Alkalinity and Calcium are two of the most important building blocks to a successful reef aquarium. Stony corals such as SPS and LPS lay down calcium carbonate skeletons by raising their internal pH through chemical processes that take place inside the coral tissue. In this elevated pH area corals are able to capture calcium carbonate from the water column and lay it down as part of their skeleton. The act of doing so necessarily consumes both Alkalinity and Calcium. 

Alkalinity and Calcium have a complex relationship and we do not claim to be the ultimate scientific authority. We highly recommend that hobbyists interested in the granular scientific details between these two parameters research and read articles by famed Reef Chemist Randy Holmes-Farley as we could never explain it more thoroughly than him. This guide is intended to be more digestible and high level overview for the average hobbyist, and, at the end of the day, is the only real information most reef keepers need to know.

Quick Index

What is alkalinity and how does it impact my reef?

Total Alkalinity is actually just a measure of how much acid it would take to lower the pH of a sample of water to the bicarbonate endpoint. In seawater that end point is a pH of 4.2, so a simple titration of Alkalinity requires adding enough acid to the water sample to reach a pH of 4.2. Much of the alkalinity in natural seawater is contributed by bicarbonate (approximately 90%), carbonate (approximately 7%) and borate (approximately 3%) with everything else occurring at irrelevant percentages although the distribution can change significantly in reef aquaria where borate can be substantially elevated and the pH can be far away from natural sea water pH of 8.2 (reef tanks tend to range from 7.8-8.6).

Alkalinity is an important surrogate measurement of the concentration of the actual building blocks carbonate and bicarbonate in your water. Corals that lay calcium carbonate skeletons (like all of the stony corals) acquire that calcium and carbonate from your reef tank's water column.

Alkalinity can be defined by a few different units of measure most commonly meq/L and dKH which stands for (degrees of carbonate hardness). At Reef Chasers, we prefer to use dKH as it is a more fine tuned measurement and is what the Neptune Trident tests for. Simple conversion calculators can be found online to convert dKH into meq/L and vice-a-versa. Natural seawater tends to have an alkalinity of around 7 dKH or 2.5meq/L but in many reef aquaria we seek to maintain this parameter at elevated levels. At Reef Chasers, we aim to keep it above 8 and below 9, typically 8.5 dKH or 3 meq/L.

Some research suggests that higher Alkalinity can lead to faster growth rates although nutrition and lighting must also be considered when considering anything above 9 dKH. It is often reported that high alkalinity can lead to 'burnt tips' in growing SPS corals because the skeleton is growing faster than the tissues. Many hobbyists suggest maintaining higher nutrients (Nitrates and Phosphates) to combat this, which will help the tissue to grow faster. However, high nutrients can lead to browning of SPS Corals and in order to combat that issue many hobbyists suggest to increase the lighting and par.  So it is in this way that Lighting, Nutrients and Alkalinity are connected to coral health and coloration. When we talk about this we are usually talking about Acropora spp corals, as most other corals do not present burnt tips, or the ability to totally change colors from brown to rainbow.

Corals, especially sensitive species like Acropora and other SPS varieties and even some LPS types can become very upset if Alkalinity is allowed to swing by more than 10% per day. In general, your entire reef system will benefit greatly from stabilizing your alkalinity and limiting the swing to < 5%, or removing any swings at all. Although zero tolerance is impossible, we should still aim to keep it as stable as possible. 

A swing of 0.5 dKh through out a 24 hour period is not of that much concern, even up to 1 dKH would not require quick and decisive actions although we would not want our Alkalinity to swing 1 dKh every day if we can avoid it.

New systems, small systems, or systems generally light in stony coral biomass often do not need much additional supplementation and the wonderful magic of weekly water changes can often maintain and stabilize the levels with relative ease. However, larger systems or systems with high demand (even small systems that are packed full of healthy thriving stony corals) will require additional supplementation and aggressive monitoring and maintenance in order to keep this important resource stable.

We rely on automated trident testing to monitor this parameter and we review tests around 4 times per day (it's an important parameter). We also do manual testing using Salifert Test kits, but other's may work as well. We tend to only take the manual tests a few times a week and this is to insure that our trident is calibrated and performing correctly. The automated trident system tends to report lower results and the manual tests higher, but in general they track closely with one another in terms of peaks and troughs.

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What is calcium and how does it impact my reef?


In reef aquaria we generally seek to maintain this in the range of 380ppm-440ppm although many salt mixes come at 450 as the baseline. The reason why salt mixes have higher calcium usually involves the replenishment of calcium to your aquarium through water changes which is one of the most effective ways of supplementing small, low demand systems.

