As coral husbandry develops, so too does the means for providing adequate calcium for corals and other invertebrates. Since corals consume calcium at a relatively rapid pace, it is necessary to replenish calcium on a regular basis. Initially, the only method available was the use of calcium hydroxide dissolved in water (kalkwasser) to replace water that had evaporated. As technology progressed and stony corals and clams became the dominant animals in these tanks, more sophisticated methods for maintaining calcium levels developed. These include balanced liquid supplements, calcium reactors, and complexed dry additives. It is now relatively easy to maintain the calcium level in the tank above 400ppm, the level of natural seawater. It is very important that calcium levels be kept this high, particularly for stony corals (SPS). If levels are less than 350ppm, coral growth will slow and what little growth does occur will result in thin, spindly branches. If calcium levels drop below 300 ppm, corals will stop growing and may even die. Furthermore, without proper calcium levels and growth levels, the coloration of the coral, particularly at the tips, will not be as vivid as when the coral is growing well.

Calcium level is not as crucial in a soft coral tank, but the corals do much better when the levels are kept at the desired 400ppm. In soft coral tanks that are not overstocked, water changes and the dissolution of the calcium substrate may be enough to keep the levels high so that no additional supplementation is necessary. This is possible since most soft corals do not take up calcium at the same rate or to the same degree as stony corals do. However, over the past ten years, it has been shown that virtually all corals, as well as coralline algae, require calcium in order to thrive.

Along with supplementation, it is also necessary to test the tank's calcium level on a regular basis. The tests available are relatively straightforward and should be done on a weekly basis. The results should be noted in a log so that the trend in calcium levels can be looked at over time. Over time as corals grow, the amount of calcium added may need to be increased to keep up with this growth. Keeping a careful log of the calcium levels can tell you when calcium levels need to be increased.

At the same time that calcium is measured, alkalinity levels should be measured as well. There is a strong relationship between calcium levels and alkalinity that should not be neglected. If the calcium levels get high (over 500) there is a tendency for alkalinity to drop. Conversely, if alkalinity levels get too high, calcium levels will tend to fall as calcium precipitates out. Therefore, check for a desired calcium level between 400 and 450 ppm, and alkalinity levels between 2.5-3.5 meq/L (7-10 dKH).

One of the more elegant means of supplementing calcium, and the one that is now becoming most widely used, is the calcium reactor. These reactors produce useable calcium by dissolving a calcareous media in low pH water. The low pH is achieved by allowing carbon dioxide to bubble into the reaction chamber at a fixed rate. The amount of calcium that is released is controlled by the flow rate of water through the chamber as well as the frequency of the carbon dioxide bubbles. When used properly, this method provides a very precise means of maintaining calcium levels within a system. However, the use of calcium reactors may increase alkalinity levels to excessively high levels if they are not monitored closely. For this reason, it is essential to test both alkalinity and calcium levels when a calcium reactor is used. When set up properly and used correctly, a calcium reactor can maintain or increase calcium levels with minimal maintenance for most tanks. Currently, calcium reactors are available in different sizes so it is possible to find a reactor for even the largest tanks.

There are numerous ways to achieve the desired result with most aspects of reef keeping; this is especially the case with calcium supplementation. As long as regular testing is done, it is relatively easy to keep calcium and alkalinity levels in their desired range regardless of which method for supplementation is chosen.

Mike Paletta is the author of The New Marine Aquarium and Ultimate Marine Aquariums. He has been in the hobby for over 15 years and has written numerous articles for Aquarium Fish Magazine, Tropical Fish Hobbyist and Aquarium Frontiers.