What happens when you put it in water? If it floats or is buoyant and takes several hours to fully wet, making a hissing sound as it does, that indicates a porous, air-filled, hydrophobic carbon. If it sinks relatively quickly and emits little or no air or hissing sound, avoid it. What about on the shelf? Compare weights and volume. Select the carbon that takes up the most volume for a given weight. The better carbons are more porous (less dense), which means that for a given volume, they weigh less. Carbon's action is a consequence of its surface area and volume, not its weight. It is inconsistent to buy or sell carbons by weight alone.

Polymeric Adsorbents
These adsorbents are synthetic porous molecular sieves based on styrene or acrylic polymers with controlled non-polar to polar surface properties. They function in essentially the same way as carbons and the operating optimum TSA, PV, ratio and particle size are the same as for carbons. Their TSA range from about 300 m2/cc to 500 m2/cc with PY ranging from about 0.4 to 0.6 ni/Icc. The TSAIPV ratio ranges from 700 to 850. Strictly speaking, only uncharged adsorbents should be considered polymeric adsorbents. Several synthetic adsorbents available for aquarium use are not uncharged, but are in fact ion-exchangers and they will be considered separately. By comparison to carbons, polymeric adsorbents generally have a less efficient porous structure, but more effective surface properties and more predictable adsorption of polar as well as non-polar solutes. Although the overall capacity of these adsorbents is less than that of carbon, they have strong affinity for some solutes of importance that are not retained by carbon. Organic acids and both organic and inorganic nitrogen compounds are good examples. Overall, carbon is superior, but there is a sound basis for using both polymers and 'carbons together. Polymeric adsorbents are usually white to tan, dull, and have the shape of small beads about the size of a pinhead. It is also possible to manufacture them as fibers.

Ion-Exchangers
There are several types of ion-exchangers. There are mineral or natural exchangers and synthetic exchangers. The mineral exchangers are, like carbon, molecular sieves, but have much less TSA, only about 5-50 m2/cc. These exchangers are zeolites, kaolins, or other type of clays. They have limited exchange capacity and are poorly defined; consequently, they have limited application. They probably would not be used at all, were it not for being very economical. Chemically, they are mixtures of aluminum, magnesium, zinc, and other metal silicates. The ion-exchange property is primarily due to surface oxygen of the silicate, making this material primarily a cation exchanger, usually exchanging ammonium ions from water for sodium or calcium ions on the exchanger. Some also have very limited anion exchange capacity. These mineral exchangers are not suitable for salt water use, because the high salt content would render them ineffective and would tend to release toxic metals into the waler. These exchangers are promoted commercially mainly for removal of ammonia from fresh-water aquaria. If you have noticed a physical similarity between your ammonia absorbent and kitty litter, the similarity is not accidental. These exchangers also have limited adsorptive capacity for polar charged groups.

Synthetic exchangers are defined as either anion or cation exchangers and are available as either microporous or macroporous types. The microporous types have only very small pores that admit only small inorganic ions. These have been used for many years to deionize water or soften water for household use. In the aquarium, however, proteins, bacteria, colloids, and other large solutes quickly plug up or "foul" the micropores of this type of exchanger and render it useless. The macroporous types are molecular sieves with TSA ranging from 25 to 506 m2/cc with PV ranging from 0.2 to 0.6 mi/cc. These macroporous exchangers are much more resistant to fouling than microporous types. There are four types of macroporous exchangers: strong anion, weak anion, strong cation, weak cation. Without getting overly technical, the main difference of importance for aquarium use between strong and weak exchangers is that only the strong exchanger is a true exchanger in that it will split off its counterions and generate and adsorb corresponding counterions from solution. Weak exchangers are not true exchangers in that they only adsorb already existing free ions without actually generating any ions themselves.

Ion-exchangers have valuable uses in the aquarium, particularly for fresh water. A mixture of strong anion and strong cation exchangers will effectively produce soft water of slightly acid pH as well as remove ammonia (ammonium) and other ionic metabolites. Weak exchangers will also produce soft water and at a controlled pH. Customized stable fresh water can easily be attained by the intelligent use of ion-exchangers.
With salt water, the high sodium, chloride, calcium, magnesium, and sulfate content quickly equilibrates with strong exchangers and renders them virtually useless as ion-exchangers. Macroporus types, however, retain their usefulness for organic removal. Weak exchangers have limited but useful applications. These exchangers can be used to effectively remove heavy metals and to remove acids. Acid removal with weak exchangers significantly promotes good pH control. Weak anion exchangers are amines and are very effective in removing copper, including many types of chelated copper, turning blue as copper Is complexed to the exchanger.

Most ion-exchangers available commercially for aquarium use are clays for fresh water use, usually, but not always, restricted to ammonia removal. There are a few sources of microporous strong exchangers for softening water. The possibilities for ion-exchange in fresh-water aquaria of aquarists have not been adequately or intelligently explored. The use and recommendations for ion-exchangers in salt water are at best confused. The combined use of carbons, polymeric adsorbents, and judicious selection of ion-exchangers results in improved water quality, which, in turn, leads to more colorful, healthier, more active fish and invertebrates than is otherwise possible. Appetites are more aggressive and one mixed blessing is the comparatively unimpeded or uninhibited growth.

Synthetic ion-exchangers are usually beads about pinhead in size ranging in color from tan or off-white to dark brown. Microporous exchangers are usually translucent and shiny, while macroporous exchangers are opaque and dull. Anion exchangers are usually off-white to tan, while cation exchangers tend to be gray or brown to dark brown.

Bottom Filtration
Although bottom filtration is primarily biological, considerable chemical activity is also involved, at least in the marine aquarium. The principal component of bottom filtration in the marine aquarium is magnesium carbonate in one form or another, either as dolomite, crushed oyster shell, or crushed coral. This material behaves as a cation exchanger and polar adsorbent, not unlike the zeolites used in fresh water. The principal recognizable chemical action of magnesium carbonate is the removal of heavy metals, including trace elements such as copper and vanadium. Many organics, including proteins, amino acids, vitamins, and medications are also adsorbed to, magnesium carbonate, although the capacity is very limited. The adsorbed material is eventually attacked biologically. Carefully controlled experiments show that a surprising amount of ammonia is retained on the surface of magnesium carbonate. This is also eventually attacked biologically.

Ammonia Absorbers
Ammonia removal is so important to aquarium maintenance that it warrants separate attention. Unquestionably, the best long-term route to ammonia removal is biological filtration. There are, however, numerous products available to aquarists to supplement the action of biological filtration. To look at these products intelligently, it is necessary to understand ammonia, the solute. Ammonia, as already indicated in Figure 1, is a covalent polar compound, not unlike water, which ionizes in aqueous solution to the positively charged ammonium ion. The interconversion of ammonia and ammonium is a reaction at equilibrium, and, with increasing pH, ammonia is favored, while, with decreasing pH, ammonium is favored. If an absorber removes one of the forms, then the equilibrium is shifted in that direction and all the ammonia is eventually consumed.

Removal of ammonia as an uncharged polar compound requires a very strong polar adsorbent, since the environment is water and, likewise, strongly polar. Some polymeric adsorbents are able to do this, but only to a very limited extent, and more so in salt water than in fresh water. The polar solvent water competes very effectively with ammonia for polar adsorbent sites, and the polar interactions between ammonia and water are effective in eluting ammonia from the adsorbent.