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How to Start Dosing Calcium, Alkalinity and Magnesium in Reef Tanks

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Last updated on December 9th, 2022


How to Start Dosing Calcium, Alkalinity and Magnesium in Reef Tanks

Alkalinity, Calcium, and Magnesium

Understanding Calcium, Alkalinity, and Magnesium dosing is an important topic if you are going to keep calcium skeleton based coral, such as SPS and LPS coral. Soft coral do not care about alkalinity and calcium nearly as much and really don't require dosing these parameters. Before telling you how to dose alkalinity, calcium, and magnesium, I will teach you what these parameters are, and why they are important for your saltwater aquarium.

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Alkalinity, Calcium, and Magnesium - Table of Contents

I'll start by saying this article is a bit long, and you are likely looking for specific details. Use these links to go to the section you are interested in reading:

What is Alkalinity

Alkalinity is the "buffering capacity" of the water, or the amount of acid (or CO2 in our case) required to lower the pH of the water to the point where bicarbonate turns into carbonic acid. But what does this mean about alkalinity in a reef tank?

The higher the alkalinity in a reef tank, the more acid, or CO2, is needed to lower your reef tank's PH. Therefore, a higher alkalinity in a reef tank means a more stable environment for your coral.

Over time, coral and other calcium based life, such as coraline algae, will use the alkalinity by absorbing the carbonate from the water. Which is why it is important to measure, track, and dose alkalinity in a reef tank, using buffering components, such as Seachem Reef Fusion Two Part System.

There are a couple of common units used to measure alkalinity in a reef tank. The most common is "Degrees of carbonate hardness", better known as dKh or Kh. dKh is the concentration of calcium carbonate and magnesium carbonate in a specific amount of water. If you're a real science geek, 1dKh is 17.86mg calcium carbonate per liter of water (17.86ppm).

Another unit for measuring alkalinity in a reef tank is meq/l or milliequivalents per liter. To convert from dKH to meq/L, multiply the dKH by 0.357.

What is the Best Alkalinity Level for Reef Tanks?

There are multiple answers to the question "what should alkalinity be in a reef tank". The different alkalinity levels have their own advantages and disadvantages. You will have to decide the benefits and risks for yourself. But I will show you a little more about the options.

The generally accepted range is between 7 and 12 dKH, with the normal target being between 9 and 10 dKH. If you keep your alkalinity between 9 - 10 dKH in your reef, you have a safety buffer if something goes wrong. If you over or under dose alkalinity, you will still be within the safe range, and will have much less stress on the corals. Most reef tank owners, especially beginners, should aim for 9 - 10 dKH alkalinity for a reef tank.

Once you are able to maintain a stable alkalinity in a reef tank, there are benefits of increasing alkalinity to between 11 - 12 dKH. However, higher alkalinity levels comes with potential risks as well. Based on an experiment by Bulk Reef Supply (BRStv), higher alkalinity has shown to be about 70% more coral growth (depending on the coral). The risk is that you could end up over-dosing the alkalinity in the reef tank, causing coral death.

An additional note to this experiment is that the higher alkalinity may have simply caused higher pH, leading ot more growth. There is not a lot or information about which of these two parameters is responsible for coral growth. For more information about stabilizing pH, check out Stabilizing Reef Tank pH with Kalkwasser. If you are new to reef tank dosing, ignore this completely!

My recommendation is to maintain an alkalinity level between 9 and 10 dKh. If you are reading this, you are likely newer to dosing and shouldn't risk killing your entire reef tank for a little growth. Leave that to the people farming coral. Once you are able to hold constant at a value of about 9.5 dKh for a few months, slowly try to increase the alkalinity (no more than 1 dKH per month) and maintain your reef at that level. I recommend going no higher than 11 dKH alkalinity for reef tanks, so that there is a little wiggle room in case anything go wrong.

To figure out how much alkalinity is used by your reef tank each day, check out the article Alkalinity in a Reef Tank - Determining Alkalinity Consumption. Where I will walk you through how to measure the alkalinity consumption in your reef tank.

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Calcium for Reef Tanks - How do Corals Grow?

Calcium is one of the main ions used by coral in a reef tank to grow their skeletons. Coral skeletons are made of calcium carbonate. For coral to grow, they pull calcium and carbonate (alkalinity) ions from the water and combine them to build their skeletons. As a result of this, you will see your coral grow.

