Because of their use by corals and other organisms,1 calcium2 and alkalinity 3 are the two most important chemical parameters other than salinity in maintaining coral reef aquaria. Consequently, aquarists are very concerned with maintaining these parameters at appropriate levels. There are many successful ways of supplementing calcium and alkalinity, and each of these systems has its relative merits for different types of aquaria.4 The best of these methods are those that force the addition of calcium and alkalinity in a balanced fashion.5 By forcing balanced additions, these additives are very successful at preventing imbalances between calcium and alkalinity that might drive one of these two parameters above or below optimal levels.

Of the balanced additives, limewater (aka kalkwasser) is one of the most popular. I have been using limewater in my reef aquarium since it was set up years ago. Recently, many aquarists have become interested in using a reactor, often referred to as a Nilsen reactor, to deliver limewater to their aquaria. One of the purported advantages of such reactors is that they are easier to use and the limewater solution is less prone to degradation by atmospheric carbon dioxide (CO2) than by dosing from still reservoirs.

This supposed advantage simply does not hold up under scrutiny, however. As will be shown in this article, the degradation of limewater by atmospheric CO2 is inconsequential in many systems. Consequently, while there are potential reasons to use a Nilsen reactor (especially if space is limited), degradation by atmospheric carbon dioxide in simpler systems is not typically one of them.

What is limewater?

Limewater can be made by dissolving either calcium oxide (CaO) or calcium hydroxide (Ca(OH)2) in water.6 When CaO is used, it first hydrates to Ca(OH)2 on contact with water (H2O):

(1)  CaO + H2  Ca(OH)2 + heat

Consequently, there is little difference between using CaO and Ca(OH)2 except that CaO gives off a substantial amount of heat when it hydrates. When these materials dissolve, they dissociate in the water to calcium ions (Ca++) and hydroxide ions (OH-):

(2)  Ca(OH)2    Ca++ + 2OH-

For those more chemically inclined it is interesting to note that limewater actually contains a substantial amount of partially dissociated, but fully dissolved calcium monohydroxide:

(3)  Ca(OH)2    CaOH+ + OH-

The calcium monohydroxide ion comprises about 25% of the total calcium at the pH of saturated limewater (pH 12.4).7 Nevertheless, that fact is not essential for the remainder of this discussion on the degradation of limewater and the monohydroxide ion will completely dissociate at tank pH.

The Degradation Reaction

When carbon dioxide is dissolved in water, it hydrates to form carbonic acid:

(4)  CO2 + H2O     H2CO3

Then, if the pH is above 11 as it is in limewater, the carbonic acid equilibrates to form mostly carbonate:

(5)  H2CO3 + 2OH-     2H2O + CO3--

It is the carbonate that we are concerned with here. It can combine with the calcium in solution to form insoluble calcium carbonate:

(6)  Ca++ + CO3--     CaCO3 (solid)

The result of this reaction is visually obvious. The calcium carbonate can be seen as a solid crust on the surface of limewater that has been exposed to the air for a day or two. It also settles to the bottom of the container. Since solid calcium carbonate is not an especially useful supplement of calcium or alkalinity,8 this reaction has the effect of reducing the potency of the limewater. With sufficient exposure to air, such as by aeration or vigorous agitation, this reaction can be driven to near completion, with little calcium or hydroxide remaining in solution.

This reaction is the basis of the claims by many aquarists that limewater must be protected from the air. It is also the basis of the claim that Nilsen reactors are to be preferred over delivery from still reservoirs of limewater.

Delivery Methods: Still Reservoirs

There are actually several different ways of delivering limewater. Some methods are primarily suited for small additions. These include the immediate addition of limewater or a slurry of lime solids in water.9 This method works fine for additions of less than 0.2 milliequivalents of alkalinity per liter of aquarium water (0.2 meq/L), but at higher additions, the pH rises too much (about 0.66 pH units on the addition of 0.5 meq/L of alkalinity via limewater, the equivalent of 1.2% of the aquarium volume in saturated limewater).10 I won't discuss these immediate addition methods further in this article.

