molybdenum, cadmium, zinc, and carbon dioxide are extremely limited and is insignificant if used only occasionally.

Carbon in planted aquariums should be limited, because many fertilizers contain minerals in cheleated form. These cheleates will be adsorbed by the carbon as organics.

In closing, a well suited activated carbon for your aquarium is coal based with an iodine number around 1000, molasses number around 230, acid washed, with a low ash content. Precautionary soaking in distilled or R/O water is recommended. Recommended dosage is about 3 tablespoons per 50 Gallons either for 24-48 hours at a time (bi-weekly) or to be replaced at least every 2-4 weeks if applicable.

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Jan

Hi Scott. I’ve heard it mentioned a few times that activated carbon should not be used in Discus tanks. From your article I cannot deduct a possible reason for this argument. Any thoughts on this?

I don’t think a quality carbon will do any harm in a discus tank. The thought is that carbon may remove important trace elements needed for healthy plant life. Some hobbyists will forgo the use of carbon in order to better balance a planted discus tank, while assuring both, plant and fish nutrient requirements are met.

A fairly common practice also seems to be to use a carbon filter before adding the water to the tank during water changes.

Ron

Scott, thanks for the great article on carbon! Can you recommend some brands that meet your recommended criteria? Ideally ones (brands) that sell in bulk. Thanks.

Thanks. Not sure it’s sold in bulk, but I always prefer Black Diamond Carbon by Marineland. A quick google search should turn up many purchase options.

Georvge B kulangara Hyderabad India.

Thanks for the knowledge given for the right one to use

Glad the information was helpful.