From famed Reef Chemist Randy Holmes-Farley: "In the ocean calcium is a major ion of seawater which makes up around 1.2% of the total weight of its solids which is 410ppm. Seawater actually contains far more calcium than carbonate or bicarbonate. Even if all the alkalinity in normal seawater was removed by precipitating calcium carbonate, the calcium would drop by only about 50 ppm. For this reason, alkalinity varies much more rapidly and extensively, on a percentage basis, than does calcium when both are over- or under-dosed, relative to their demand."

That is to say that if an overdose of Calcium occured it would likely only result in a small uptick in your calcium measurements, say 10ppm calcium. So your 410 ppm Calcium tests 420 PPM the next day. Clearly not that big of a deal. However if that same overdose occurs on the alkalinity side of things, it can lead to a 1 dKH swing or over 10% swing in total alkalinity. It is important for all hobbyists to remember this principal and realize that alkalinity is much harder to control and far more sensitive than calcium. This also means that alkalinity is easier to correct both up and down than calcium since it's rate of change and the impact of dosing is much sharper.

Water Changes are a great choice for supplementing calcium in small systems with low demand. If you start at 440 Calcium, and drop to 430 during the week due to coral consumption and coralline growth then you can do a 10-20% water change with a salt mix at 450 and you will see that some calcium will be supplemented to your system through that water change. Frequent water changes can help small systems with light biomass maintain the stability of this parameter without the need for any other forms of supplementation. We recommend 10% per week.

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    From the famed Reef Chemist Randy Holmes-Farley (read more):

    "In surface seawater, calcium carbonate is supersaturated. It also means that calcium and carbonate are poised to precipitate any time they are given the opportunity. If the pH rises from 7.5 to 8.5, there is approximately a ten-fold increase in the carbonate concentration. From pH 8.0 to 8.5, the increase in carbonate is about threefold.

    Reef tanks often have higher alkalinity and higher calcium than seawater, and hence are more supersaturated than seawater. In tanks with a high pH (such as many tanks using limewater/kalkwasser) the supersaturation is also higher than in seawater. At the same alkalinity, if you raise the pH, you convert some of the bicarbonate into carbonate:

    Combining these various factors, here are some combinations of calcium, alkalinity, and pH that have equal supersaturation with respect to aragonite:"


    Risky Parameters (Water is exactly saturated, any lower and calcium carobnate skeletons will start to dissolve and your corals will die)

    • pH = 7.7, Calcium = 410 ppm, Alkalinity = 2.5 meq/L
    • pH = 8.2, Calcium = 340 ppm, Alkalinity = 1.0 meq/L

    Normal Seawater Parameters (Water is supersaturated such as the case with normal seawater)

    • pH = 8.2, Calcium = 410 ppm, Alkalinity = 2.5 meq/L
    • pH = 8.0, Calcium = 410 ppm, Alkalinity = 4.0 meq/L
    • pH = 8.4, Calcium = 260 ppm, Alkalinity = 2.5 meq/L

    Risky Parameters (Water is too supersaturated and the chance for precipitation of carbonate increases)

    • pH = 8.2, Calcium = 410 ppm, Alkalinity = 5.0 meq/L
    • pH = 8.2, Calcium = 820 ppm, Alkalinity = 2.5 meq/L
    • pH = 8.0, Calcium = 410 ppm, Alkalinity = 8.0 meq/L
    • pH = 8.7, Calcium = 410 ppm, Alkalinity = 2.5 meq/L
    • pH = 8.4, Calcium = 410 ppm, Alkalinity = 4.2 meq/L


    When using supplementations such as Limewater/Kalkwasser, an overdose can lead to too much pH, calcium and alkalinity and such an increase can lead to a massive precipitation event of carbonate and bicarbonate which will turn your Reef Aquarium into a snow globe. Take our word for it -- we know a thing or two because we've seen a thing or two! It is important to be careful not to over-saturate your water too much. This is like goldilocks, too much or too little is no good, we want it just right.

    If this happens, do not panic and stop dosing supplements for a bit. These situations typically resolve themselves as the precipitation leads to a decreases in saturation levels.

    At Reef Chasers we aim for 8.2 pH, 420-430 Calcium and 4.0 meq/L or 8-9 dKH (typically 8.5) and we try to keep these parameters as stable as we possibly can. Of course are pH does fluctuate and can hit peaks as high as 8.35 and troughs as low as 8.15, on the good days we try to keep the range stable and tight at 8.2-8.3

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    Well, we don't have a great answer for that one other than to say it's not something we ever have to supplement and the value of it rarely ever changes or moves around much. The parameter seems to stay stable without much needed in the way of supplementation beyond what is provided by simple water changes. Our recommendation to the average hobbyist is simply to not worry very much about magnesium. If you feel compelled to test for it, I wouldn't do so all that often. Perhaps just once per month.