For larger additions, most aquarists use either slow addition from a reservoir, or a Nilsen reactor. Slow addition from a reservoir can be accomplished using a gravity driven dripper, or using a slow pump to spread the additions out throughout the day (and night). In it's simplest form, a gravity system can be comprised of a suitable large container set above the aquarium or sump, with a hose running from near the bottom of the limewater container to just above the water line of the sump, where it slowly drips into the water. There are a number of commercial products designed for the purpose, such as the AquaDoser by Kent.

More sophisticated systems can involve a large holding reservoir for limewater (up to 55 gallons or more) coupled to a delivery pump and a float switch in the aquarium or sump that controls the delivery to match the evaporation rate. This is the type of system that I use. I make up limewater in a 44-gallon Rubbermaid Brute trashcan by putting the CaO in the bottom, and pouring in water by 5-gallon buckets. That process takes about 5 minutes once every 2-3 weeks. The trashcan is closed by simply putting on its lid. The pump that sends the water to the sump is a Reef-Filler pump (maximum pumping rate 3 gallons per day), which is controlled to match the evaporation rate using a float switch in my sump. The entire limewater system is located remotely from my aquarium (in my basement), so the size of the reservoir is of no consequence. In my case, I often do not use saturated limewater because my aquarium does not need that much supplementation of calcium and alkalinity. Consequently, I add less CaO than would be required to produce saturated limewater. If an aquarist wants saturated limewater, there is no real reason to try to add a specific amount. Any extra solids just sit on the bottom and wait for the next water refill (these solids also absorb impurities like copper out of the water, but that's the subject of a different article).

This type of limewater system is the type that most often comes under fire for being prone to degradation problems by reaction with atmospheric carbon dioxide. In this type of system, limewater is made up once, and then allowed to sit unstirred for as long as it takes the delivery system to send it to the aquarium. Since this type of reservoir can deliver limewater to the aquarium for several weeks, many aquarists have incorrectly concluded that substantial potency is lost as the limewater degrades, and that such a system will fail. Moreover, this assertion is why many aquarists claim that Nilsen reactors are simpler: because the simple delivery from a large reservoir won't work and that only daily mixing of limewater can be successful. In truth, it takes me five minutes to make up limewater every 2-3 weeks, so the idea that some other system is easier to use is simply unfounded. Later in this article I will show that such simple systems do not lose substantial potency, and hence should be considered by aquarists who have the space for large reservoirs.

Delivery Methods: Nilsen Reactors

It is not the purpose of this article to review Nilsen reactors in detail, but how they work is essential in understanding the debate about degradation of limewater. In short, a Nilsen reactor involves a closed chamber where solid lime (calcium hydroxide) is allowed to mix with incoming fresh water. After mixing, the limewater then continues on its way to the aquarium, and is often controlled by float switches to match evaporation. In the mixing chamber, a stirrer periodically turns on, mixing the incoming water with the solid lime, helping it to dissolve. Since the reactor is largely closed to the atmosphere, reaction with atmospheric carbon dioxide is minimized. One potential advantage of Nilsen reactors is that one does not need significant holding reservoirs, and so they are easily kept hidden underneath aquaria (much like CaCO3/CO2 reactors).

Measuring the Potency of Limewater

Measuring the potency of limewater can be complicated. Limewater often has suspended particulates in it. These particulates can include both Ca(OH)2 and CaCO3. With certain methods used to measure potency, these solids can become problematic. For example, alkalinity tests typically involve measuring the amount of acid required to lower the pH to about 4.3 At that pH, particulates of both Ca(OH)2 and CaCO3 will dissolve, potentially giving false high readings. Likewise, measuring calcium may suffer a similar fate with many test kits where solids may dissolve and be detected. Other techniques, such as Inductively Coupled Plasma (ICP) used for calcium and impurities will also detect the solids. Filtration can reduce the particulate load, but many of the particulates that form when limewater interacts with carbon dioxide will be smaller than any normal filters (less than 0.1 mm).11

Two techniques that are largely unaffected by the presence of solids are pH and conductivity. Of the two, pH is much less useful because the change in pH that comes from a small change in potency is hard to properly quantify. Nevertheless, aquarists can monitor the pH of limewater to see if it still retains most of its potency. Instead of comparing to an absolute number, aquarists should compare the pH of the limewater in question to limewater that is known to be saturated (for example, two teaspoons dissolved in a cup of pure fresh water). While exactly how much the pH drops with a drop in potency is complicated due to the presence of CaOH+, as a rough guide a drop of 0.3 pH units is equivalent to a drop of a factor of two in hydroxide concentration (that is, a drop of a factor of two in potency).

