she used it to make pickles. And they were darned good. Maybe
she also used it to make butter
and tortillas. So why didn’t she tell you that you
could use it in your reef aquarium? Probably because you never
asked. I’ll bet she knew you could, but just wanted
you to discover it for yourself!
This article describes everything you need to know about
using limewater (aka kalkwasser) to maintain calcium,
alkalinity, and pH in reef aquaria. Topics include what it
is, how it supplies calcium and alkalinity, where to get it,
how to use it, and what impurities it may contain. Much of
the information has been collected from other articles and
online sources, and links to these references are scattered
throughout the article. They can be useful for those interested
in further detail on a particular topic, or who want to see
more of the reasoning behind a particular claim.
Since some aquarists may be interested only in particular
attributes of limewater, the table of contents below shows
how the article is laid out:
What Is Lime?
According to the National
Lime Association, “lime” is defined as either
quicklime or hydrated lime. These materials are made by heating
calcium carbonate until the carbon dioxide is driven off,
forming quicklime (calcium oxide):
CaO + CO2
Water can then be added to form hydrated lime (calcium hydroxide;
referred to subsequently in this article as just lime):
2. CaO +
Both lime and quicklime are suitable for making limewater
(kalkwasser) and otherwise supplementing calcium and
alkalinity in reef aquaria. There are some important differences
between the use of lime and quicklime that will be discussed
in subsequent sections. These differences relate to the fact
that quicklime is slightly more potent and gets hot when water
is added to it (equation 2).
A couple of other definitions are worth noting. Limewater
is the solution that forms when lime (or quicklime) dissolves
in fresh water. The solution is exactly the same using lime
or quicklime, as long as the same amount of calcium is added.
Kalkwasser is simply the German word for limewater.
Neither term is ever correctly applied to a solid, so any
solid material sold as “kalkwasser” is
either lime or quicklime.
Pickling lime is, in most cases, food grade calcium hydroxide.
Mrs. Wages website
(a place to buy pickling lime online) refers to it as calcium
oxide in some places, but my understanding is that this
product is calcium hydroxide. [As an aside, some of Mrs. Wages
other information about pickling lime makes no sense at all,
such as saying the reason that it contains lime is proprietary,
but in the line above, showing how it works.]
Where To Get Lime
can buy lime from a variety of different aquarium supply companies.
Most often, these companies sell calcium hydroxide and call
it kalkwasser or kalkwasser mix. Some of these
companies claim that their lime meets a particular standard
for purity (Warner, ESV, and Seachem, for example), and these
standards are detailed in later sections of this article.
Others (Kent, Coralife and Two Little Fishies, for example)
do not appear to make specific claims other than claiming
that their products are pure.
Pickling lime can often be found at large grocery stores,
especially in the Fall. Apparently, some folks still do use
it for canning, although one company (Ball) has apparently
stopped selling it. However, you may still find their product
occasionally. Another major brand is Mrs.
Wages, which can also be obtained online for $1.85 per
pound or less. Any brand of pickling lime is likely fine to
use as a base for limewater solutions, as it has to be food
grade to be sold commercially (the implications of which are
Many aquarists have recently turned to
buying bulk brands of food grade lime or quicklime. Some primary
manufacturers such as the Mississippi Lime Company sell a
variety of different grades of lime and quicklime, including
food grades of each. Unfortunately, they do not sell directly
to the public, and even their distributors will sell only
large amounts (many hundreds of pounds). Still, many reef
clubs or local reef stores have organized purchases for their
members or customers. There is no beating bulk lime for cost
when compared to other balanced methods of supplying calcium
and alkalinity to aquaria. I bought 100 pounds of quicklime
this way several years ago for less than $0.50 per pound,
and expect it to last a considerable time into the future.
One should be careful using agricultural lime, such as the
products sold by Home Depot or other home improvement stores.
In many cases this material, despite being called lime, is
actually calcium carbonate. If so, the term lime is simply
short for limestone. Even when the bag gives concentrations
of CaO and/or MgO, that statement is a unit of measure of
calcium or magnesium, not an indication that there is really
CaO in the bag. Limestone is not suitable for making limewater
since it is insoluble. Further, in agricultural grades of
calcium hydroxide or oxide, the purity may not be adequate
for a reef aquarium.
Purity Of Commercial Lime
Lime is graded in
a variety of ways. One way has to do with the amount of magnesium
it contains. High calcium quicklime (and lime) is derived
from limestone (calcium carbonate) containing 0-5% magnesium
carbonate. Magnesian quicklime (and lime) is derived from
limestone containing 5% -35% magnesium. Dolomitic quicklime
(and lime) is derived from limestone containing 35% - 46%
magnesium carbonate. Most reef aquarists use high calcium
limes since the magnesium is not generally soluble in limewater
(a fact discussed in detail below).
Another way of grading lime has to do with impurities present
in the lime. Manufacturers may refer to grades in a wide variety
of ways, and some of those that are worth knowing are detailed
below, along with four brands that claim to meet these specifications
(Warner, ESV, Seachem, and Mrs. Wages). Other companies that
sell to aquarists (Kent, Coralife, and Two Little Fishies,
for example) do not specify the grades used (at least that
I can determine). If you use those brands, then you are relying
on the manufacturer’s claims to ensure suitable purity.