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    Below we discuss different strategies for maintaining and supplementing Alkalinity and Calcium in Reef Aquaria. Often times large, high demand systems will require at least 2, if not all 3 of these solutions and at Reef Chasers often rely on a combination of all of them. It goes without saying that for small, low demand systems often times WATER CHANGES are all that are needed to supplement Alkalinity, Calcium and Trace Elements. It also goes without saying it's only as effective as you are consistent. Weekly 10% water changes is ideal if the goal is to maintain and stabilize these parameters in a smaller, low demand reef tank.

    For larger and especially higher demand reef tanks, water changes are often more difficult and costly while being less effective.



    At Reef Chasers we heavily rely on Kalkwasser (also known as Limewater) for our supplementation strategy. We dose the kalkwasser only AFTER the main photo period has ended (typically 1 hour into the rampdown) and we stop dosing it as soon as the photo period begins (typically one hour after ramp up starts). By dosing the Alkalinity and Calcium supplements at night time, you can offset the diurnal swing of pH caused by photosynthesis. This will help further increase AND stabilize your systems overall pH. Kalkwasser is the most potent supplementation of Calcium and Alkalinity for affecting pH. The pH effects of Kalkwasser dosing are much higher than that of twp part dosing.

    At Reef Chasers we use a fifty gallon 'infinite kalkwasser' reservoir which is fed constantly with RODI water using a float valve, which means it is infinitely topped off. We super-saturate the container with wayyyy too much kalkwasser (which leads to most of it just settling on the bottom of the container, undissolved) and we activate a flow pump using a mechanical timer for 1 hour each day to mix the "infinite reservoir" up and keep it's saturation potent. This requires very little maintenance on our part and allows us to dose a LOT of kalkwasser.  We simply add another bag of kalkwasser powder to the reservoir as needed (about once every month).

    ** We would like to note that one time during a power outage the mechanical timer failed and the pump remained on inside the infinite kalkwasser reservoir. This was bad news when the system began dosing at 5pm and it was a highly concentrated slurry coming through the line. To keep our systems safe, we made sure to implement some programming on our neptune that will turn off the kalk doser if pH rises above 8.4 to avoid any disastrous precipitation events**

    Some of our 1,200+ gallon systems receive over 10 gallons of Dosed Kalkwasser per day! Obviously most home aquariums will not have this level of demand for supplementation, but any reef tank regardless of size can benefit from the use of kalkwasser.  Kalkwasser is a great choice as it contains both Calcium and Alkalinity as a balanced additive. That is to say they are in the correct ratios so that an imbalance of these parameters will not occur by dosing it.  Kalkwasser also has the added benefit of being superb at increasing your systems overall pH. In many cases kalkwasser is the only supplementation needed to get pH up and maintain stable Alkalinity and Calcium parameters but can pair well with other forms such as Calcium Reactors and 2 Part Dosing. 

    For small home aquariums, a simple solution can be to add a tiny (think: teaspooon or less) amount of kalkwasser powder to the ATO Top Off Reservoir. If doing this strategy, we recommend starting very small indeed and monitoring the effects on your system. Increasing the amount slowly over time once you understand how it works. We do not like this implementation at Reef Chasers because it lacks control although I have done this on smaller systems that I keep personally at my home. There is no way to dial back or control the amount of evaporation of your system and there for the dosing through the ATO will not be stable or consistent. If alkalinity or calcium begins to rise above desirable levels, there is little you can do besides emptying/cleaning your ATO reservoir to remove or reduce the kalkwasser.

    Even for small home aquariums 30 gallons or more we would recommend setting up a doser for Kalkwasser to have the fine control necessary to stabilize and maintain these important parameters. On small aquariums a container as small as 1 Liter can be used to dose kalkwasser, which may only take 100ml or less per night. Alternatively, a 1 gallon container, 5 gallon bucket, or 10 gallon trash can make good choices for dosing reservoirs based on your systems over all volume, the amount needed for nightly dosing, and the amount of time you want to spend refilling/maintaining such a reservoir. In our case we opt for one that infinitely tops itself off.

