Calcium, Magnesium, and Alkalinity in Natural Sea Water

Most people know that salt is a natural feature of sea water. Salt’s formal name is sodium chloride (NaCl), and not surprisingly sodium and chloride are two of the most significant elements in sea water. Others at the top include magnesium, sulfate, calcium, and potassium. Another prominent feature is alkalinity; not an element in itself but a combination of carbonates (made of carbonic acid) and boric acid. Both together make up carbonic alkalinity and boric alkalinity. What alkalinity actually measures is the ability of a liquid to neutralize acids. The higher the alkalinity, the better of an acid buffer the liquid is. In sea water, the carbonates act to buffer out acids.

The ratio of magnesium to calcium to alkalinity is key in sea water. In most of the world’s saltwater bodies, the ratio remains the same. To be specific, natural sea water has 412 parts per million (ppm) of calcium, 1280 ppm of magnesium, and 172 ppm total alkalinity (carbonate and borate combined). Even as specific gravity changes (ratio of the density of, in this case sea water, to a reference substance), the amounts of each of these elements will change but will remain relative to one another.

Reef Tank Water Parameters Chart

Calcium, Magnesium, and Alkalinity in Saltwater Aquariums

In the field of saltwater aquarium maintenance, measures must be taken to achieve an ionic balance that is close to that of natural sea water. Low magnesium levels can cause low alkalinity levels, which can make it difficult for an aquarium owner to balance calcium and alkalinity levels. With low magnesium, one may attempt to increase calcium levels, only to have alkalinity levels drop. Then, attempting to increase the alkalinity levels may only cause the calcium levels to drop! This can be very frustrating, but can be remedied by increasing magnesium to appropriate levels, which will allow for a much easier balance of calcium and alkalinity. The reason this happens is because calcium and magnesium are both positively charged and have very similar molecular structures, and carbonate will readily attach to magnesium instead of calcium. This bind will create a precipitate that is not biologically available for uptake, and which can also no longer be tested.

Some of the guidance one could find online may be misleading as far as how to balance calcium and magnesium levels. It’s common to find suggestions that magnesium levels must be raised first in order to raise calcium levels. This is not the case if your magnesium levels are at a proper level to begin with; you should be able to increase calcium without making any changes to magnesium levels. That being said, if your magnesium levels are too low to begin with, then you would indeed need to raise them in order to raise calcium.


This understanding of the role of calcium, magnesium, and alkalinity in saltwater shows the importance of maintaining an ionic balance between the three. As they stay within ratio to one another, the levels of one affect the levels of the others. Having balance between the three promotes healthy pH levels, which affects the health of everything living in your tank. It is recommended that magnesium levels are tested at a minimum once a month, but ideally every two weeks.

Interesting Fact: Corals create skeletons made of calcium carbonate, and they can actually use calcium and magnesium ions interchangeably during this process.

Interesting Fact: Corals create skeletons made of calcium carbonate, and they can actually use calcium and magnesium ions interchangeably during this process.



Alkalinity, Calcium, and Magnesium. 4 December 2010.  ReefGrow. Web.

Ions in Sea Water