The most common and widely recognized grades of lime are:
1. . FCC, which stands for Food Commercial Codex.
It states that the lime or quicklime is food grade, and meets
minimum standards to be a food product. In that sense, FCC
has purity assurances that other industrial and agricultural
grades may not have. The impurities regulated and the actual
impurity levels in typical batches from the Mississippi
Lime Company are shown in Tables 1A, 1B, and 1C. The requirements
for Ca(OH)2 are slightly more stringent
than for CaO, but Ca(OH)2 is slightly
less potent so an aquarist would need to use more. Any food
grade lime, or pickling lime (such as Mrs. Wages or Ball’s)
should meet the specifications in Tables 1A and 1B. As seen
in Table 2, the actual impurity levels in commercial FCC products
can be much lower than the FCC limits. Seachem
also claims to meet this specification.
and Alkali Salts
than 150 ppm
than 5 ppm
than 3 ppm
than 30 ppm
and Alkali Salts
than 50 ppm
than 10 ppm
than 3 ppm
than 30 ppm
& Alkali Salts
2. USP stands for United States Pharmacopoeia and
NF stands for National Formulary. USP/NF calcium hydroxide
can have the impurities shown in Table 2. This rating is similar
to, but requires slightly fewer tests than does the food grade
claims to meet this specification.
2. USP Grade Calcium Hydroxide
and Alkali Salts
than 3 ppm
than 40 ppm
3. AR (ACS) stands for Analytical Reagent grade,
as described by the American Chemical Society. Reagent grade
calcium hydroxide can have the impurities shown in Table 3.
The Reagent grading is focused on attributes that are important
to chemists, but that may not matter much to reef aquarists,
such as low magnesium, sodium, and potassium. It also does
not focus on toxic impurities (no testing for arsenic or lead,
for example). The total amount of heavy metals allowed is
similar to food and pharmaceutical grades. Warner
claims to meet this specification.
3. AR (ACS) Grade Calcium Hydroxide Specifications
compounds (as sulfate)
than 30 ppm
4. Water Chemicals Codex is a grade that is described
by the Committee on Water Treatment Chemicals of the National
Research Council. The grade is specifically listed as suitable
for treating potable (drinking) water. Consequently, it focuses
on toxic impurities, as shown in Table 4. The specification
is the same for calcium oxide and calcium hydroxide. ESV
claims to meet this specification.
What Is Limewater?
have used limewater
very successfully for a number of years, and it is the system
that I use in my aquarium. It is comprised of an aqueous solution
of calcium and hydroxide ions that can be made by dissolving
either quicklime or lime in fresh water. Note that the water
must be freshwater. Combining lime with seawater will result
in a mess of precipitated magnesium and calcium carbonates
The only inherent difference between calcium oxide and calcium
hydroxide is that adding a molecule of water to quicklime
produces lime, and that a great quantity of heat can be generated
when that happens.
3. CaO +
Consequently, dissolving quicklime can make water quite warm,
especially if an excess of solids is added. Some aquarists
have damaged equipment by adding a large amount of quicklime
to a small amount of water in a plastic reactor. The heat
released can easily boil the water, and some plastic devices
may not be able to withstand that hot, corrosive mixture.
The calcium ions in the solution obviously supply calcium
to the aquarium, and the hydroxide ions supply alkalinity.
Hydroxide itself provides alkalinity (both by
definition and as measured with an alkalinity test), but
alkalinity as bicarbonate, not hydroxide. Fortunately,
when limewater is used in a reef aquarium, it quickly combines
with atmospheric and dissolved carbon dioxide and bicarbonate
to form bicarbonate and carbonate:
+ CO2 à
OH- + HCO3-
CO3-- + H2O
In an aquarium with an acceptable
pH, there is no concern that the alkalinity provided by
limewater is any different from any other carbonate alkalinity
supplement. The hydroxide immediately disappears into the
bicarbonate/carbonate system. In other words, the amount of
hydroxide present in aquarium water is really a function of
only pH (regardless of what has been added), and at any pH
below 9, it is an insignificant factor in alkalinity tests
(much less than 0.1 meq/L). Consequently, the fact that alkalinity
is initially supplied as hydroxide is not to be viewed as
problematic, except as it impacts pH (see below).
Limewater that is saturated with calcium hydroxide has a
pH of 12.54 at 25ºC. It is actually recognized as a secondary
pH standard. The pH is substantially higher at lower temperature
(12.627 at 20ºC and 13.00 at 10ºC), and lower at
higher temperature (12.289 at 30ºC; 11.984 at 40ºC).
Saturated limewater has a conductivity of about 10.3 mS/cm
at 25ºC, and contains about 808 ppm of calcium and 40.8
meq/l of alkalinity. Slightly more calcium and alkalinity
dissolve at lower temperatures, and less at higher temperatures.
Of interest to chemists, a large fraction of the calcium in
saturated limewater is present as the ion CaOH+,
with the remainder being Ca++.
