Some chemical filtration media:

Activated carbon. Granular activated carbon is still the most common medium used for adsorptive "chemical" filtration. Carbon selects for molecules with a higher affinity for the carbon than they have for water. Though its action is generally described as "chemical" filtration, adsorptive filtration is actually a physical process, not a chemical one. There are no ion exchanges or chemical bonding between the adsorbed molecules and the surfaces of the carbon pore spaces that adsorb them.

Activated carbon adsorbs dissolved organic molecules. These include natural byproducts of the partial decomposition of plant and animal materials, such as organic acids (humic acid, fulvic acid, etc.), phenols and polyphenols, proteins and carbohydrates, hormones and many medications. The more soluble an organic molecule is, however, the less adsorbable it is likely to be. Though many of these dissolved substances remain in the water precisely because they are resistant to microbial breakdown, the bacteria that colonize carbon surfaces will consume some of the organics sorbed to the carbon surfaces. Organic molecules trapped in carbon pores are still available to nitrifying bacteria also, as long as there are sufficient local supplies of oxygen.

Fresh activated carbon will also adsorb a wide variety of noxious molecules that originate outside the aquarium: insecticides, volatile chemicals like dry-cleaning fluids, PCBs in your tapwater, dyes (including some used for medications), perfumes, etc. Granular activated carbon will also adsorb some things you might not consider so undesirable: EDTA and other chelating agents, heavy metals etc. Chemist Shawn Keslar experimented and found that in the first 100 hours, carbon adsorbed over 60% of the iron and almost 50% of the manganese added to water. His report to the Aquatic-Plants Digest is archived at theKrib.com.

Some kinds of granular activated carbon are sold in packages as "pre-washed." This is not a genuine convenience, because you are buying water at an activated-carbon price. What percentage by weight of a "pre-washed" carbon is represented by water? Is it as acceptable as a "water-added" packaged ham, or is it more like the frozen-food industry, where selling packaged ice as product is considered disreputable?

The best articles describing activated charcoal were written by Tim Hovanec, for Aquarium Fish, May 1993 and again in June 1998. (There was a further recap in the June 2000 issue.) Though none of this matter was archived at the Aquarium Fish website, you can find condensed versions of his two carbon articles among Hovanec's library at the Marineland website. Those versions don't reprint Hovanec's useful table of what substances activated carbon adsorbs well, moderately, only fairly, or not at all: the table is reproduced at the Discus-L mailing-list site. Hovanec's assessment of carbon's adsorption of complexed copper and ferric iron is only "fair." I can't square these two assessments, but I feel that no one currently arguing about adsorption of desirable trace elements seems to have looked at this table.

North Dakota State U Extension Service offers a series of easily-understood papers "Treatment systems for household water supplies" one of which give basic information on activated carbon filtration. The paper stresses that activated carbon is most effective in adsorbing organic contaminants with fairly large molecules that get trapped in the pores: pesticides, aromatic hydrocarbons, industrial solvents, PCBs. They emphasize that activated carbon does not remove microbes, sodium, nitates, fluoride, nor hardness, nor lead and other heavy metals. "Generally, the least soluble organic molecules are most strongly adsorbed. Often the smaller organic molecules are held the tightest, because they fit into the smaller pores." Adsorption of many organic molecules is stronger at lower temperatures (your aquarium, rather than your water heater) and at lower pH, the paper points out.

AmQuel. AmQuel binds free ammonia in the water, part of the nitrification "cycle." I mention it here because its action is really an aspect of chemical filtration.

Phosphate and "Heavy Metal" adsorbers. Products like Phos-Guard and Phos-Zorb package granules of aluminum oxide in a nylon pillow that you pre-soak and then place in your filter, where they adsorb phosphates. They don't work overnight, but they offer an excellent way to achieve low levels of phosphates, which encourage algae. They have certain drawbacks, though, so you can't just pack a pillow into your filter canister and forget it: when bacterial biofilm coats their surfaces, bacteria may scavenge the adsorbed phosphate and make it available to algae once again. A periodic overnight soak in a bleach solution (followed by de-chlorination) will keep the phosphate-adsorbing granules from bio-fouling in this way.

