Calcium In The Reef Tank

Calcium is used by corals for growth, with calcium carbonate forming the building blocks of coral skeletons.  Calcium is also utilised by many types of algae including hard calcerous green macroalgae as well as highly-desirable coralline algae.

clams need calcium for growth

Clams need calcium for continued growth

As mentioned on Ideal Water Parameters For A Reef Aquarium, calcium is at an ideal level at 400 to 480mg/l.  At the minimum, calcium levels should not fall below 380mg/l in a reef tank with LPS stony corals.

The Relationship Between Calcium, pH and Alkalinity

Calcium, pH and alkalinity should form a natural balance in the reef aquarium.  As mentioned in our pH and Alkalinity page, carbonate is the buffer that maintains alkalinity which in turn keeps pH within an ideal range.

And since calcium carbonate makes up coral skeletons, high alkalinity (carbonate) works together with calcium to ensure that corals will grow and thrive.   Consistently low levels of both alkalinity and calcium will result in the gradual demise of corals.

Precipitation of Calcium

When calcium levels exceed 550mg/l, precipitation of calcium results.  This is seen as a dense, milky white clouding of the water.  Precipitation of calcium is usually accompanied by a drop in alkalinity.

Startling though it may be for the beginning saltwater aquarist, the remedy for this is a partial water change of 15 to 20% with a reputable synthetic salt mix.  Wait 24 hours and the cloudiness should clear up.   If the cloudiness persists after 24 hours, another 10% water change is in order, which should address the problem entirely.  Check pH and alkalinity levels during and after the precipitation event.

Calcium Reactors

SPS corals especially, require readily available calcium for growth.  While it is certainly possible to add a powdered calcium additive by hand every other day, it is tedious.  For this reason, most reefkeepers with SPS-dominated reef tanks prefer to employ a calcium reactor.

How A Calcium Reactor Works

Commercially available media for calcium reactors are made up of calcium carbonate, mostly crushed coral and aragonite.  Aquarium water is pumped and made to flow through the media in the reactor chamber.   But since calcium carbonate does not dissolve readily at regular reef tank pH levels, carbon dioxide is injected into the water in the reactor to lower the pH.  The low pH of the water in the reactor chamber causes the media to dissolves, steadily releasing calcium carbonate which then enters the aquarium.   As an added benefit to the reef tank, minerals and trace elements from the crushed coral media also enter the aquarium

It is best to continuously monitor pH and calcium levels with calibrated electronic meters when using a calcium reactor in your setup.

Aragonite Sandbeds and Calcium

One added benefit of having an aragonite sandbed in the saltwater aquarium is the the gradual dissolution of aragonite over time, releasing calcium and carbonates into the water.  In a small way, this dissolution contributes to calcium and alkalinity levels, and is also the reason why sandbeds in saltwater aquariums seem to get thinner with each passing year!

Links to more pages on Water Parameters:

Ideal Water Parameters For A Reef Aquarium

pH and Alkalinity

Ammonia and Nitrite

Nitrate In The Aquarium

Phosphates and Phosphate Adsorbing Media

New Tank Syndrome

Water Parameters | Danger Levels

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