Calcium is an essential building block for wild and captive reefs alike. Calcium combines with carbonates (alkalinity) and other elements in the water to create calcium carbonate. Stony corals synthesize calcium carbonate along with trace elements from the water in order to build rigid skeletons which ultimately end up creating a foundation for wild reefs.

In captivity, SPS and LPS corals utilize calcium carbonate and trace elements in the same way in order to grow and build skeletons. Other organisms, such as coralline algae, Halimeda, and even giant clams, also use calcium carbonate. Experienced hobbyists know that keeping their calcium and alkalinity levels stable ensures plenty of calcium carbonate is available for their corals, which ultimately results in a flourishing reef.

This article covers everything you need to know in order to maintain consistent calcium and alkalinity levels, including (but not limited) to testing your water, dosing your tank, and running a calcium or kalkwasser (calcium hydroxide) reactor with your system.

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Water Testing

The first (and most important) step in calcium and alkalinity maintenance is to test your tank water. Quality test kits are important! The results must be accurate in order for you to keep levels within ideal ranges (see Reef Tank Parameters Chart). We carry several liquid titration-type test kits which are great for most hobbyists and tend to be the most affordable option. Red Sea, Salifert, AquaForest, and Seachem are all popular choices among our customers.

Hanna Instruments recently released a line of Checker Colorimeters that display digital readings of individual water parameters. This technology certainly makes water testing less labor-intensive by providing near-instantaneous readings and removes much of the risk of human error from the process. The Alkalinity Checker specifically is our absolute favorite for monitoring dKH.

The desired calcium level in reef aquaria is 380 to 450ppm. The ideal alkalinity level in most reef aquariums is between 8 and 12 dKH. Try to maintain the most consistent levels possible on a daily basis. Less fluctuation equals less stress on your system. Do your best to keep fluctuations to a minimum and within the aforementioned ranges.

During the first few weeks, you will probably end up testing daily in order to become comfortable with the test kits and monitor your aquarium closely. I generally recommend charting/recording these daily tests. Once you establish a good system of maintaining calcium levels, daily tests are often not required for success and can be reduced to weekly or as needed.

Reaching the Proper Levels

It is important to understand you must begin with proper calcium and alkalinity levels before you attempt to maintain the levels on a daily basis. If you start with calcium levels that are either too low or too high, it will be difficult to properly maintain the levels. Most solutions in the hobby are designed to maintain calcium and alkalinity versus dramatically adjusting their levels.

Frequent, small water changes with good quality saltwater will typically provide your tank with a balanced level of calcium and alkalinity. One key fact to understand about calcium and alkalinity is that if one level is out of range, the other will move in the opposite direction. For example, if your calcium level is 500ppm or more, your alkalinity will likely test quite low. If your alkalinity tests over 13 dKH, your calcium will most likely be below 380. The best way to correct this problem is small frequent water changes with well balanced saltwater.

If you need to boost calcium, you can use a supplement like BRS Bulk Pharma Calcium Chloride or E.S.V. Calcium Chloride. If you need to boost alkalinity, try BRS Bulk Pharma Alkalinity Supplements. Start with the minimum recommended dosage and continue until desired calcium and alkalinity levels are reached. These products can also be used to maintain calcium and alkalinity but may lead to spikes or fluctuations if not dosed correctly.

Maintaining Proper Levels

2-Part Calcium and Alkalinity Supplements

The easiest and most popular way to maintain calcium and alkalinity levels in a reef aquarium is to use balanced 2-part calcium and alkalinity supplements. We recommend 2-part supplements, such as BRS Bulk Pharma, that contain calcium and alkalinity in balanced ratios, along with trace elements. This maintenance method has been widely adopted by hobbyists because the dosage calculations are simple and it is difficult for your tank to become unbalanced when dosed as instructed.

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Start out with the minimum recommended amount. Dose each part at least 10 minutes apart and test 24 hours later. If you find your calcium and alkalinity levels are not ideal, increase the daily dosage in small amounts until desired levels are achieved. We have some handy aquarium calculators on our website that can help you calculate gallons and make metric conversions if need be, since different products use different units of measure.

Calcium Reactors

Next, we will take a look at calcium reactors. Calcium reactors are generally found on more advanced reef aquariums as they are a bit more difficult and expensive to apply. They do, however, provide a constant level of calcium carbonate in your system and eliminate the need for daily dosing. This is the most effective way to maintain consistent levels of calcium in a captive reef.


Calcium reactors are essentially a pressurized, fluidized chamber full of dead coral skeletons (aragonite). This chamber allows you to safely mix your aquarium water with CO2 to create an acidic environment that melts the aragonite, in turn enriching the water with calcium carbonate. You then slowly drip this calcium carbonate-rich water back into your aquarium system to maintain proper levels. Read "What Is a Calcium Reactor" to learn more!

With a pH controller and solenoid, you can basically automate the input of CO2. After setup, you dial in your calcium reactor to find the proper balance of CO2 injection and drip-rate. Every tank differs as far as how much calcium will be needed, so tuning a reactor varies from user to user. Remember, our team of aquarium experts is always available to answer questions, offer advice and walk you through the installation of new equipment.


The last subject we will cover today is calcium hydroxide, also known as kalkwasser. Kalkwasser is a very popular way to maintain calcium and alkalinity in a reef aquarium. This method can be a little tricky since it requires frequent testing, but it can also be a much less expensive way to maintain calcium and alkalinity compared to the methods described above.

First, kalkwasser is dissolved into freshwater. Once dissolved, the solid white precipitate will settle on the bottom of the mixing chamber and the clear remaining liquid on top is slowly dripped/dosed into the aquarium. Kalkwasser is designed to maintain both calcium and alkalinity levels in your reef tank.

The use of a Kalkwasser reactor is the most efficient method of delivery but you can also manually dose Kalkwasser and even mix the powder into your top-off water for delivery via the ATO. Kalkwasser is great because it is affordable and easy to use. If too much is added too quickly, however, kalkwasser can drastically increase pH and result in improperly balanced calcium and alkalinity levels. It is very important to test your pH, calcium, and alkalinity levels daily in order to find out how much kalkwasser you need.

Some hobbyists only dose kalkwasser in the evening when pH is naturally lower. This helps maintain a more constant pH in your system and prevents pH from increasing to dangerous levels during the day. As long as you start with the lowest recommended dosage and slowly increase in very small increments, you should not have a problem.


Reef aquariums can certainly be intimidating with all of the different products, supplements and techniques out there. Hopefully this article has narrowed things down a bit. Remember, calcium and alkalinity are among the most important water parameters in a captive reef environment. By combining one of the maintenance methods in this article with regular water changes and testing, you should have little trouble keeping your calcium and alkalinity levels stable and your coral happy.