I'll give it a stab. As stated above:
alkalinity ~= [(OH)-]- + 2[(CO3)2-] + [(HCO3)-]
or in other words, alkalinity is approximately equal to the hydroxide concention (OH-) + the carbonate concentration (CO3)2- + the bicarbonate concentration (HCO3)- of the solution. The hydroxide concentration is reflected by the pH (pH is really a representative of H+ concentration of the solution, but that concentration is proportional to the OH- conectration.) of the solution. Once you have the hydroxide concentration (or pH) where you want it, the carbonate and bicarbonate ions are what will keep it there. The carbonate and bicarbonate ions are used by the system in chemical reactions. They are still in the system, but have just altered their forms. The alkalinity can be maintained by chemical additions.
There are two main methods to do this.
1. Kalkwasser (lime water) or calcium hydroxide Ca(OH)2. When calcium hydroxide is dissolved in water, it breaks down into Ca2+ ions and OH- ions. The Ca2+ ions are used by corals, and other calcium loving organisms. The (OH-) ions convert dissolved CO2 into carbonate and bicarbonate ions.
2. "Buffer" compound. The basic buffer compunds add sodium carbonate [Na2CO3] and sodium bicarbonate [NaHCO3]. This type of product is available from many different manufacturer's. Each manufacturer may or may not add other compounds to "help" your reef. Beware, and always read the label. If you only want to raise your alkalinity, then make sure that the product you are interested in only contains the two elements listed above. Often times, the "buffer" compund will be a part of a two part solution to help maintain calcium and alkalinity.
3. Calcium reactor. This is a device that uses CO2 to lower the pH of the tank water, making it more acidic. This acidic solution is used to dissolve Calcium carbonate. In a nutshell, it gives the tank calcium, carbonate, and bicarbonate ions. (You can see some of the formulas above. I am not sure how many of these are desired.)
The above solutions are designed to keep alkalinity levels stable. At the same time, they are working to add calcium (#2 requires a two part additive) to the system. They all have their relative benefits, and short comings. I personally use the two part additive, on my small tank. I don't want the hassle of dosing Kalk, and the calcium reactor seems like overkill for me. I am planning a new tank, that will use a calcium reactor. I think that the reactor will require less daily work which should lend to more enjoyment time. I am not qualified to compare their realtive costs. Anyone else?
How's that?
-ignatz
P.S. I realized I didn't answer the question, and that is why I edited the post.
[This message has been edited by ignatz (edited 10-25-1999).]