The CaOH+ will instantly
dissociate into Ca++ and
OH- upon its addition to
How To Dose Limewater
The fact that limewater
is very basic (the pH is typically above 12) demands that
the limewater be added slowly to an aquarium unless very small
additions are made. The reason for this is two-fold: to prevent
the local pH in the area of the addition from rising too high
(slow addition permits more rapid mixing with tank water to
reduce the pH), and to prevent the overall tank pH from rising
too high (slow addition allows the tank to pull in CO2
from the atmosphere during the slow addition, mitigating the
pH rise). Some aquarists advocate
rapid addition, and that is fine for small additions that
would add less than 0.2 meq/L of alkalinity to the aquarium,
but larger additions will drive the pH too high, as detailed
Consequently, limewater is most often added slowly, by dripping
or slow pumping. Often it is added as the top off water, replacing
most or all of the evaporated water. Such pumps add cost and
complexity to the system, especially if combined with a float
valve or switch (I use the latter and a Reef Filler pump that
I bought from Champion Lighting).
The suitable delivery methods for lime and limewater are:
1. Slow dosing clear, settled limewater to replace evaporated
water. This can be attained with drippers (homemade
or commercial) or slow pumps (including diaphragm pumps like
the Reef Filler or peristaltic pumps like the Litermeter).
The delivery can be controlled by float switches or valves,
or matched to the evaporation rate by controlling pump speeds.
Using pumps and float switches raises the costs considerably,
but also reduces the time spent dealing with limewater. I
spend only five minutes once every three weeks to replenish
my limewater delivery system. Sophisticated float switches
and slow pumps also lessen the likelihood of overdosing that
can come with other methods.
Some aquarists have tried to use powerheads as part of such
a limewater delivery system. Frequently, these add too much
limewater at once before a float switch turns them back off.
Aquarists designing such systems should keep in mind the dosage
limitations described below.
Drippers often hold only a gallon or two of limewater, and
so need to be refilled frequently. Delivery with pumps, however,
can be from as large a container as space permits. I also
use a 44-gallon Rubbermaid Brute trash can that I bought at
Home Depot to store limewater (I use them to store RO/DI water
and artificial salt water, and have two connected to form
my sump). Some aquarists even use 55-gallon plastic drums.
Obviously, such large limewater reservoirs need to be placed
in basements, garages, or “fish rooms.”
2. Dosing milky limewater, to get more lime into the aquarium
than is available in clear, settled limewater. A drawback
is the delivery of impurities in or on the solid particles,
and the possibility that some solids may interact with organisms
before they dissolve.
3. Dosing limewater mixed with vinegar. The vinegar allows
more solid lime to dissolve into the limewater, and limits
the maximum pH that the aquarium will attain. Drawbacks include
the possibility that bacterial growth will be driven by the
acetate in the vinegar; the fact that some of the measured
alkalinity may be acetate, which is not used by corals and
coralline algae to calcify as far as I know; and that some
impurities in the lime may dissolve in the presence of vinegar
when they might settle out in its absence. Details behind
the use of vinegar are given below. Limewater containing typical
amounts of vinegar is still very high in pH and must be dosed
slowly. Much of the vinegar’s pH reduction comes after
it is metabolized by bacteria, as is shown later in this article.
4. Delivering a small amount of limewater all at once. Adding
1.25% of the aquarium’s volume (1.25 gallons of limewater
per 100 gallons of aquarium water) as saturated limewater
all at once raises the pH
by 0.6 to 0.7 pH units. Such an increase is clearly too
large. Adding a smaller portion all at once can, however,
be acceptable. Adding, for example, 0.25% of the aquarium
volume (0.25 gallons or 1 L of limewater per 100 gallons of
aquarium water) will raise the pH by only 0.1 to 0.2 pH units.
Unless the pH is high (>8.4) before the addition, that
amount is likely acceptable. The other concern with all-at-once
dosing is that the local pH in the area of the addition will
rise considerably higher than the values above. So dosing
must be done far from living organisms, and in high flow areas
that will facilitate fast mixture. In some aquaria, such restrictions
make all-at-once dosing of limewater prohibitively risky to
5. Delivering a small amount of solid lime slurried (dispersed)
in a small amount of water. Adding one level teaspoon of solid
lime (Ca(OH)2) slurried in a cup of water to 40 gallons of
aquarium water all at once raises the
pH by 0.6 to 0.7 pH units. That is clearly too much. Adding
a smaller portion all at once can, however, be acceptable.
Adding, for example, 1/4 teaspoon to 40 gallons will raise
the pH by only 0.1 to 0.2 pH units. Unless the pH is high
(>8.4) before the addition, that amount is likely acceptable.
The other concern with all at once dosing is that the local
pH in the area of the addition will rise considerably higher
than the values above. Moreover, dosing a slurry raises the
added concern that the solids must dissolve before encountering
organisms that may take them up and be harmed. So it is best
to dose such materials to a sump, and watch that they completely
dissolve before reaching the main aquarium or a refugium.
In many aquaria, such restrictions make all–at-once
dosing of a slurry prohibitively risky to living organisms.