Laterite. Certain components of your mixed substrate will also pull phosphates out of the water. Cations associated with colloidal clay will attract the negatively-charged phosphates, according to Diana Walstad. Thus baked or unbaked laterite in your substrate mix is in effect an aspect of your adsorptive, or "chemical" filtration.

PolyFilters. PolyFilters adsorb and absorb a wide range of contaminants, including dissolved organic substances. They have been made since 1976 only by Poly-Bio-Marine Inc. Their website at www.poly-bio-marine.com is a model of accessible scientific clarity. PolyFilter is one of the few aquarium products that scarcely needs to be marketed with hype. It's single drawback is that it's more expensive than activated charcoal. Since the white spun polyester pad turns dark mahogany brown from organics it has sorbed, you can see when it's time to replace the pad. Polyfilter also selectively sorbs toxic ammonia at levels above 0.1 mg/L but doesn't adsorb the harmless ionized form, ammonium, which is used by plants. It adsorbs phosphates and some nitrates (in freshwater, though not if you were using it in saltwater).

Zeolites. Zeolites have varied industrial careers as sorbents, ion-exchangers and catalysts.

"Zeolite is a generic name for a group of hydrous silicates in which the bases are alumina and the alkalis and alkaline earths, generally characterized by swelling up and fusing to a glass or enamel under the blowpipe; commonly found in the cavities of igneous rocks,"

Geologists have broken down naturally-occuring zeolites by the variations in their molecules and structure into 48 minerals, but zeolites are artificially produced, too. All the zeolites adsorb ions and molecules, that is they bind them to their enormous surfaces. They contain large numbers of molecular sites occupied by cations, often sodium (Na+), which they are ready to exchange for an ion in the water that has a more powerful positive charge, calcium (Ca++), for example. When a zeolite exchanges two sodium ions for a calcium ion, the water is being softened, but total dissolved solids (TDS) are not being reduced. Zeolites will bind ammonium (NH₄) but they have less effect on un-ionized ammonia (NH₃), so you can see that the higher your pH is-- and proportionally the more NH₃ you have-- the less useful zeolites are going to be for you. In saltwater, zeolites don't work at all.

Taking advantage of the differential in a particular zeolite's affinity for one molecule over another, industries use selected zeolites to separate out many kinds of chemicals; buried beds of zeolite can adsorb the radioactive elements in an underground plume spreading from a nuclear waste site. That same clinoptilolite you may use in the aquarium for some temporary ammonia sequestering is also the strongest adsorber of strontium and cesium in radioactive wastes! Or zeolites can be made part of the grit in poultry feed to adsorb toxins and improve the animals' health. You can control the desirable properties of zeolites best by containing some in a bag in the filter system rather than by incorporating it permanently in the substrate.

The cation exchange capacity ("CEC") of a particular zeolite can become exhausted. The adsorption of ions by zeolites is reversible, a fact that gives rise to worries and some confusion. Though some strongly charged ions in the water can "bump" some others, you can't very effectively recharge zeolites, except in concentrated brines or with powerful acids or alkalis.

One common species of zeolite is clinoptilolite which has a high affinity for ammonia and other positively-charged ions. It can even adsorb some sodium from the water. Clinoptilolites have a microscopic sheeted structure with ringlike channels through the sheets, like restacked deli slices of swiss cheese. The resulting openings and channels act as ion-exchange sites and also as molecular sieves that permit movement, entrapment and adsoption of relatively large molecules like ammonia, and even carbonates and nitrates. Most clinoptilolites exchange sodium ions, which limits their usefulness in water softening, as far as aquaria are concerned. But "potassium clinoptilolite," or Clinoptilolite-K, has unrealized aquarium potential, it would seem.  Have a look: run a search on "clinoptilolite-K" at http://www.google.com. In ion-exchange, the released potassium ion would be taken up by plants as fertilizer, and the resulting growth spurt should even help reduce phosphate levels. GSA Resources (see their informative website at www.gsaresources.com) is a corporate leader in developing applications for zeolites as ion-exchange media and filtration employed in aquaculture and water treatment facilities. Besides their more usual sodium clino, they market a Clinoptilolite-K they call "Cabsorb," ZK406H. They sell samples in quantities that would permit some aquarium experimentation.

