The Delicate Balance of Major Elements in Sea Water

As you stir up your reef tank’s weekly water change, do you ever what is in those white grains as you see them dissolve in your RO/DI water?  Do you wonder if everything that your corals will need to grow is in the right balance as you stir up 10% of your water volume to be replenished?  Are your corals actually using up elements that can magically be replaced by adding 10% of your volume weekly?  Where did that # even come from.  Today we will look at these questions and examine them a bit through the science and honestly, even a little magic of what is in sea water.


The history of Ionic balance

Taking a step back in time a bit, a renowned chemist named William Dittmar made an important discovery during the expedition of the HMS Challenger in 1873. For three years, Dittmar sailed around the world, collecting water samples from every ocean for chemistry testing. He found that, though the salinity levels changed throughout the bodies of water, the ratios of the major ions did not. This became known as the “rule of constant proportions”, or more commonly known as “ionic balance”.

For approximately one and a half billion years, the composition of natural sea water has remained constant. About 96.5% of the solution is water, and the remaining 3.5% is comprised of sea salts. Corals, among other marine species, have grown in these stable conditions for billions of years and are highly sensitive to even the slightest disruption in ionic balance.

Sea Salt and Ionic Balance Explained

Most people think of salt as simply “table salt”, or sodium chloride. While these two elements are the most abundant in the sea salt compound, there are actually many more. In the field of chemistry, a “salt” is any compound formed from an acid/base reaction where an acid’s hydrogen ion is replaced by a metal or another cation. So elements like potassium, magnesium, calcium, and sulfate are all actually salts too!  Basically there are many salts that make up the water in our aquariums.

When examining the composition of sea salts, we break the list into 2 camps to make it easy to manage.  Any element that is 1% or greater of the total salts is a major element, and the remaining are trace elements.  This helps for the illustration here, but will also make managing your aquarium that much easier.

The most abundant element in the salt compound is chloride, which makes about 55% of the total salts. Sodium follows at 30.6%, then sulfate at 7.7%, and magnesium at 3.7%. Calcium makes up 1.2%, and finally potassium comes in at 1.1%, just barely qualifying as a major element!  Many of these compounds should look familiar to you as they make up most of the test kits that we should be running weekly on our tanks ( Mg, Ca, K).


These ratios hold true in most of the world’s saltwater bodies, not including brackish waters. The term “salinity” refers to the volume of salts in the water. Even in waters with higher salinity (greater amounts of salts), the ratio of the elements that make up those salts remains the same. This is called ionic balance, and it is just as important for the health of organisms living in a saltwater aquarium as those living in the ocean.

Measuring Salinity in Saltwater Tanks

Various things can affect the salinity of a particular body of water, including temperature. At the same specific gravity (the volume of a given substance related to a reference substance), waters of a higher temperature will have a higher salinity than cooler waters. Therefore specific gravity may not always be the most accurate way to measure salinity. For saltwater tank maintenance, having a refractometer that’s calibrated to 35% (target salinity level of natural sea water) can be handy. As the temperature of your tank water rises, specific gravity should be kept lower in order to stabilize salinity. The same will hold true if you keep a tank say at 76 degrees, naturally you will want a higher specific gravity in order to maintain a constant salinity.

How to use this science when shopping for salt

ionic balance

This graphic compares two water treatments, and assumes both to be at 1.024 specific gravity, 80.2°F, and 35% salinity. The column on the left is perfectly balanced, and the column on the right shows what might happen if even a small change in the sea water “recipe” is made. A common practice by some saltwater aquaria companies is to advertise products that increase calcium or alkalinity levels. This might seem appealing, but because of the principles of ionic balance, it should be noted that unnaturally increasing one element will sacrifice others.  If your corals thrive under nature’s magic ratios of ionic balance,   a good rule of thumb is to do your best to provide them with this ratio when you are replenishing the elements in your tank through dosing and water changes.  This is what makes reading labels and doing research all the more important when you are shopping for salt.



The chemical compounds of sea water are in a delicate ionic balance that has held constant for billions of years. Chemistry and biology are tightly connected, and unnatural changes in an organism’s abiotic environment can have serious consequences on health and survival.  You can now use your understanding of ionic balance to keep the organisms in your own aquarium healthy with naturally-balanced water.  Maintaining this balance is ongoing; corals actually absorb the many elements in seawater to grow and thrive, so it is important to test often and maintain balance as elements are used up by corals in the tank.  To dive into the major elements in sea water and how they relate to coral health, check this out.


“Major Elements”. 28 Oct 2014. ReefGrow. Web.

“Ionic Balance”. 9 Jun 2015. ReefGrow. Web.