6. Delivering limewater via a reactor, sometimes called a
Nilsen reactor (picture right). In this setup, fresh
water is added to a small chamber containing solid calcium
hydroxide. After mixing with the lime and becoming “limewater,”
the fluid portion then continues on to the aquarium. The mixing
is often performed by a magnetic stirrer, with a magnet inside
the chamber driven by a rotating magnet outside. These systems
normally mix the lime and fresh water several times per day,
but not continually. It can be difficult to get such systems
to continually deliver saturated limewater, and it is technically
challenging to use vinegar with them. They are, however, well
suited to use under aquaria or otherwise where space is limiting.
Vinegar And Limewater To Reduce
The reason that limewater
raises the pH of aquarium water so considerably is because
of the hydroxide that it adds. As described above, the hydroxide
can combine with carbon dioxide to form bicarbonate and bring
the pH back down. In many aquaria, however, the aeration is
not great enough to bring in carbon dioxide fast enough to
meet all this demand, and the pH rises. There are several
ways to add additional carbon dioxide to meet this demand,
including delivery from a carbon dioxide cylinder. Many aquarists,
however, choose to add
carbon dioxide in the form of vinegar. Many of them choose
to add the vinegar directly to the limewater, although if
pH reduction is the goal, it can also be added directly to
a high flow area of the aquarium.
When vinegar is added directly to aquarium water, the active
ingredient is acetic acid. The first thing it does is ionize
into acetate and H+:
CH3COO- + H+
Bacteria can then metabolize the acetate to gain energy in
the reaction shown below:
+ 2O2 à
2CO2 + H2O +
On balance, the H+ released
in (6) and the OH- released
in (7) offset each other, and the net addition is simply carbon
+ OH- à
+ 2O2 à
2CO2 + 2 H2O
One of the potential side effects of this metabolism is
that the bacteria performing the transformation may grow faster
because of it. This growth may have positive or negative outcomes.
One potentially positive outcome is that as they grow, they
will necessarily consume nitrogen and phosphorus, possibly
lowering nitrate and phosphate levels in the aquarium. Another
is that the bacteria may be a suitable food source for other
Potential drawbacks can include reduced oxygen as the bacteria
use it to consume the acetate, and the appearance of unattractive
bacterial mats in the aquarium (reported by some, but not
by the majority of vinegar users).
Vinegar And Limewater To Boost
useful attribute of vinegar is that it can be used to help
dissolve additional solid lime into limewater. It does this
by reducing the hydroxide concentration in the limewater:
CH3COO- + H+
The H+ combines with OH-
in the limewater:
H+ + OH- à
The actual dissolution of Ca(OH)2
is limited by the multiplication product of the calcium and
hydroxide concentrations in the limewater as shown below:
Ca++ + 2OH-
x [OH-] x [OH-] £
5.5 x 10-6
where [Ca++] is the concentration
of calcium (in moles/L) and [OH-]
is the concentration of hydroxide (in moles/L). Consequently,
if you reduce the concentration of OH-
via equations (10) and (11), then more Ca(OH)2
can dissolve into solution and still meet the equation (13)
This would seem like a concern, however, since losing OH-
might reduce the amount of alkalinity delivered by the limewater.
Luckily, this is not the case. While the OH-
is temporarily reduced by the acetic acid in the vinegar,
when bacteria metabolize the acetate, they release it back
to the water:
+ 2O2 à
2CO2 + H2O +
Consequently, additional solid lime can be dissolved into
limewater using vinegar.
How much can be used? The more vinegar that is used, the
lower the pH of both the limewater and the aquarium will be.
One reasonable point to shoot for is to add about the same
amount of total CO2 via the vinegar
as is needed by the lime to form HCO3-.
balance is roughly matched by using three level teaspoons
of solid lime per gallon of limewater, and 45 ml of vinegar
per gallon of limewater. For those aquarists choosing to use
vinegar in limewater, these values are a suitable starting
point. Note that the pH of the limewater is still quite high,
so slow dosing is usually required.
What kind of vinegar should be used? Luckily, cheap distilled
white vinegar is likely the best. More expensive flavored
and colored vinegars, such as red wine vinegar, will deliver
other unnecessary organic molecules to the aquarium, and are
What Else Is In Limewater Besides
Calcium And Alkalinity? Metallic Impurities
One interesting aspect
of limewater is its ability to self purify before being added
to the aquarium. This happens in several ways, but all relate
to the fact that most aquarists dissolve it and then let any
undissolved solids settle out. Few, if any, of these solids
are then dosed to the aquarium. It turns out that these solids
can contain many of the impurities that came to the limewater,
either in the solid lime, or in the water itself. In a recent
article I showed experimentally and theoretically how
this process works for a variety of metals, including copper,
nickel, and cadmium.
Figures 1 and 2 show experimentally what happens when solid
lime is added to water that contains a significant amount
of copper. In the high pH of limewater, copper precipitates
from solution as copper hydroxide. It also turns out that
excess lime solids themselves can help remove additional metals
from solution, as those metals bind to the surfaces of the
undissolved lime. Besides metals, other impurities can also
be precipitated from limewater as calcium salts, including
Figure 1. Fresh water containing copper, giving it
a slight blue color (left).
