At Seachem, we get many questions from hobbyists, not all directly related to our product line. We do our best to be helpful and answer questions without bias. We do not, however, follow the crowd, so that sometimes our views may seem contrarian. Here are some of the issues that come up repeatedly.

Calcium Supplementation

The most frequently recommended practice for reef maintenance is the addition of limewater or calcium hydroxide solution either in a continuous drip or when replacing water of evaporation. Because of the very high pH (>12) of this material, the addition has to be done slowly and carefully. For example, Tullock (MFM, Oct., 1993, p 4.) recently recommended one fluid ounce per 50 gallons each day of limewater containing 1.8 g of calcium hydroxide per gallon. Aside from the potential pH hazards to the aquarium, the zone of high pH that is formed when the limewater mixes with the tank water creates a chemical environment ideal for precipitation of both the calcium being added and the carbonate buffer present in the tank. Beyond this, it is chemically The most frequently recommended practice for reef maintenance is the addition of limewater or calcium hydroxide solution either in a continuous drip or when replacing water of evaporation. Because of the very high pH (>12) of this material, the addition has to be done slowly and carefully. For example, Tullock (MFM, Oct., 1993, p 4.) recently recommended one fluid ounce per 50 gallons each day of limewater containing 1.8 g of calcium hydroxide per gallon. Aside from the potential pH hazards to the aquarium, the zone of high pH that is formed when the limewater mixes with the tank water creates a chemical environment ideal for precipitation of both the calcium being added and the carbonate buffer present in the tank. Beyond this, it is chemically improbable that this addition of minute quantities of calcium has any significant impact on calcium concentration. The addition of one ounce of limewater per 50 gallons per day amounts to less than 0.04 mg/L (ppm) per day. Put another way, it would take 25 days at this rate to increase the calcium concentration by 1 ppm. In sea water containing some 380 mg/L calcium, 1 ppm is insignificant. Calcium hydroxide, although itself rich in calcium and economically available from most food stores as pickling lime, is poorly soluble and caustic and so must be used in the form of limewater, which is an exceptionally poor source of calcium. This, coupled with the undesirable side effects of pH hazard and alkalinity depletion, suggests that its recommendation or use is not well thought out.
Calcium chloride and organically chelated calcium are clearly better sources of calcium without the inherent inadequacies of limewater. Calcium chloride is readily soluble, does not notably alter pH, calculating exacts amounts to use is easy, and it is readily available from many home or hardware stores as 96% pellets. It is used mainly as road salt. The 4% impurities are mainly other calcium, magnesium, and sodium salts, which pose no hazard to the reef aquarium. The chloride ion already predominates in sea water, so that the addition that comes with calcium chloride supplementation is relatively minor. Like limewater, calcium chloride will precipitate alkalinity as calcium carbonate, but this is less severe than with limewater, since calcium chloride does not produce zones of high pH that favor such precipitation.

Organically chelated calcium has the advantages of being both more readily available biologically and more stable in sea water, requiring smaller and less frequent dosing than other additives, as well as diminishing the need to maintain calcium at 380 mg/L or higher. As for calcium chloride, calculating proper dosage is simple. Some prefer not to use chelated calcium, because of concern about adding organics to the aquarium. This is not a valid concern. The amount of organics added with chelated calcium is insignificantly small when compared to the organics released by most reef creatures, even in a no feed, no nutrients approach. Since chelated calcium products are lactate, gluconate, or cross-linked gluconate chelates, the naive misconception that these products contain sugars has arisen. While these components are related to sugars, they are oxidized aldehydes and do not react or behave as sugars any more than chloride ions, predominant in sea water, react or behave as chlorine. When regularly monitored by redox measurements, the addition of chelated calcium has no significant impact on redox.

Alkalinity

Alkalinity is another area of confusion for aquarists and recommendations about this are often equally confused. The first problem with alkalinity is the term itself. It is not well defined and means different things to aquarists, chemists, environmental engineers, and water treatment engineers. It is often confused with calcium content and hardness. Some units used to report concentration of alkalinity do not help. Expressing alkalinity as grains/gallon calcium carbonate or, in the hobby, as German degrees (KH), defined as parts calcium oxide per 100,000 parts water, only adds to the confusion. Calcium concentration is measured as mg/L (ppm) and it is not hardness nor is it alkalinity. Hardness is a measure of divalent metal cations and primarily reflects the combined calcium and magnesium concentration in freshwater. Hardness is not a measure of carbonate, even though it is often expressed as some unit related to calcium carbonate. Alkalinity would be better called buffer capacity. Alkalinity is the ability of the water to resist changes in pH on the addition of acid. Most of the buffering in sea water comes from its bicarbonate, carbonate, and borate content. It makes little sense to express buffering capacity as some unit related to calcium oxide or calcium carbonate. The value should be expressed in terms consistent with sound chemistry (related to the amount of acid required to measure it) and the S.I. (international) system of units. The expression meq/L satisfies these criteria. Even without defining terms, it is almost intuitive that 1 meq of alkalinity will neutralize 1 meq of acid. Alkalinity is measured by titrating a known volume of sea water with a known concentration of acid until a pH indicator marks the complete consumption of alkaline components by the acid: the amount of acid required is equivalent to the amount of alkalinity or buffer capacity.
How much alkalinity does a reef aquarium require? Natural sea water has an alkalinity of about 2.5 meq/L. Recommendations range from the same as sea water to as much as 4 times that amount. It has been suggested that to deviate in any way from natural sea water is an experiment and should not be done. While this is true to some extent, it is not sound for all parameters of sea water chemistry. An alkalinity of 2.5 meq/L is fine for the open ocean, but a reef tank is not the open ocean.