Conductivity, on the other hand, is ideal for measuring the concentration of dissolved ionic material in the presence of solids. I use it, for example, to determine the concentration of dissolved salts in the presence of particulate pharmaceuticals. It has also been used to measure the potency of limewater as it reacts with carbon dioxide.12 In a previous article I showed how and why conductivity can be used to measure salinity13 and the basic explanation is the same here. In short, conductivity is a measure of the charged ions in solution as they respond to an electric field. In limewater without impurities we have:

Ca++, CaOH+, OH-, and H+

The concentration of H+ is so low as to be insignificant in terms of conductivity. However, all three of the remaining chemical species are significant. When an electric field is placed on these ions, the Ca++ and CaOH+ move in one direction (toward the negative pole), and the OH- moves in the other (toward the positive pole). The amount of current flow for a given electric field strength indicates how many of these ions must be in solution. The details of conductivity probes are a bit more complicated than this description (e.g., the electric field is actually an alternating electric field, not a static one, and many probes actually have four electrodes) but those details are unnecessary for understanding their use in this article.

When limewater undergoes the degradation described by equations 5 and 6, the calcium and hydroxide ions are effectively removed from solution, and are replaced by uncharged calcium carbonate solids (which are not conductive). Consequently, the conductivity declines when limewater reacts with carbon dioxide. How low the conductivity gets as the limewater degrades may depend on the nature and concentration of other impurities present, either in the lime or the water, but in general the contribution to conductivity from these impurities will be small relative to the conductivity provided by the species above. It is this method that I used to measure the potency of limewater under a variety of conditions.

Conductivity Measurements

The units of conductivity are traditionally milliSiemens per cm (1 mS/cm = 1,000 microSiemens per cm = 1000 mS/cm) and are always reported with the values temperature corrected to 25°C (because ions conduct more as the temperature rises). In all data reported in this paper, I used an Orion Model 128 conductivity meter. Aquarists who want to test this for themselves can use any conductivity meter that can read in the appropriate range of 2-11 mS/cm. The conductivity of saturated limewater at 25°C is about 10.3 mS/cm (a little higher at lower temperatures due to increased solubility of limewater and lower at higher temperatures due to decreased solubility). This value (or something close to it) is easily reproduced by any aquarist with a suitable conductivity probe: add a teaspoon of lime to a cup of pure water and look at the conductivity after a few minutes. This procedure is also a good way to see how fast the lime actually dissolves. In my case, it is very fast. Figure 1 shows the change in conductivity as a function of time after calcium oxide is added to pure water. Clearly, the dissolution is fast.

Figure 1. The conductivity of limewater as a function of the time after the addition of
calcium oxide at 21 °C.

A solution that is less than saturated with lime will have a conductivity less than 10.3 mS/cm. I use such a solution to dose my aquarium, where I do not need to replace all evaporated water with saturated limewater. Depending on the time of year, and hence on the evaporation rate, I increase or decrease the amount of lime added to maintain appropriate levels. This March and April (2003), I monitored the conductivity in the limewater that I dosed. Figure 2 shows the change in conductivity of the water in the 44-gallon trashcan that I use for dosing. Over the three weeks of the test, the conductivity did not significantly drop from the initial value of 3.8 mS/cm. Over the years, I have repeated this experiment a number of times at different initial conductivities, and have always obtained the same result: no significant degradation.

Figure 2. The conductivity of the limewater in my dosing reservoir as a function of time.

To ensure that the 3.8 mS/cm measurement in Figure 2 is really representing calcium and hydroxide in solution, it is important to show that the conductivity drops when CaCO3 precipitates. For example, the measured conductivity might be due to conductive impurities in the lime, and not the calcium and hydroxide themselves. Since impurities would not precipitate on degradation of the limewater, it is important to show that the conductivity does decline under some conditions to bolster the claim that it does not do so under other conditions. To confirm this, I aerated a 1 liter sample of the same limewater using an airstone connected to an air pump. Figure 3 shows the conductivity as a function of time in this solution. Clearly, the conductivity drops significantly in an hour, and the conductive species are essentially gone in 10 hours.

