All You Need to Know About: Calcium

All You Need to Know About: Calcium

Stony corals such as Large Polyp Stony Corals (LPS) and Soft Polyp Stony Corals (SPS) need calcium to grow and build their coral skeleton. As these corals grow, they consume more significant amounts. Therefore, regular replenishment is generally needed to keep levels stable.

What is Calcium?

Calcium is one of the most common elements in both the land and the sea. In seawater, corals combine calcium with alkalinity to form calcium carbonate which is an essential building block of coral skeletons.


Why is it Important?

Just as calcium is essential for building strong bones in the human body, it is also vital for making the skeletons and shells of many marine organisms. Invertebrates (i.e. snails) as well as algae (i.e. coralline algae) use it for their protective shells. Fish also use calcium for their bones, while corals use it for their skeletons. Without the proper amount, the growth of corals, invertebrates and coralline algae will slow considerably or stop altogether. Corals can also brown-out and recede if there is a lack of it.

What is the Optimal Range?

Reef Aquariums: 400 ppm – 450 ppm

Tools for Measuring

Hand-held Tester:

A hand-held tester, also known as a colorimeter, is easy to use and relatively inexpensive. Place a small water sample in a chamber for analysis.

Test Kit:

An inexpensive and commonly used method for measuring calcium. Simply count the number of drops of reagent needed for a water sample to turn a specific color, usually blue, and then compare the number of drops to the provided chart levels.

Water Quality Testing

How to Maintain Levels:

A regular weekly water change can generally keep up with demand in a reef tank with few small hard corals. However, the need for calcium quickly increases as the number and size of corals in the aquarium increases. When regular water changes can no longer keep up with the demand, it needs to be dosed either manually or automatically with a dosing pump.

To calculate an aquarium’s required daily dosage, bring calcium to the desired level (we recommend somewhere in the 400 – 450 ppm range) and then do not dose for three days. After 3 full days (72 hours), test calcium level and calculate the dosage required to raise calcium back to the desired level. Divide this dosage by 3 to determine the daily dosage. Continue testing calcium levels at least once a week and adjust as needed to maintain the desired calcium levels. Calcium levels can fluctuate throughout the day, so always test at the same time each day.

The dosage will vary depending on the product being used. Please follow the manufacturer’s recommendations for calculating the dosage or refer to one of the following handy calculators.

For Bulk Reef Supply Calcium: or

For Most Other Calcium Additives:

The Relationship Between Calcium, Alkalinity & Magnesium + How to Prevent Calcium From Precipitating Out of the Water:

In saltwater, calcium combines with alkalinity to become calcium carbonate. Corals use this to build their skeletal structure. If magnesium levels are too low, calcium carbonate will precipitate out of the water. Make sure to keep magnesium levels between 1250 – 1350 ppm.

To sum up everything, calcium is an essential element of a reef tank to grow coral. If not kept within range, corals are unable to grow. Additionally, if it’s not kept stable in conjunction with alkalinity and magnesium, corals are unable to thrive and grow as well.

Have a question or feedback for this article? We’d love to hear from you! Please feel free to drop us a line here. If you are interested in products for your saltwater aquarium, please click here to visit ATI North America.

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