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    Two part dosing can be an effective means at stabilizing and controlling alkalinity and calcium and is often implemented with an automated doser for consistency and stability. Soda Ash for alkalinity and Calcium Chloride for calcium can be prepared at home using DIY recipes (Thanks Randy!) or bought cheaply online from popular vendors like Bulk Reef Supply (I am pretty sure they used Randy's recipe!).

    It is best to concentrate the dosing of these two parts during the night time hours just as with limewater in order to best offset the diurnal pH swing of your reef aquarium although it should be noted that there is less of a pH boosting effect with two part supplementation than there is with Kalkwasser/Limewater. Even still, I have seen nice effects from dosing a "little bit" of soda ash alongside limewater even though I do not need to. I don't know what it is, but there does appear to be some different affect on the pH when introducing a bit of soda ash even if you are primarily supplementing with Kalkwasser/Limewater. I can't quite put my finger on why this would be, but chemical processes are very complicated in reef aquaria and sometimes there is no telling. Perhaps Randy would know!

    Also it is important to note that due to biological processes that take place in your reef tank alkalinity can shift out of balance with your calcium. In these scenarios the easiest way to make corrections to one and not the other is by using 2 part. Lets say for example that my alkalinity has risen disproportionately to my calcium then in this case I can simply dose a little extra calcium to get that balance back in line and then allow them to both come down naturally and slowly by reducing or restricting my supplementation for a night or two. Likewise if my alkalinity has depleted more and has fallen out of balance with calcium, I can dose Soda Ash to raise it independently from the calcium.

    Even with good strategies using limewater or calcium reactor, 2 part dosing will still have a place as corrections are sometimes needed. As with all supplementations, you should start slow and conservatively with your dosing, gauge its impact on your reef and your parameters, and adjust upwards from there.

    If using automated doser we highly recommend the neptune ecosystem and we recommend putting some programming protections in place to prevent over-dosing. With the neptune this can be as easy as adding a line of programming that says when Alk is too high, stop or likewise for Calcium.

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    Calcium reactors can be complicated and expensive, or simple and cheap (even made DIY). Like with most things you get what you pay for. Often times the simple and cheap implementations require constant fiddling to get them performing just right. The more expensive and complicated automatic solutions often require a lot less maintenance. 

    How do they work?

    Calcium reactor's work by using crushed coral sometimes called argonite in a chamber. Water is pumped to the chamber from the aquarium, where it is infused with CO2 (requires a co2 tank) to drive the pH down to approximately 6.8. At this low pH the water inside the calcium reactor is acidic, and the dissolution of the crushed coral releases Calcium, Alkalinity and all minor and major trace elements present in the argonite.  

    This is obviously fantastic. No need to worry about minor elements like lithium, iron, borate, rhubidium and other unknowns. You're getting everything you need in a balanced fashion including calcium and alkalinity. 

    Drawbacks to Calcium Reactors

    The one drawback? The water being returned to your aquarium is at 6.8 pH and systems that run calcium reactors often suffer from chronic low pH. It has been theorized that low pH results in slower coral growth although the jury is still out on that one as coral growth is complicated and is a measure of Alkalinity, Calcium and pH not just pH alone.

    People who use calcium reactors often try to degas the effluent by shooting it directly into their skimmer intake, which will help to remove some of that CO2 and raise the pH of the water returning to the system. Another strategy is to dump the low pH, high co2 water into the refugium where your macro algae is growing and photosynthesizing at night. The photosynthesis taking place in the macro algae refugium will effectively degas some of the co2 present in the water and your algae will also like it!

    How we implement Calcium Reactors at Reef Chasers

    Three of our main coral farm systems are equipped with a large, automated calcium reactor which we use only during the day time for 8 hours each day, by running our calcium reactors during the day and our Kalkwasser at night, we are able to achieve better overall pH stability (no sharp daily rises, and limited night time dips). We mainly use them for their ability to restore all the minor trace elements that a large, high demand coral system needs. 

    In the case of smaller systems or the average hobbyist, water changes alone can serve the purpose for restoring trace elements as all salt mixes tend to contain balanced editions of the necessary ones.

    We highly recommend the neptune ecosystem and we recommend putting some programming protections in place to prevent over-dosing. With the neptune this can be as easy as adding a line of programming that says if the pH falls below 8, turn the calcium reactor off. Or perhaps a threshold for Alkalinity or Calcium.

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      Chris swain

      Chris swain

      Hi guys I have a few gonipora corals and one day one will shut and reopen the next day my parameters are ok I do an icp test every month,I keep my manganese up,can you give me any advice on this thank you.



      We love to discuss all things reef and we never stop learning and growing! If you have any feedback, questions or concerns about this article post them here. We are continually improving every day and knowledge is key!

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