Ideal Calcium Level for Reef Tank

Unlike alkalinity levels in a reef tank, the ideal calcium level is much more simple to decide. The best amount of calcium for a reef tank is anything between 400-450 ppm.

There does not seem to be any advantage to higher calcium in reef tanks like there are for alkalinity. If your calcium level is too low, there may not be enough available calcium ions for your corals to grow. If your calcium level is too high for reef tanks (over 500ppm) then it can cause the alkalinity to precipitate out and make it difficult to raise the alkalinity of the reef tank. Once your alkalinity is at the level you want, just make sure your calcium level is between 400-450 ppm and everything should be okay.

Magnesium for Reef Tanks

Magnesium in reef tanks is another important ion for the growth of corals. Magnesium is less important than proper alkalinity and calcium levels for reef tanks and coral growth however. Magnesium allows for higher calcium levels and alkalinity without precipitating. It is important to maintain magnesium between 1250 and 1350 ppm, otherwise your alkalinity and calcium can react with each other and remove themselves from your reef tank. The consumption of Magnesium is fairly small which is why it is not measured as much as alkalinity and calcium.

Additional Information

If this doesn't quite satisfy the detail of the chemistry you're looking for, this video by Advanced Reef Aquarium gets into more detail.

Dosing Calcium, Alkalinity, and Magnesium in Reef Tanks

Reef Chemical Interactions

Calcium, Alkalinity, and Magnesium ions will all interact to balance each other in your reef tank. Changing the levels of one of these 3 parameters will affect the other 2 in some way. High calcium levels in reef tanks will bind with alkalinity and magnesium and precipitate, leading to a drop in all parameters and possibly a white "snow" in your sump or aquarium.

Why Dose Alkalinity and Calcium in a Reef Tank Infographic

Having higher magnesium levels can allow for a higher calcium levels and alkalinity in reef tanks before precipitating. The take away is that calcium binds with the ions from alkalinity buffers as well as magnesium. Higher magnesium will allow for a higher alkalinity before calcium cancels it out, as magnesium ions will be canceled out first.

Measuring and Tracking Reef Parameters

The first rule of dosing, or any other ways of modifying reef chemicals is:

Don't dose ANYTHING unless you are measuring it.

Modifying any parameter too much can lead to problems such as coral or fish death, and this is much more likely to occur when you are not measuring and tracking the parameters you're modifying. If you are increasing calcium levels in a reef tank that already has high calcium levels, you will likely cause a massive drop in alkalinity, leading to coral death. Additionally, adding alkalinity to a tank that is already high in alkalinity can cause coral death as well.

How Often to Measure Calcium, Alkalinity, and Magnesium in Reef Tanks

Everyone has their own opinion on how often to measure calcium, alkalinity, and magnesium in reef tanks. The obvious answer is that the more often you measure calcium, alkalinity, and magnesium, the better off you are. So measure all of your reef tank parameters daily, right? While ideal, this may not be realistic for most reef tank owners. Measuring calcium, alkalinity, and magnesium daily takes more time and money than most people can give. Measuring all of your reef tank parameters this frequently is even worse. Most of the time, these parameters do not change often, or at least not enough, to be worried about measuring daily. But naturally, some of them do. So what do I recommend for How Often to Measure Calcium, Alkalinity, and Magnesium in Reef Tanks?

How Often to Measure Calcium, Alkalinity, and Magnesium in Reef Tanks
Parameter Frequency
Alkalinity Daily
Calcium Weekly
Magnesium Bi-Weekly / Monthly

Reef Tank Chemistry: Understanding Alkalinity, Calcium, Magnesium, and pH in the Reef Tank

Reef Chemistry Book

Get it On Amazon

If you're a reef tank nerd like me, you probably want to learn as much as you can about alkalinity, calcium, magnesium, and pH in the Reef Tank. Though reading blog articles can get you the information you're looking for, it can change or be hard to find later. As such, there is nothing better than having a book or eBook to reference later. Check out Reef Tank Chemistry: Understanding Alkalinity, Calcium, Magnesium, and pH in the Reef Tank on Amazon.

Best Reef Tank Test Kits

Equally as important to measuring often is using a high quality reef tank test kit. Using the cheapest test available will give you a rough idea of your calcium, alkalinity, and magnesium levels, but they may not be accurate as you need for a reef tank. For that reason, these are the best reef tank test kits recommended by Reef Stable.

