Calcium is one of the major building blocks of aquatic life. It is the primary element of exoskeletons for invertebrates and endoskeletons for vertebrates. It is vital for corals too! Corals uptake calcium in the form of calcium carbonate (CaCO₃). Hard coral species (Large Polyp Stonies (LPS Corals) and Small Polyp Stony Corals (SPS Corals)) require large quantities of calcium, which is vital in order for them to form their hard calcium carbonate skeleton. Soft corals also need some calcium, although to a much lesser extent than hard corals. Soft coral’s uptake calcium to form sclerites within their softer tissue. A saltwater reef tank dominated by hard corals will therefore use up significantly more calcium than a soft coral tank. 

Why Is It Important To Test Calcium Levels?

Maintaining the right calcium levels in your reef aquarium is critical for the fundamental survival of your corals, as well as your invertebrates too. Calcium is used to shape exoskeletons and forms an important part of water hardness. Successful coral aquariums will use the remaining calcium in the water and the levels will eventually deplete. It is important to achieve the right concentration of calcium. Within your tank, calcium levels should be maintained at 400-450 mg/l, a bit lower than natural seawater which has calcium levels of approximately 420 mg/l. If your calcium concentration levels fall below this range, this will result in slow growth of your corals. On the other hand, if the amount of Ca²⁺ is too high, the calcium binds to carbonates and precipitates, rendering it inaccessible for invertebrates and depleting carbonate hardness (KH). 

Why Is Calcium Hardness (KH) Important?

Carbonate hardness is the measurement of the combined presence of carbonates and bicarbonates in your tank water. Carbonate hardness is important to measure as it provides a buffering capacity to the pH of the water. A correct KH value provides a stabilizing effect to your water pH. For hard coral to build their calcium carbonate exoskeletons they use carbonates, and to a lesser extent, bicarbonates, as their predominant form of carbon. Through using carbonates or the processing of bicarbonates, their relative abundance is reduced. This process results in KH being consumed, which therefore leads to a reduction in pH. Basically, the absorption of carbonates and bicarbonates will reduce carbonate hardness levels. 

As we now know, when KH is decreased, pH will also tend to decrease. It is therefore important to monitor your tank KH weekly to ensure adequate KH values are maintained. To best replicate natural sea water, the KH should be kept at approximately 7.5 dKH. 

What Is Magnesium?

Magnesium is another very important element in any reef aquarium tank too. It is highly abundant, so typically you do not need to supplement magnesium in a marine aquarium.  However, magnesium can sometimes become reduced when manipulating the calcium concentration in your reef aquarium. Because of this, we recommend that you also regularly monitor your tank magnesium concentration. In a reef aquarium, a low magnesium concentration can affect the health and growth rate of your corals.

How To Maintain Correct Calcium Concentrations

If you have a lot of hard corals, your calcium levels are likely to drop much more quickly. Maintaining the right calcium levels is accomplished by regularly changing your tank water or using calcium additives. Calcium supplements would be the most common option in reef aquariums as they can deplete rapidly. It is vital to test your calcium levels before using any calcium supplement products to ensure you are not overdosing your tank. 

Remember, it is important to also test for magnesium when considering calcium. The ideal ratio of 3 parts Mg²⁺ to 1 parts Ca²⁺ should be adhered to ensure calcium is kept available and prevented from depleting carbonate hardness (KH).

How Can I Add More Calcium?

If your tank only has a low calcium demand, then simply changing your tank water regularly can help to boost calcium levels. However, continuous dosing is often required if you have lots of hard corals or other organisms that take-up calcium. Here are some products that we recommend to effectively and safely increase calcium levels:

• Seachem Reef Kalkwasser™
• Seachem Reef Advantage Calcium™
• Red Sea Reef Foundation CALCIUM+ | KH/ALKALINITY | MAGNESIUM Supplement
• API® KH/Carbonate Test Kit and Calcium Test Kit
• Hanna Instruments Marine Calcium Checker HC

Summary Of Ideal Water Conditions For Good Coral Growth:

Calcium level: 400-450 mg/l

Alkalinity: 7-10 dKH 

Magnesium: 1200-1300 ppm

pH: 8.2-8.2 

Salinity: 35-36 ppt

Understanding and implementing proper calcium and alkalinity supplementation may seem difficult at first, but with proper testing and patience, keeping these compounds at their proper levels will become quite simple.