Immediately after addition of calcium hydroxide (right),
the solution is
cloudy and darker blue.
Figure 2. Fresh water containing copper, giving it
a slight blue color (left).
After addition of calcium hydroxide and allowing time to settle
of the visible blue color has settled out of solution.
This purification is also seen in practice by many aquarists
who have noticed the solids on the bottom of their limewater
containers discolor, often to a bluish/green color suggesting
copper. For these reasons, I recommend that lime solids not
be dosed to aquaria when it is possible to avoid it. Letting
the limewater settle for a few hours to overnight will permit
most of the large particles to settle out, and whether it
looks clear at that point or not, it is likely fine to use.
In general, it is a good practice to leave residual solids
on the bottom of limewater reservoirs rather than cleaning
them out every time, as they may actually help purify the
water by these precipitation mechanisms. Once the solids discolor,
or have been collecting for 6-12 months, however, they should
What Else Is In Limewater Besides
Calcium And Hydroxide? Mg++
As discussed above,
solid lime can contain considerable material in addition to
calcium and hydroxide. In particular, most grades allow a
significant amount of magnesium and alkali salts (which include
sodium, potassium, and lithium). Of these, many aquarists
would be most concerned with magnesium. In order to better
assess how much strontium and magnesium would be delivered
by the use of lime, I determined how much calcium, magnesium,
and strontium were present in the solid quicklime that I use.
Details of the testing method were presented in a previous
article. The results are shown in Table 5.
5. Alkaline earth metals in quicklime.
Concentration in solid (weight %)
Concentration (by weight)
= Mg/Ca ratio
= Sr/Ca ratio
As expected, the primary ingredient is calcium. Magnesium
is fairly low, and strontium is quite low. This material has
a Mg/Ca ratio of 0.0038. That is at the low end of the Mg/Ca
ratio found in corals, and well below that found in coralline
algae. It has a Sr/Ca ratio of 0.00037. That Sr/Ca value is
far below the 0.02 ratio of Sr/Ca found in typical corals.
I dose my aquarium with limewater made from this quicklime.
I typically use limewater at less than saturation because
my reef aquarium does not need full strength limewater. In
order to test for magnesium and strontium in the limewater
that I dose, I made 44 gallons of limewater, and dosed it
for about three weeks. I then took a sample of the clear limewater
that remained. It had a conductivity of 7 mS/cm, indicating
that it is not saturated (saturated limewater usually has
of about 10.3 mS/cm). This limewater sample was analyzed
(the details of which were presented in a previous article)
and the results are shown in Table 6.
6. Alkaline earth metals in limewater.
Concentration (by weight)
Relative to Solid Lime
It is interesting to note that relative
to calcium, magnesium is greatly underrepresented compared
to that in the starting quicklime. The reason for this result
is the well-known insolubility of magnesium hydroxide at high
pH. Any magnesium ions released into the solution rapidly
combine with hydroxide to form insoluble magnesium hydroxide
In a previous article on the solubility
of metals in limewater, I showed a graph
of the theoretical solubility of magnesium as a function of
pH. At the pH of limewater (low 12’s) the solubility
is between 0.01 and 0.001 ppm. The experimental solubility
here is a tad higher (0.017 ppm), presumably for one of two
reasons: some particulates of magnesium hydroxide may have
been present in the solution which were detected as soluble
magnesium when in fact, they were not. A second possibility
is that the solution simply had not reached thermodynamic
equilibrium, and the theoretical limit of solubility had not
yet been reached. Nevertheless, the point is that it is expected
that magnesium hydroxide will precipitate from such a solution,
and this did, in fact, happen. The magnesium in solution was
depleted by a factor of more than a hundred compared to what
would have been in solution had it all been soluble.
Relative to calcium, strontium was nearly unchanged in the
limewater compared to the solid quicklime. The reason for
the slight elevation is that strontium is even less likely
than calcium to precipitate onto the bottom of the limewater
reservoir, and so stays more in solution than calcium.
What Gets Left Behind On The
Bottom Of The Limewater Container?
solids on the bottom of a limewater reservoir contain everything
that did not dissolve, or that dissolved and later precipitated
from solution. Such solids could contain magnesium hydroxide
and carbonate, calcium hydroxide and carbonate (though calcium
hydroxide is fairly unlikely in unsaturated limewater) and
a variety of other impurities, such as alumina, silica, etc.
In order to determine what was in it, I tested a sample of
the white solid material that had been collecting for months
on the bottom of my limewater reservoir, and detailed the
results in a previous
article. The white sludge was removed along with some
limewater. The mixture of solid and liquid was acidified to
dissolve it, and it was tested for calcium, magnesium, and
strontium. The results are shown in Table 7. Only relative
concentrations are shown as no effort was made to dry the
sample prior to analysis, making absolute concentrations meaningless.
7. Alkaline earth metals in limewater sludge.
Concentration (by weight)
Relative to Solid Lime
As anticipated, relative to calcium, magnesium is enriched
by a factor of 13 in the sludge compared to the solid starting
quicklime. This magnesium may be present as both magnesium
hydroxide and magnesium carbonate, but because magnesium carbonate
is fairly soluble compared to calcium carbonate, it is most
likely that the primary magnesium salt is magnesium hydroxide.
