Allowing organic substances to decompose increases the activity of many undesirable microorganisms in the aquarium, increases the total oxygen demand (BOD - biochemical oxygen demand, COD - chemical oxygen demand), reduces the redox potential and causes excessive production of CO2 through bacterial respiration, which reduces the pH.

In addition, there will be an increase in nitrogen and phosphorus compounds, which promote the growth of nuisance bacteria and possibly algae. The resulting organic matter also has a significant impact on the penetration of light, especially in the blue and near-UV spectra, which stimulate coral growth.

Removing organic matter from the water is therefore crucial to the success of the aquarium. By using strong protein skimming, fine mechanical filtration and healthy biological filtration (nitrification and denitrification) the large organic particles and the end products of the organic decomposition processes are treated. However, there are many small organic particles that can only be removed with very fine mechanical microfiltration using activated carbon.

How does activated carbon filter water?

Scientifically speaking, carbon removes organic matter from the water through a physical process called adsorption. Within a reef aquarium, the submicroscopic pores of the activated carbon are the final stage of mechanical filtration, adsorbing the organic molecules that are too small to be removed from the water by even the finest mechanical filters or physically by protein skimming.

Adsorption can be thought of as the adhesion of a molecule to the surface by attractive forces. This occurs within the submicroscopic pores of the coal because the attractive forces between the organic molecules and the internal surfaces of the coal are much greater than the attractive forces that would keep them in solution. This continues until all of the surface area inside the coal is used up and it is consequently saturated with organic matter.

What is activated carbon?

Activated carbon is an extremely porous material with a sponge-like structure, but with a very large internal surface area in relation to its actual physical size. It is made from many organic materials that have a high carbon content, such as coconut shells, nut shells, lignite and bituminous coal or anthracite. The carbon-based material is "activated" by physical modification and thermal decomposition in a furnace under a controlled atmosphere and temperature.

The activation process significantly increases the surface area per unit volume of a fine network of submicroscopic pores, which gives activated carbon the ability to be used as a filter material. The specific source of the carbon and the activation process used determine the physical properties of the final product. Since there are many different grades of activated carbon, they must be specifically selected for each application.

What makes a specific carbon ideal for use in reef aquariums?

There are a number of internationally recognized tests for evaluating and comparing activated carbon. However, the most important parameters for selecting a carbon for reef aquariums are the  iodine and methylene blue number. These two parameters measure the ability of the carbon to absorb all the organic waste from a reef aquarium that has not been removed by mechanical filtration and protein skimming. The higher the number, the more organic matter is removed from the aquarium per gram of activated carbon used. Both numbers are approximately equally important and should always be considered together when evaluating test data.

Some manufacturers provide a molasses number However, molecules in the molasses size range are usually removed by protein skimming, so this number is not relevant to evaluate the suitability of activated carbon for reef aquariums.

In the past it was known that coal phosphate into the water, which is undesirable for a reef aquarium and therefore the lower the phosphate release, the better the carbon was considered. In reality, as long as the level of leachable phosphate is below a baseline threshold, no differences are measurable and are therefore irrelevant.

The cleanliness or dustiness of the product is also important to consider and is measured as the ash content. The lower the better, as higher levels represent a greater amount of particles being released into the water, leaving a lower actual mass of carbon to contribute to filtration. ash level are often stated by the manufacturers, but this is most easily seen by looking at the turbidity when you add the carbon to the water.
In addition to these important indicators, the overall efficiency of activated carbon is influenced by physical parameters such as Product density, TSA , the pore volume and the mesh size.

Activated carbon produced from lignite and anthracite is not recommended for reef aquariums due to an excessive amount of fine powder (which can cause lateral line necrosis "HLLE syndrome" in some fish) released into the water. The preferred sources are coconut shell or bituminous coal.