Chemical Filtration is the use of a chemical resin, usually some form of activated carbon to bind pollutants on a molecular level. Organic pollutants are particles and nutrients from metabolic waste, organic acids, proteins, hormones, antibiotic compounds, and organic compounds. In addition other pollutants from the surrounding air such as pesticides, nicotine, and perfumes enter the aquarium at the water surface.
While ill suited as a primary filter, chemical filtration has its benefits as a supplemental filtration technique. It is excellent at binding up yellowing agents that tend to give the water a greenish yellow cast. Aside from large water changes, chemical filtration is the most effective means to polish the water in aquariums under 200 gallons. Unless the reef system is very small, it is not practical to run a chemical filter as the main source of nutrient removal. It depletes too quickly, and in large tanks, the amount you would need could be excessive.
Carbon is the most widely known and used media for chemical filtration in the reef hobby. Activated carbon works through a three-step process. First, static forces attract particles absorbing them onto the surface of the chemical media. The second process is the diffusion of gases. Once absorbed into the carbon, the gases are detoxified. The last process is called chemo-sorption, where particles are irreversibly bound to the carbon.
As the activated carbon media absorbs compounds in the water, it loses its effectiveness. Periodically you have to discard the old media and replace it with new chemical media. Some products are designed to absorb some compounds better than others. For example, media is available to absorb more phosphate a known contributor to algae blooms. Beware of ridiculous product claims though, chemical media has its uses, but it does not perform miracles.
Ion Exchange Resins
Ion Exchange resins work by adding or subtracting an ion from the exchange medium. The medium is either a cation or anion (positive or negative) and each serve to remove complementarily charged compounds. Negatively charged compounds are attracted by the positively charged medium and vice versa.
Both are rechargeable, anionic in a base solution and cationic in an acid solution.
Ion Exchange is used mainly for the purification of tap water before mixing with salt water. It can be used on the tank's water, but it is not recommended since it will remove the salt as well as any other pollutants. The reef keeper would have to constantly monitor the tank's salinity and add more salt if necessary.