Hanna Checkers

Hanna checkers are an electronic reef tank test kit, allowing better accuracy and/or ease of testing. Hanna Checker electronic reef tank test kits are generally a bit more expensive up front, but the reagents are relatively inexpensive. This way you don't need to buy a new test kit every time your chemicals run out. The accuracy and ease of use of the Hanna Checker electronic reef tank test kits is well worth the up front cost. Hanna Checkers also remove the uncertainty of trying to match a color to the closest ink box on a piece of paper.

I should also note, the Calcium Reef Tank Test Kit by Hanna often gets scrutiny for having less accuracy than other test kits. Though this is true, the value given by the Calcium Hanna Checker is fairly close, better than many of the cheaper reef tank test kits. The Hanna Checker is much faster most other test kits and is honestly close enough for most reef keepers.

Recommended Reef Tank Test Kits:

Red Sea Reef Tank Test Kits

Red Sea Reef Tank Test Kits are another accurate method of testing these calcium, alkalinity and magnesium in reef tanks, though not as easy to use as the Hanna Checker. Red Sea Reef Tank Test Kits are a bit more involved, but much cheaper up front. The Calcium Red Sea Reef Tank Test Kit is also more accurate than the Hanna Checker version. This reef tank test kits for all three parameters can be purchased for roughly the same price as one Hanna Checker. The Red Sea Reef Tank Test Kit would be a good place to start while getting used to dosing and measuring if the Hanna Checkers are not in the budget.

Recommended Reef Tank Test Kits:

How to Balance Calcium, Alkalinity, and Magnesium in Reef Tanks

It's important to remember that when you increase the alkalinity in reef tanks, it may decrease the calcium level and vice verse. For that reason, you need to focus on one parameter at a time.

Alkalinity is always the hardest to stabilize in reef tanks. Alkalinity is also the most important of the three reef tank parameters. This makes Alkalinity the best place to start getting to the desired level.

Make sure that while adjusting alkalinity, you are monitoring calcium levels to make sure it's not outside the safe range. If Calcium levels do leave the safe range, you will need to adjust the calcium levels.

If the calcium is too high, you can lower calcium levels with water changes. If calcium levels are too low, it can be raised by dosing a small amount of calcium daily until the ideal level is hit.

If you're new to dosing and adjusting parameters, you may not know what stable looks like. Many people have different ideas about what is an acceptable swing in alkalinity from day to day. In general, I try to ensure that alkalinity levels stay within 0.5 dKh of my target value. Here is an example of a week of measurements.

Stable Alkalinity in Reef Tanks - Reef Stable Aquarium Parameter Log

As you can see, most of the alkalinity values are consistent, with one point one that seems very high. This is because that value was measured at a different time, shortly after dosing an alkalinity buffer. All of the other data points are measured one hour after adding an alkalinity buffer to the reef tank. Allowing time for the alkalinity buffer to dissolve.

When you see something like this, it is important to understand if the reason is due to timing or other factors. Measuring at the right time may fix the problem. Don't jump too quickly and try to change the alkalinity or other parameters' level. Acting too quickly will lead to more problems. Measuring at the same time daily will help show stable reef tank measurements.

Once your alkalinity level is in check, you can start trying to focus on your calcium levels. It is best to ensure Calcium is in the safe range. Beyond that, there is not much else you need to do. Additionally, check the magnesium levels and make sure it is also in the good range.

Stable Magnesium and Calcium Levels in Reef Tanks - Reef Stable Aquarium Parameter Log

As you can see, calcium levels and magnesium are not measured as often. The key is to just make sure they are within the target range. Calcium levels and Magnesium are not measured as often because slightly high or low results generally do not tend to lead to coral death. Instable Alkalinity in reef tanks has potential to kill corals quickly.

Methods of Dosing Calcium, Alkalinity, and Magnesium in Reef Tanks

2-Part Dosing

Dosing Calcium, Alkalinity, and Magnesium, best known as 2-Part solutions, is the most popular way to maintain control over Calcium, Alkalinity, and Magnesium. Alkalinity Buffers and Calcium supplements are separated in two or three containers and you will determine the amount to dose by tracking the values over a few days or a week and increase your 2-part dosing amount accordingly. Some of the 2-part solutions are split to 3 parts. The third is a magnesium supplement, allowing greater control over the parameters. If this is not an option, then it is generally mixed in with the alkalinity buffer.

It is important to have these chemicals added to high flow areas and separate the 2-part dosing times by at least an hour to prevent the ions from precipitating out.

