"Alkalinity and pH are distinctly different from each other, although their definitions and functions can be easily confused. For those of you as uninformed about water chemistry as I was when I first began, alkalinity is essentially a measurement of water's ability to neutralize acids. It is a measure of the buffering capacity of a system while pH is basically the measurement of the concentration of hydrogen ions in water, in terms of acidity or alkalinity. The alkalinity of water regarding pH issues merely refers to the basic end of a pH scale (alkaline) in contrast to the acidic end of the scale and does not reflect the buffering capacity of a system. It is easy to believe that water with alkaline pH is likely to be high in alkalinity (buffering capacity). However, this is not necessarily true. Water with a high pH, but a low alkalinity is regarded as unstable. Such water will quickly decline in pH with the natural accumulation of organic acids in aquarium systems.

Regarding the care of reef invertebrates, water that is low in alkalinity but high in pH is generally undesirable. Unfortunately, it is not uncommon for aquarists to test their systems and report such conditions. Alkalinity depletion is caused sometimes by the misapplication of calcium supplements and/or a lack of water changes. It occurs naturally, as stated, from the neutralization of acids and removal of carbonates for calcification primarily. Ironically, it is obvious to aquarists that corals need calcium to grow, but carbonates are often ignored. Calcium supplements are some of the first and only products that many aquarists use for reef invertebrates culture. It is often forgotten, however, that coral skeletons are comprised of calcium carbonate. Calcium additions without balanced carbonate additions are about as useless as the keys to one thousand cars in an empty parking lot. And so, misinformed aquarists may continue to dose calcium without noticing any significant growth among corals in the collection (and in observance of an alkalinity that continues to fall). To some extent, an imbalance between free calcium and carbonate levels is natural, although unfriendly at times to successful reef aquariology. Grossly stated, high alkalinity and high calcium levels are mutually exclusive. Simply stated, seawater can only hold so many dissolved solids. As alkalinity increases (the levels of carbonates and bicarbonates) there is less "room" for the saturation of other dissolved elements such as calcium. Calcium and alkalinity in practical applications exist in a tenuous Hi-Lo relationship. Until recently, systems with high alkalinity and low calcium were uncommon because of a poor understanding about alkalinity and the popular application of calcium supplements.

The advent and success of calcium reactors has made some aquarists change their thoughts on ways to maximize calcification. Aquarists are divided on which Hi-Low methodology for maximum calcification works best. Some aquarists dose kalkwasser aggressively to raise calcium levels and accept the coincident drop depression of alkalinity. This technique has grown many corals to impressive size with numerous other benefits from the supplementation of calcium hydroxide. The intent of some aquarists is to maintain calcium levels above 400 ppm. 

As such, calcium hydroxide indirectly contributes to the alkalinity of a system by neutralizing acids that would otherwise exhaust buffers from the system. Hydroxide molecules are "spent" rather than carbonate molecules. And so, high calcium and adequate alkalinity can be maintained with the proper application of kalkwasser. While I am inclined to favor this technique for simplicity and the small expense incurred relative to reactor set-ups, it is admittedly difficult to maintain and potentially dangerous when pushed to extreme. It is an error to think that if the addition of x grams of calcium is a good, then 2x grams are better. At calcium levels extending beyond saturation, or during events when calcium is added quickly, it is possible to disturb the balanced relationship between calcium and alkalinity and cause a sudden precipitation of calcium carbonate, commonly known as a "snowstorm", which can have tragic ramifications. Spontaneous precipitation of calcium carbonate occurs when pH levels rapidly climb beyond a certain threshold, which causes crystalline carbonate "snow"  to fall out of solution in an essentially insoluble form. The tragedy of the event for a system suffering from this condition is that the reaction must run its course before corrective measures can be taken. The addition of buffers in an attempt to counter the declining alkalinity serves only to feed the precipitous reaction. An aquarist is resigned to watch the spawn of his error to completion, which leaves the buffering
capacity of the system at a dangerously low level.  The stress of the sudden change in water quality can be significantly harmful to marine organisms as well. Water changes and any methods of damage control that insure stability in the environment will be necessary. Despite the inherent risks, I strongly favor and recommend the use of properly dosed calcium hydroxide in at least small quantities for most systems. 

One of the very best ways to maintain alkalinity in reef invertebrates systems is the employment of a calcium reactor. Calcium reactors are vessels filled with calcium carbonate material that is slowly dissolved with a supply of carbon dioxide. Lingering or accumulating carbon dioxide in aquarium systems depletes alkalinity and lowers pH, as the presence of carbon dioxide is neutralized by carbonate ions in seawater. It can also contribute to undesirable growths of algae if neglected. Carbon dioxide and carbonic acid in solution are easily driven off with vigorous aeration. Properly operated, calcium reactors do maintain high alkalinity and reasonably good calcium levels (although they are significantly and proportionately lower). Although they are no less dangerous to use than calcium hydroxide for supplying calcium and alkalinity, calcium reactors are convenient and efficient devices. I strongly recommend the use of calcium reactors for aquarists favoring stony corals in display or culture, and for aquarists displeased with the tedious application of calcium hydroxide. Calcium hydroxide, however, does have additional benefits such as saponification (improving protein skimmer performance) and phosphate precipitation. Many European hobbyists rely on calcium reactors to maintain high levels of alkalinity with supplementation from calcium hydroxide. This has proven to be an excellent methodology for promoting the growth of calcareous organisms.

The oldest and most common method of increasing the buffering capacity of salt water is the addition of sea buffer. Sea buffer is basically (no pun intended) a powdered mix of bicarbonates, carbonates, and borates. Such mixes are designed to increase the alkalinity (buffering capacity) of seawater without raising the pH beyond a set point. Some buffering products do raise the pH of seawater and should only be used with caution. Baking soda, sodium bicarbonate, is a significant portion of most dry mixes of sea buffer. I do not recommend using sodium bicarbonate alone for most aquarists, especially new and less experienced individuals, without the strong admonition that it can raise pH quickly and dangerously without due caution. Baking soda should only be used in small portions when water quality can be tested frequently. Concentrated liquid buffer solutions are becoming popular and seem to be most useful when dosed with proportionate amounts of calcium supplementation in commercial two-part mixes.

It is recommended that alkalinity in captive systems be maintained between 7-12 dKH. There are, in fact, several ways to test for alkalinity in seawater. Some aquarists prefer to measure alkalinity in milliequivalents per liter [meq/L] (the target is more than 3 meq/L). Buffering capacity is also described as carbonate hardness, measured in ppm, but this is only a measure of carbonates and bicarbonate components. Total alkalinity measures all buffers and is higher than carbonate hardness. Commercial test kits for
testing alkalinity are sometimes difficult to read. Aquarists who have color blindness, vision impairment or other difficulties in reading colorimetric charts should consult distributors or fellow aquarists (marine aquarium societies, Internet, etc.) on brands of test kits with conspicuous color changes at the titration point.

Lastly, there are implications that difficulties maintaining calcium and alkalinity may be linked to inappropriate magnesium levels. Magnesium should be maintained at roughly three times the level of calcium. High magnesium levels are encountered by inappropriate supplementation and can be lethal to some reef invertebrates. Aquarists have most often reported sensitivity in mollusks and starfish such as "turbo" snails and brittle/serpent starfish. Low magnesium levels, as in economy brand synthetic sea
salts have likewise been implicated in difficulties maintaining free calcium and alkalinity adequately. This is yet another reason for maintaining proper water quality through water exchanges and testing with supplementation."