It may also be mixed calcium and magnesium carbonates.
Interestingly, strontium is actually depleted by a factor
of two relative to solid starting quicklime, indicating that
it is less likely than calcium to end up on the bottom of
the reservoir. While strontium carbonate is somewhat less
soluble than calcium carbonate, the strontium concentration
in the limewater is so low that SrCO3 may not actually be
saturated, so it precipitates less. The strontium that is
there may simply be copreciptiated with calcium carbonate.
Does Limewater Degrade Over Time?
The Degradation Reaction
When carbon dioxide
is dissolved in water, it hydrates to form carbonic acid:
CO2 + H2O
Then, if the pH is above 11, as it is in limewater, the carbonic
acid equilibrates to form mostly carbonate:
H2CO3 + 2OH-
2H2O + CO3--
It is the carbonate that we are concerned with in the degradation
of limewater. It can combine with the calcium in limewater
to form insoluble calcium carbonate:
Ca++ + CO3--
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 (do not
bother to remove this crust, it may actually be protecting
the underlying limewater from further penetration by carbon
dioxide). The formed solids also settle to the bottom of the
container (as described above). Since solid calcium carbonate
not an especially useful supplement of calcium or alkalinity,
this reaction has the effect of reducing the limewater’s
potency. 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
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. Neither
of these claims, however, stands up to experimental scrutiny,
as I showed in a previous
Conductivity is perhaps the easiest way to monitor the potency
of limewater, with a conductivity of about 10.3 mS/cm detected
in saturated solutions at 25ºC. Figure 3 shows the potency
over time of limewater in an aerated 1-gallon container. Clearly,
the potency drops rapidly due to the formation of nonconductive
calcium carbonate precipitate.
Figure 3. Conductivity as a function of time over two
days in my standard
44-gallon limewater reservoir with its lid on (red) and in
an open 1-gallon
container with an airstone (black).
In a still reservoir that is not aerated, however, the potency
is stable for a period adequate to permit dosing. Figures
3 and 4 show the conductivity of the limewater in my 44-gallon
limewater reservoir over a three-week period. It was simply
covered with a loose fitting plastic lid. It is apparent from
the conductivity that the potency does not decrease significantly
Figure 4. Conductivity as a function of time over three
weeks in my
standard44-gallon limewater reservoir with its lid on.
For those interested in dosing saturated limewater, Figure
5 shows the potency of limewater in a still but uncovered
1-gallon container with excess solid lime on the bottom. In
that configuration, any calcium and hydroxide that is taken
away via precipitation to form calcium carbonate is apparently
replaced by dissolution of more solid lime from the bottom.
Consequently, even a simple unstirred one-gallon container
can be used without fear of loss of potency, if there is solid
lime on the bottom.
Figure 5. Conductivity as a function of time over 10
days in an open 1-gallon
container of limewater with excess solid lime on the bottom.
To summarize the degradation issues:
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 trash cans with loose fitting lids, however,
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, or more so than, dosing using
any other scheme.
What Else Does Limewater Do In
An Aquarium? Raise pH Whether You Want It To Or Not
limewater has a pH above 12 (even when a reasonable amount
of vinegar is added), it causes a substantial rise in pH when
added to a reef aquarium. This attribute has both positive
and negative aspects. It limits the speed at which limewater
can be added without raising the tank’s pH too much
(discussed above). It also can be a serious problem in accidental
overdosing, where the pH can rise very high. Often, this overdosing
can lead to the aquarium turning white like milk as calcium
carbonate precipitates throughout the water column.
In such cases of acute overdose, here is my advice:
1. If the pH is 8.5 or lower (as it often is since a precipitation
event itself reduces pH even if it was much higher to start
with), there is little that can or needs to be done. Just
wait a few days for the white calcium carbonate to slowly
disappear. A water change is not necessary, although once
the water is clear, testing calcium and especially alkalinity
is in order (don’t bother to test the cloudy water as
it will give false high readings as these tests detect solids
even though they are not truly in solution). Few aquarists
suffer the loss of organisms from such events. I’ve
had several such events without any apparent losses.
2. If the pH is above 8.5, some action to reduce the pH is
warranted. The higher it is, the faster and greater the needed
action. Since such events may happen when few tools are available
to solve them (e.g., New Year's morning when few stores are
open), I’ll provide a number of options, although some
are better than others. In all cases, reduce the pH only to
8.5 to avoid overshooting.
The best option is to add carbon dioxide, either by bubbling
the gas directly, or by adding soda water/seltzer (or blowing
into a skimmer inlet if it is your only option). At least
in the normal aquarium pH range, a teaspoon of soda water
per gallon of tank water will lower pH by a couple of tenths
of a pH unit. Overshooting with carbon dioxide, while undesirable,
is less of a concern than is overshooting with any other option.
A second option is to add vinegar. Be especially careful
to not overshoot pH 8.5 or so, because when bacteria begin
to metabolize the acetate, the resulting CO2 will further
lower the pH, and oxygen will be consumed (equation (14)).
