Calcium, Alkalinity and Magnesium

Calcium, Alkalinity and Magnesium in the Reef Tank.

Ca, Alk and Mg are inter related and optimum levels are essential for optimum coral growth. If not maintained at or near optimum levels organisms will not grow, become stressed and can even die. Ca, Alk and Mg are the primary ingredients in creating Calcium Carbonate (CaCO3) which is the skeleton of LPS and SPS corals.

Calcium Ca

Calcium is one of the major ions in seawater. Natural seawater has calcium levels around 420 ppm. Changes in salinity will cause calcium levels to fluctuate. A calcium ion carries two positive charges in seawater and is written as Ca++.  Calcium is very important in a reef aquarium as many organisms, including corals and coralline algae, use it to create calcium carbonate skeletons. If it is not maintained at adequate levels, such organisms become stressed and can even die.  Calcium levels between 360-450 ppm are necessary for coral growth. A good indication of proper calcium levels outside a Ca test kit is the rate of growth of coralline algae. We keep our levels at 420 ppm and find the hardest part of maintaining a reef aquarium is cleaning the coralline off a glass or acrylic tank.  Boosting the calcium concentration above natural levels does not enhance coral skeletal growth. Experiments on several SPS corals have shown that low calcium levels below 360 ppm will limit coral and algae growth  Levels above about 360 ppm do not increase calcification. In fact if you recklessly add calcium to your reef you may do more harm than good as it will precipitate (come out of solution) and foul your pumps and heater.

Alkalinity Alk

Alkalinity is a rather complex measurement of bicarbonate. Bicarbonate (HCO3-) is what corals take up and process into carbonate (CO3–) in order to build their calcium carbonate skeletons. Normal to high alkalinity implies adequate bicarbonate, while low alkalinity implies bicarbonate is in short supply. Without constant additions or water changes aquarium water can rapidly become depleted of bicarbonate. Alkalinity’s depletion from normal to unacceptable levels can take only a day or two in some reef aquariums. This of course can take longer in reefs with a lower demand for carbonate. A reef with only soft corals will require much less bicarbonate than a reef with clams, SPS or LPS corals. When the water’s bicarbonate is depleted, corals that deposit calcium carbonate can become stressed and even die. We at reef ecosystems recommend that reef aquarists maintain an alkalinity of 2.5-4 meq/L (7-11 dKH, 125-200 ppm calcium carbonate equivalents). Natural seawater has a KH of 7-8.


Magnesium is the third most abundant ion in seawater. It is involved in a great many biological processes in every living organism. Reckless additions of  Magnesium and other supplements with out constant monitoring will often exasperate a chemical imbalance and more often than not do more harm than good. Natural Sea Water has Magnesium levels of 1280ppm. We recommend keeping levels between 1250ppm-1350ppm. Magnesium in seawater is an ion that carries two positive charges, just as calcium does. Generally magnesium is a free ion, with only water molecules attached to it. Proper levels of Magnesium are important because it stabilizes carbonate and allows it to be present in far higher concentrations. This allows seawater to act as a buffer in the pH range of 8.0-8.5. This ion pairing also keeps seawater pH from getting too high and reduces diurnal pH (daily) swings. Magnesium is also extremely important in the process of calcification and many organisms (if not all) take up magnesium from seawater. To ensure you have a proper balance it is extremely important you do regular tests of you levels with a quality test kit like Salifert. To keep things simple we recommend using a calcium reactor to supply your reef aquarium with the proper balance of Ca, Alk and Mg. Some manufacturers like Seachem add magnesium to some of their calcium products like Reef Complete and Reef Advantage Calcium. Other three part products are also a good way to keep your levels in check.

For more information on Calcium, Alkalinity and Magnesium click on the following links.

Reef Chemistry Calculator

Calcium Reactor Calculator

Relationship Between
Calcium, Alkalinity, Magnesium and pH

Relationship Between Alkalinity and pH

Relationship Between Calcium and Alkalinity

Chemistry and the Reef Aquarium

When Do Calcium and Alkalinity Demand Not Exactly Balance

Chemical and Biochemical Mechanisms of Calcification


What is Alkalinity?