Calcium: A Critical Element in Reef Aquariums

Popular calcium test kit

Popular calcium test kit

The stony corals, crustaceans, mollusks, and echinoderms in our marine aquariums depend on it to build their skeletons/shells/tests. Soft corals use it to build supportive structures, called sclerites, in their tissues. Of course, without it, you’ll never get a nice patina of coralline algae on your rockwork.

The “it” I’m referring to is calcium, and reefkeepers need to monitor the level of this element in their systems closely and possibly supplement it if they hope to maintain healthy invertebrates.

What’s the correct calcium level?

The appropriate range for calcium in a marine aquarium is somewhere between 380 and 450 ppm. But keep in mind that it’s more important to maintain a stable value somewhere within that range than to hit a specific target value.

If you read my prior post on alkalinity, you understand that there’s an interdependent relationship between calcium and alkalinity. Because a given volume of water can hold only so many dissolved solids, increasing the level of one tends to drive down the level of the other. Thus, it’s not always possible to keep both calcium and alkalinity at the high end of their respective ranges simultaneously.

Calfo’s Marble Analogy

The best explanation I’ve read for this phenomenon is Anthony Calfo’s Marble Analogy—primarily because he put it in such simple terms that even my lone functioning brain cell can comprehend it. Here’s how he phrased it in his Book of Coral Propagation (Reading Trees Publications, 2007), which, by the way, I strongly recommend for your reference collection:

Imagine a bowl that holds one hundred marbles representing the total dissolved solids in seawater in a given system. If red marbles represent calcium, and blue marbles represent alkalinity, the bowl can still only hold one hundred marbles no matter what mix of colors they are. Thus, if seventy red marbles are the equivalent of 400ppm calcium and the remaining marbles are blue, the only way to increase calcium is to displace alkalinity (remove blue marbles).

I get that!

Replenishing calcium

Different marine aquariums can have very different calcium demands depending on the organisms kept. Also, keep in mind that a reef system’s calcium demand can change significantly over time as the corals and other invertebrates contained within it grow.

For marine tanks with a low calcium demand, such as fish-only systems or reef tanks that are lightly to moderately stocked with soft corals, it’s usually possible to maintain proper calcium and alkalinity levels with routine partial water changes. However, in systems containing stony corals and/or Tridacna clams, mixed stony/softy systems, or even more heavily stocked softy tanks, some form of calcium supplementation—in conjunction with alkalinity supplementation—is often necessary. Regular testing will help you determine whether supplementation is needed to keep pace with your system’s calcium demand.

Supplementation techniques

Here are the more common methods reefkeepers use (either alone or in combination) to supplement calcium/alkalinity along with a brief description of each:

Balanced two-part supplements

The easiest method, this simply involves dosing the tank with equal amounts of liquid or dry calcium and alkalinity supplements according to the manufacturer’s recommendations. Two-part supplements are very convenient but can get prohibitively expensive when used for larger tanks.

Calcium reactor

This device consists of a reaction chamber filled with a calcareous medium (typically aragonite), through which aquarium water flows. Carbon dioxide is introduced into the reaction chamber, lowering the pH of the water and dissolving the calcareous medium, which liberates calcium and bolsters alkalinity. The initial setup for a calcium reactor can be somewhat costly and complicated (for those who confuse easily, such as myself), but once everything is properly tweaked and running properly, ongoing costs aren’t too bad and you’ve essentially automated the chore of calcium/alkalinity supplementation.


Kalkwasser, or limewater, supplementation is the cheapest method of maintaining calcium/alkalinity, but it can also be somewhat labor intensive depending on the delivery method used. The low-tech approach involves mixing calcium hydroxide into purified tap water in a glass or plastic container, allowing time for any undissolved powder to settle to the bottom of the container, decanting off the clear to somewhat milky water above the settled powder, and then dripping it very slowly into your aquarium.

You can drip the kalkwasser from a simple DIY doser (such as the one made from a milk jug, described in “My Top Six Simple Accessories Repurposed for Marine Aquariums”) or one of the various commercially manufactured dosers on the market. Kalkwasser can also be delivered via a kalkwasser reactor in conjunction with an automatic top-off system if you’d prefer to automate the process. There are numerous options—and widely varying degrees of complexity—for dosing kalkwasser, so you’ll need to do a little research to find the method that fits your reefkeeping style and budget.

Because kalkwasser is dosed in place of regular freshwater top-offs, the biggest drawback to this method is that the amount you can add at any given time is limited by your system’s rate of evaporation. If the evaporation rate tends to be relatively slow, it’s possible for the system’s calcium demand to exceed the amount of calcium that can be replenished via kalkwasser top-offs.


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About Jeff Kurtz

Jeff Kurtz is the Co-founder/Editor of Saltwater Smarts, former Senior Consulting Editor for Tropical Fish Hobbyist Magazine, and the aquarist formerly known as “The Salt Creep.” He has been an aquarium hobbyist for over 30 years and is an avid scuba diver.

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