For this reason, it is especially important to maintain aeration
when using vinegar in such a fashion. I’ve added vinegar
to my aquarium in similar situations without difficulty, although
the pH was only marginally high and I did not need to add
A third rung of options involves adding a mineral acid such
as muriatic acid (HCl or hydrochloric acid) or sulfuric acid.
I’ve added HCl to my aquarium in similar situations
without difficulty. When performing such a mineral acid treatment,
be very careful not to overshoot, and to monitor the pH during
any acid additions. I would intervene in this fashion only
if I could monitor the pH in real time, and could add the
acid to a high flow area far from any organism. Diluting the
acid in water (say, 20:1 or 100:1) prior to adding it to the
tank is highly recommended for the safety of both the aquarist
and the tank’s inhabitants (diluting vinegar, which
is already dilute, isn’t necessary). One other drawback
to adding a mineral acid is that it reduces the alkalinity.
In such a case, the result may be elevated calcium and reduced
alkalinity that will require
What Else Does Limewater Do In
An Aquarium? Raise pH When You Need It
In a great many cases,
reef aquaria have pH values lower than aquarists might prefer.
This low pH comes about from excess carbon dioxide in the
water, often either from a CaCO3/CO2
reactor, or from excess carbon dioxide in the home air.
In both of these cases, limewater is arguably the best way
to raise pH, and I have recommended it for this purpose in
articles. Figure 6 shows how adding limewater can raise
the pH by taking up excess carbon dioxide and adding alkalinity,
all without raising alkalinity relative to calcium (since
it also adds calcium).
Figure 6. The relationship between alkalinity and pH,
showing the effect of limewater on pH by
both reducing the excess carbon dioxide (the hydroxide combines
with it to form bicarbonate
and carbonate) and increasing the alkalinity.
What Else Does Limewater Do In
An Aquarium? Reduce Magnesium
Despite old beliefs
that using of limewater depletes magnesium, the truth is somewhat
more complicated. As was shown above, magnesium is not dosed
in typical settled limewater because it is insoluble; nor
is it present in very high concentration even in undissolved
Bingman showed that precipitation of magnesium carbonate
and hydroxide in aquaria using limewater was unlikely to be
significant. More likely, such depletion is simply the result
of not delivering as much magnesium to the aquarium as is
being “exported” during calcification.
In a previous article I used the data presented earlier in
this article to develop models of how magnesium might be depleted
from reef aquaria over the course of a year using limewater
based on magnesium incorporation rates typical of corals and
coralline algae. Table 8 shows some of the data generated.
8. Magnesium depletion with dosing of settled limewater.
of limewater added daily
Added over 1 year (ppm)
Removed over 1 year (ppm)
of tank volume
of tank volume
of tank volume
As expected, the magnesium depletion is significant. While
this depletion may be mitigated in a number of ways (including
water changes), it shows that aquarists using limewater should
consider monitoring magnesium over long periods of time. The
previous article also compared magnesium depletion via limewater
to magnesium depletion using a variety of different calcium
carbonate materials in CaCO3/CO2
Using limewater can also deplete strontium, not because it
is insoluble in limewater, but because there isn’t much
in at least some brands of lime. Since strontium may or may
not be beneficial, this may or may not be a concern.
What Else Does Limewater Do In
An Aquarium? Reduce Phosphate
reefkeepers accept the concept that adding limewater reduces
phosphate levels. This may be true, but the mechanism remains
to be demonstrated. Craig Bingman has done a variety of experiments
related to this hypothesis, and has published them in the
old Aquarium Frontiers. While many aquarists may not care
what the mechanism is, knowing it would help to understand
the limits of this method, and how it might best be employed.
Habib Sekha (Salifert) has pointed out that limewater additions
may lead to substantial precipitation of calcium carbonate
in reef aquaria. This idea makes perfect sense. After all,
it is certainly not the case that large numbers of reef aquaria
will exactly balance calcification needs by replacing all
evaporated water with saturated limewater. And yet, many find
that calcium and alkalinity levels are stable over long time
periods with just that scenario. One way that can be true
is if the excess calcium and alkalinity that such additions
typically dump into the aquarium are subsequently removed
by precipitation of calcium carbonate (such as on heaters,
pumps, sand, live rock, etc.).
It is this ongoing precipitation of calcium carbonate, then,
that may reduce the phosphate levels: phosphate binds to these
growing surfaces, and becomes part of the solid precipitate.
absorption of phosphate from seawater onto aragonite is
pH dependent, with the binding maximized at around pH 8.4
and with less binding at lower and higher pH values. If the
calcium carbonate crystal is static (not growing), then this
process is reversible, and the aragonite can act as a reservoir
for phosphate. This reservoir can inhibit the complete removal
of excess phosphate from a reef aquarium that has experienced
very high phosphate levels, and may permit algae to continue
to thrive despite having cut off all external phosphate sources.
In such extreme cases, removal of the substrate may even be
If the calcium carbonate deposits are growing, then phosphate
may get buried in the growing crystal, which can act as a
sink for phosphate, at least until that CaCO3 somehow dissolves.
Additionally, if these crystals are in the water column (e.g.,
if they form at the local area where limewater hits the tank
water), then they may become coated with organics and be skimmed
out of the aquarium.