Automatic dosing pumps are frequently used to do this with built in timers to remove the human error and timing components. You can also measure and add Calcium, Alkalinity, and Magnesium supplements by hand with simple measuring utensils and containers.

If you choose a 2-part dosing brand, and decide to use a different brand later on, don't worry. I have a guide here, How to Switch 2-Part Solutions, to help you with just that!


Kalkwasser is the use of lime water to maintain alkalinity and calcium levels in reef tanks. Again you will need to track the levels over time to determine if you need more or less based on coral consumption. Once dialed in, simply adding the kalkwasswer to your auto top-off (ATO) or top-off water if you are topping off the tank manually, will maintain your parameters.

There are thought to be additional benefits of kalkwasser such as phosphate removal. One of the proven benefits of kalkwasser is that it will raise your pH! The difficulty is that there is a maximum saturation, meaning large tanks or tanks with high alkalinity consumption would require a large amount of kalkwasser. There is a way to super-saturate kalwasser if you face this problem. It is also important to mention that kalkwasser does not increase magnesium, so you will need to dose that seperatly.

Calcium Reactors

Calcium reactors are the more advanced method of maintaining calcium, alkalinity, and magnesium in reef tanks. The upfront cost will deter most from making the investment however. Calcium reactors work by dissolving coral skeletons. Because of this, you will not need to add media frequently. You will need to monitor and replenish a CO2 tank though which does add a bit more complexity than dosing 2-part.

In addition to complexity, Calcium reactors do lower your reef tank pH because of the added CO2. This may not be as bad depending on how it is dialed in. Some ways to fix this include using an additional reactor chamber, supplementing with kalwasser, or even using a recirculating skimmer.

Which Dosing Method is Best?

Like anything else in this hobby, there are no definite answers. You need to decide the method that works best for you. 2-Part dosing and Kalkwasser dosing can be used to supplement each other or even a calcium reactor.

In general, kalkwasser dosing is the easiest all-around method because it is 1 chemical for alkalinity and calcium. 2-part gives you better control over the individual parameters. Calcium reactors are good for large aquariums with a lot of calcium based coral. Make sure to do your research and decide what method is best for you!

Solving Alkalinity and Calcium Problems

Depending on which one(s) of the parameters and what direction the are off, there are different ways to bring the chemicals back in balance.

High Alkalinity and Normal or High Calcium

Slightly reduce the amount of alkalinity being added to the tank. The coral will consume the alkalinity and bring the alkalinity level back down. You will need to re-determine the alkalinity consumption rate.

Normal Alkalinity and High Calcium

This is the easier of the problems. Simply stop adding calcium to the tank and the corals should consume the excess. Then reduce the amount of calcium dosed and re-determine the consumption rate.

Low Alkalinity and/or Calcium

If either alkalinity or calcium levels are low, simply dose either an alkalinity buffer or calcium supplement to bring the levels back in check.

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I went over how to balance alkalinity, calcium and magnesium using a variety of methods. I also looked at the benefits and difficulties of these different dosing methods, and how to solve a variety of reef tank parameter problems such as high alkalinity and low calcium. If you're looking to learn more about how alkalinity, calcium, magnesium, and pH affect your reef tank, check out my book on Amazon, Reef Tank Chemistry.

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About the Author

Reef Stable Founder John Krenzer

John is a Software Engineer with a passion for saltwater aquariums, as well as the founder and president of Reef Stable. He started in the aquarium hobby as a child with a 20 gallon freshwater aquarium. His interest in aquarium life grew and in 2008, John set up his first saltwater aquarium.

Today, John maintains an over 300 gallon reef tank system, consisting of a 120g reef and a 210g reef. These large tanks are contained within the same system, sharing a sump as a means to reduce total maintenance and increase total water volume.

John writes articles for the blog as a means to learn about more reef aquarium topics. These articles act as a reference for the readers as well as himself. John updates these articles frequently to provide additional information or make corrections as new information becomes available.

If you would like to request an article, tank tour article, or to colaborate, let me know via the Contact Me Page!

About Reef Stable

Reef Stable was initially founded in 2019 as a reef tank parameter log to fill a need. Reef Stable quickly grew, becoming a location to solve all of your reef tank problems as well as a place to learn.

Reef Stable now provides a Reef Blog, Reef Aquarium Guides, Coral Care Guides, Identification and Solutions for Pests and Algae, and Reef Dosing Calculators, in addition to the original Reef Parameter Log.

Reef Stable continues to grow, striving to provide a single location for all your reef tank needs!

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