Figure 3. The conductivity as a function of time in my standard trashcan reservoir
(red, reproduced from Figure 2) and in a small container with an airstone (black).

One additional control experiment is important to ensure that conductivity is a useful measure of limewater potency. Figure 4 shows the effect on conductivity of diluting the limewater with pure water. The limewater started with a conductivity of 3.8 mS/cm, and then dropped roughly linearly with the dilution. This result indicates that conductivity is an adequate indicator of the potency of limewater. Taken together, the results shown in Figures 3 and 4 demonstrate that the conductivity value of 3.8 mS/cm in the large reservoir (Figure 2) is representative of calcium and hydroxide in solution. Moreover, it confirms that it is accurate to say that no depletion in the potency has taken place during the period shown in Figure 2.

Figure 4. The conductivity of limewater as it is diluted. The starting limewater (3.8 mS/cm) is diluted with varying amounts of pure water, and the new conductivity is plotted against the relative concentration based on the known dilution (e.g., starting limewater = 1.0 relative concentration; 50 mL limewater + 50 mL pure water = 0.5 relative concentration, etc.).

In the first of a final pair of experiments, I placed 1 liter of limewater in an open plastic container. The top opening of the container was about 6 inches across. In one test, this batch of limewater contained 4 teaspoons of calcium oxide, which is significantly more than is necessary to saturate the limewater. Consequently, this batch has solids on the bottom as they settle from solution. Over time, this solution gathered a significant surface coating of solids, presumably calcium carbonate. Figure 5 shows the conductivity of a probe placed (and left) in this solution. Over the course of the test (10 days), the conductivity did not drop measurably. Consequently, just about any still container of limewater (that is, not stirred or aerated) can be kept near full potency simply by adding excess lime solids. Any precipitation of calcium carbonate is apparently offset by dissolution of Ca(OH)2 from the bottom. For aquarists that demand that their limewater be full strength, adding excess lime solids is the simple route to success.

Figure 5. Conductivity as a function of time for limewater in an open container with
excess lime on the bottom.

In a related experiment, a limewater solution with excess solids was allowed to settle for 24 hours and the liquid was decanted from the solids. This liquid was then monitored by conductivity while stored in an open container. In this case, the probe was generally not left in the solution, but was added for each measurement, breaking the solid crust and permitting much of it to settle to the bottom. Figure 6 shows that the conductivity does decline slightly over a period of several days. The drop in potency here is likely due to both the fact that there is no excess solid calcium hydroxide on the bottom that dissolves as potency drops, and because the crust was protecting the solution from penetration of carbon dioxide.

Figure 6. Conductivity as a function of time for initially saturated limewater that has no excess lime solids present. The liquid was kept in an open container and the surface crust that forms was broken by the conductivity probe where indicated. The data in red is reproduced from Figure 5 (where there is excess lime and no breakage of the crust). Note that the conductivity scale is blown up considerably (compared to all other figures) to see the drop.

Effects of Vinegar

Some aquarists add vinegar to their limewater in order to increase it potency.14 This addition is readily accomplished using reservoir delivery, but is not readily automated for use with a Nilsen reactor. In terms of the degradation of limewater by atmospheric CO2, the addition of vinegar is not expected to have a big impact. The vinegar lowers the pH of the resulting solution, and the lower pH tends to decrease the driving force for CO2 to enter the solution, and for the CO2 in the solution to show up as carbonate (as opposed to bicarbonate at lower values of pH; bicarbonate is less of a concern from a degradation standpoint). Nevertheless, these effects will be small for the amounts of vinegar that aquarists typically use, and the end result is that limewater and vinegar mixtures will typically have about the same reactivity with atmospheric CO2 as will ordinary limewater. The use of very large amounts of vinegar, where the pH drops below about 11, would be expected to reduce the likelihood of precipitation of calcium carbonate. In no instance should vinegar make this problem worse.