An alternative mechanism for phosphate
reduction via limewater may simply be the precipitation of
calcium phosphate, Ca3(PO4)2.
The water in many reef aquaria will be supersaturated with
this material, as the equilibrium saturation concentration
in normal seawater is only 0.002 ppm phosphate. The supersaturation
of calcium phosphate will be even higher in the high pH/high
calcium fluid present where limewater enters reef aquaria.
The locally high pH converts much of the HPO4--
to PO4---, and it is the
concentration of PO4---
that ultimately determines supersaturation. That high supersaturation
may tip the balance to precipitation of calcium phosphate,
just as too much limewater all at once can tip the balance
to precipitation of calcium carbonate. As with CaCO3,
the precipitation of Ca3(PO4)2
in seawater may be limited more by kinetic factors than by
equilibrium factors, so it is impossible to say how much might
precipitate under reef tank conditions (without, of course,
somehow determining it experimentally).
As with the precipitation of CaCO3
containing some phosphate, if these calcium phosphate crystals
are in the water column (e.g., if they form at the local area
where limewater hits the tank water), then they may become
coated with organics and be skimmed out of the aquarium.
Limitations To Limewater: Limits
To The Addition Of Calcium And Alkalinity
important consideration for limewater is the upper limit of
the amount that can be added to an aquarium. This limitation
exists simply because both the amount of water that can be
added to an aquarium each day (to replace evaporation), and
the amount of solid lime that can be dissolved in that water,
are finite. Using lime slurries eliminates this concern, but
brings its own issues that were discussed above.
If an aquarium’s calcium and alkalinity demands are
near the high end, then replacing all of the evaporated water
with saturated limewater may not be adequate. In the case
of my reef aquarium system, however, it is more than adequate.
I do not even use saturated limewater (typically, I aim for
a conductivity of about 7 mS/cm), and I still meet my aquarium’s
demand. However, many aquaria have higher, maybe even much
higher, demands for calcium and alkalinity than mine does.
One way to enlarge the limewater’s impact is to add
fans to the aquarium to increase evaporation. A second involves
vinegar as described above. Many aquarists have successfully
employed both of these methods. Additionally, aquarists often
use a small amount of one of the other balanced additive systems
(especially the two-part additive systems) to give a little
boost to aquaria that need a small amount of extra calcium
and alkalinity beyond what limewater can supply, without incurring
significant capital costs. Likewise, these two-part additive
systems can be successfully combined with limewater during
periods of low evaporation when limewater may be temporarily
limited and not meet demand (as during rainy cool weather).
Dosing Other Additives In Limewater
frequently ask if they can mix other additives into their
limewater. For some additives, the answer is clearly no. These
(which precipitates as magnesium hydroxide), calcium (which
limits the dissolution of calcium hydroxide), and alkalinity
supplements (which will precipitate as calcium carbonate).
supplements can be combined with limewater, although there
may be no real need for it. Silicate
also can likely be dosed in this manner. Even though I use
limewater and silicate
supplements, I do not combine them.
Other additives fall into a grey area, where they may or
may not be damaged by combination with limewater. These include
iodine and iron supplements, some forms of which may not permit
mixing without causing problems, and I would not recommend
the negative side, limewater does have some safety concerns
that don’t apply to most other calcium and alkalinity
additive systems. The high pH of the liquid and the dust hazard
of the solid are not to be treated lightly. Inhalation of
the dust is to be avoided. Splashing limewater onto skin is
also to be avoided, and should be followed by extensive rinsing
with tap water if it happens. Splashing limewater into the
eyes is especially to be avoided, and the use of safety goggles
is prudent when using large amounts or in situations where
exposure is likely. Extensive and immediate rinsing with tap
water, followed by professional help, would be advised in
the case of eye exposure. Keep in mind that the slippery feeling
that a high pH liquid such as limewater causes on your hands
is due to the breakdown of the fats in the skin into fatty
acids (which are soaps).
Quicklime has some special hazards beyond normal lime and
limewater. Specifically, these relate to the heat produced
when calcium oxide hydrates to form calcium hydroxide. A small
amount of water added to a significant amount of quicklime
will get very hot. It can even boil. Some aquarists have melted
Nilsen reactors this way, and some have had such reactors
“explode,” presumably through rapid heating and
pressure buildup. So when using calcium oxide, be sure to
add a small amount of lime to a significantly larger amount
Limewater is one of
the most useful solutions for aquarists looking to maintain
calcium and alkalinity in reef aquaria. I have used it for
many years to supply my reef aquarium system. It can be inexpensive,
is not too difficult to use, and can maintain the pH of reef
aquaria even when it is otherwise reduced by calcium carbonate/carbon
dioxide reactors or excess carbon dioxide in home air. Limewater
does, however, have a number of eccentricities that aquarists
need to be aware of when using it. These include high pH,
limitations on how much can be added based on evaporation
rates, and considerations of what else is or is not dosed
along with it (such as magnesium). Hopefully, this article
will provide aquarists with the information they need to effectively
use limewater for their aquaria.
Oh, and the next time you see your grandmother, you might
mention what a cool use you have found for pickling lime!