OPTIMAL CALCIUM AND CARBONATE SUPPLY IN THE REEF TANK

It is a particular joy for every saltwater aquarist to watch how the corals develop a beautiful color and grow. The growth of stony corals depends primarily on two ions: calcium (Ca2+) and carbonate (CO32-). These two components are stored in the coral skeleton in a ratio of 1:1 as calcium carbonate (CaCO3).

Determination of calcium and carbonate concentration

You can easily determine the concentration of the two ions using commercially available drop water tests. The test used to determine the concentration of carbonate ions is called the KH test (carbonate hardness test; unit: °dH).

A KH test can be used to determine the concentration of carbonate ions in seawater.

In order for calcification, i.e. the formation of the coral skeleton, to proceed optimally, the calcium concentration should be in the range of 400 to 450 mg/l and the carbonate hardness value should be 7 to 9 °dH.

Methods for calcium supply in saltwater aquariums

If your corals grow well, the two ions will be used up and they must be replenished at regular intervals to maintain their concentration. Different supply methods have proven effective in practice. The most commonly used include the lime water method, the Balling method and the lime reactor.

#1 Lime water

The lime water method is the oldest method and was developed in the 1970s by Peter Wilkens. In this method, calcium hydroxide (Ca(OH)2) is dissolved in osmosis water and then added drop by drop to the aquarium.

Since the resulting solution is highly alkaline (basic), the addition must be made very slowly so that the pH of the tank does not rise too quickly. The addition should also primarily be made at night, when the pH is usually lower due to reduced photosynthesis activity.
However, the lime water method mainly leads to an increase in calcium concentration. Carbonate ions are only created indirectly with the lime water method through the addition of hydroxide ions (OH-). These then react with the carbon dioxide (CO2) dissolved in the water. The lime water method is therefore only partially suitable for supplying calcium to a fully stocked and well-functioning saltwater aquarium.

Calcium hydroxide is available as a powder from specialist retailers - when dissolved in osmosis water, you get what is known as lime water.

#2 Calcium reactor

The calcium reactor is a technical element that dissolves calcium carbonate and supplies the resulting ions to your aquarium.

The calcium reactor consists mainly of a reactor tube filled with calcium carbonate. With the help of carbon dioxide (CO2), the pH value within the reactor is lowered to such an extent that the calcium carbonate material begins to dissolve. This process produces both calcium and carbonate ions in a balanced ratio.

Calcium carbonate granules are a good alternative to coral rubble for calcium reactor filling.

A major disadvantage of this method is the low pH value required to dissolve the calcium carbonate. The water leaving the reactor now has a high content of calcium and carbonate ions, but also an excess of carbon dioxide.

When operating a lime reactor, you should therefore always ensure that the amount of water leaving the reactor is as small as possible. This increases the residence time of the carbon dioxide in the reactor and reduces the concentration in the outgoing water. In contrast to the lime water method, the pH value in the aquarium does not increase, but rather decreases. If possible, the reactor should therefore be operated primarily during the day. Another option is to combine the lime reactor with the lime water method, as the resulting pH value changes balance each other out.

#3 Balling

The Balling method was developed in 1994 by Hans-Werner Balling.

For this method, two salts containing calcium and carbonate ions are dissolved in osmosis water. Two solutions are created. The salts used are calcium chloride (CaCl2) and sodium hydrogen carbonate (NaHCO3). If you now add these two solutions to your aquarium, sodium (Na+) and chloride ions (Cl-) are formed in addition to the desired calcium and carbonate ions.

Sodium and chloride are the main components in sea salt (about 70%). In addition, a commercially available sea salt mixture contains other important macro and trace elements (about 30%).

The elements contained in sea water are therefore present in a certain ratio to one another. If you add the two Balling solutions to your aquarium over a longer period of time, this ratio of the elements to one another will shift. Therefore, with the Balling method, depending on the amount of the solutions added, the missing macro and trace elements are added in the form of a third solution. This is made using a mixture of various macro and trace elements, which is known as sodium chloride-free sea salt.

The coordinated addition of these three solutions does lead to an increase in the salt content, but not to a shift in the ions. You can counteract this increase by simply using less sea salt when preparing sea water for the next water change.

That was a brief overview of calcium and carbonate supply in saltwater aquariums.