Limewater can lose potency by reacting with carbon dioxide in the air, forming insoluble calcium carbonate. Since calcium carbonate is not an effective supplement of calcium and alkalinity in reef aquaria, the limewater can become less useful through this process. The rate at which this happens in large containers, such as plastic trashcans with loose fitting lids, is much less than many aquarists expect. There is, in fact, little degradation under typical use conditions. Consequently, the dosing of limewater from such large still reservoirs can be just as effective as dosing using any other scheme, and may have substantial advantages. These advantages include simplicity of the system and the ability to use organic acids such as vinegar to boost the potency. The use of a reactor to dose limewater has the advantage of requiring less space, but does not have the oft-stated advantage of eliminating degradation by atmospheric carbon dioxide that is reported to plague delivery from reservoirs.

If you have any questions about this article, please visit my author forum on Reef Central.


1. The Chemical and Biochemical Mechanisms of Calcification by Randy Holmes-Farley, Advanced Aquarist, April 2002

2. Calcium by Randy Holmes-Farley, Advanced Aquarist, March 2002

3. Alkalinity by Randy Holmes-Farley, Advanced Aquarist, February 2002

4. How to Select a Calcium and Alkalinity Supplementation Scheme by Randy Holmes-Farley, Advanced Aquarist, February 2003

5. Calcium and Alkalinity by Randy Holmes-Farley, Reefkeeping, April 2002

6. Precipitation of phosphate in limewater and in the aquarium by Craig Bingman, Aquarium Frontiers, Fall 1995

see also: Limits To Limewater...Revisited by Craig Bingman, Aquarium Frontiers, August 1999

7. Aquatic Chemistry Concepts. Pankow, J. F. (1991), 712 pp. Publisher: Lewis Publishers, Inc.

8. Calcium Carbonate as a Supplement by Randy Holmes-Farley, Advanced Aquarist, July 2002

9. Jaubert's Method, the "Monaco System," Defined and Refined by Julian Sprung, Advanced Aquarist, July 2002

10. The Relationship Between Alkalinity and pH by Randy Holmes-Farley, Advanced Aquarist, May 2002

11. Preparation and application of the various crystal form modified calcium carbonate with nanometer particle size. Liu, Shejiang; Yu, Yuanwen; He, Yuji; Xie, Yinghui; Ren, Baoshan. College of Chemical Technology, Hebei University of Technology, Tianjin, Peop. Rep. China. Wujiyan Gongye (2002), 34(2), 11-13. New technology for the preparation of nanometer calcium carbonate by internal circulating carbonator. Wang, Shui; Hu, Qingfu. Hebei University of Science and Technology, Shijiazhuang, Peop. Rep. China. Wujiyan Gongye (2002), 34(3), 8-10. Synthesis of nanometer-sized calcium carbonate. Xie, Yinghui; He, Yuji. College of Chemical Engineering, Hebei University of Industry, Tianjin, Peop. Rep. China. Haihuyan Yu Huagong (2002), 31(2), 14-16. Studies on the crystallization of nano calcium carbonate in the reaction system Ca(OH)2-H2O-CO2. Zhang, Shi-cheng; Han, Yue-xin; Jiang, Jun-hua; Wang, Hong-kuan. School of Resources Civil Engineering, Northeastern University, Shenyang, Peop. Rep. China. Dongbei Daxue Xuebao, Ziran Kexueban (2000), 21(2), 169-172.

12. Synthesis of basic calcium carbonate from the reaction of the system calcium hydroxide-water-carbon dioxide. Yamada, Hideo; Hara, Naomichi. Gov. Ind. Res. Inst., Kyushu, Japan. Gypsum & Lime (1985), 196 130-40. Formation process of colloidal calcium carbonate in the reaction of the calcium hydroxide-water-carbon dioxide system. Yamada, Hideo; Hara, Naomichi. Gov. Ind. Res. Inst. Kyushu, Japan. Gypsum & Lime (1985), 194 3-12.

13. Using Conductivity to Measure Salinity by Randy Holmes-Farley, Aquarium Frontiers, 2000

14. Expanding the Limits of Limewater: Adding Organic Carbon Sources by Craig Bingman, Aquarium Frontiers, October 1999.

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The Degradation of Limewater in Air by Randy Holmes-Farley -