One zeolite is packaged for the aquarium market under the name Ammo-Chips. Since zeolite adsorbs ammonium, it is often added to the filter media (about a teaspoon per 20 gallons will do) in order to reduce an expected "pulse" of ammonia when new fish are added to the tank. As the chemical filtration capacity of the zeolite quickly diminishes during the first few days, biofiltration takes up the increased ammonia resources. The result, if you do this right, eliminates even a small ammonia "spike." If zeolite were used continuously, however, it would be competing with aquarium plants for the ammonia, though ammonia ions adsorbed to zeolites in a filter are still available to nitrifying bacteria. One drawback is, you'll be exporting those bacteria when you renew the filter media. Another drawback is, that as nitrifying bacteria using the zeolite as a substrate oxidize ammonium to nitrite, they rob the molecule of its positive charge, and the zeolite may release nitrite or nitrate anions into the water.

Ion-exchange resins are another part of the chemical filtration arsenal. The phenomenon of ion-exchange was first recognized in the 19th century when certain clay minerals were found to remove potassium and ammonium ions from water, with the release of an equivalent number of calcium ions.

Most of the adsorptive "chemical" filtration media mentioned up to now don't actually chemically modify the water; ion-exchange processes do. The Brits call the kinds I've been talking about "adsorptive" filtration rather than "chemical" filtration.

There are several kinds of synthetic adsorbents in bead form or woven pads available at your LFS. Any of them are more expensive than activated carbon, but some can be regenerated and reused, which helps amortize the initial cost. Generally they are white or cream-colored when new and show you when they're exhausted by discoloring to dark brown. Some can be regenerated in a strong solution of household hyperchlorite bleach. Other less do-able recharging techniques would involve steeping in caustic alkali or strong acid solutions. Perhaps you can spend your energy and your cash more effectively than in attempting to re-use any chemical filtration medium.

In ion exchange, ions of the same net charge are exchanged, a free ion in the water taking the place of an equivalent ion bound to a solid, such as a zeolite as I was just explaining, or to a bead of artificial resin.

Resins in bead form contained in a nylon flow-through pillow are sometimes used to exchange calcium and magnesium ions for sodium ions, thus softening the water. This is great for doing the laundry but never a real improvement in aquarium water, especially since two positively-charged sodium ions (Na⁺) have to be released for every Ca⁺⁺ or Mg⁺⁺ ion taken out of the water. Those Na ions are more stressful than the ions responsible for hardness, so you're really not ahead of the game. Resins that are said to be partly rechargeable in a salty brine, according to the manufacturer's instructions, are giving off sodium ions, which makes them undesirable for the freshwater aquarium, in my opinion. However, if potassium chloride is substituted for sodium chloride (common salt) in recharging such resins, which seems to be acceptable procedure, then a K⁺ ion will be released instead of a Na⁺, and the plants will benefit. Apparently it doesn't matter which cation is used.

De-ionizing resins. Don't confuse ion-exchange resins with de-ionizing resins that extract ions from the water, exchanging them for H+ ions and lowering the total dissolved solids (TDS). These desirable kinds of ion-exchange resins you can use in the aquarium are termed acid/base resins. They take up calcium and magnesium, releasing a proton (H+) in exchange. One is marketed for the hobby as Tapwater Purifier.

De-ionized water ("DI Water"). Wastewater managers can get water of great purity, with ions and organics at the parts-per-billion level, by using a cation exchanger in series with an anion exchanger. First the cation exchange resin replaces all positively-charged ions with hydrogen ions, then the anion exchanger replaces negatively-charged ions with hydroxide (OH). The two freed ions combine to form molecules of water. A final run through activated carbon removes big organic molecules with approximately neutral charges. In industrial use, the resins are re-charged with sulfuric acid and sodium hydroxide.

Peat filtration. Peat filtration is one way to use humic substances released by peat-- but also by leaf litter, coconut shells, Osmunda fiber, used green teabags, even bogwood-- to achieve some softening and to chelate various substances. Rather than repeat this material here, I suggest you check it out in the "Softening" page.