are supplied to reef aquaria in order to balance the losses
caused by the formation of calcium carbonate. This formation
takes place in hard corals to form their skeletons, and in
other internal structures such as spicules in certain soft
corals. It also takes place in a wide range of other organisms,
ranging from coralline algae to snails to clams. Deposition
of calcium carbonate also takes place outside of biological
systems, such as on heaters and pump impellers, where the
increased temperature results in decreased
solubility of calcium carbonate, and hence a greater likelihood
In each of these cases, what is being deposited is largely
calcium carbonate. Since calcium and carbonate are present
in pure calcium carbonate at exactly equal concentrations
(one ion of calcium to one ion of carbonate), the removal
rate of calcium and carbonate by all of the mechanisms described
above should be the same. To a great extent, aquarists use
as a surrogate measure of carbonate (and bicarbonate). The
exact balance between calcium and carbonate demand in a reef
aquarium is therefore equally well described as a balance
between calcium and alkalinity demand.
Reef aquarists take great advantage of the 1:1 matching of
calcium and alkalinity demand in reef aquaria by using additives
that supply calcium and alkalinity in this same ratio.
In this way, over- or under-dosing of such balanced calcium
and alkalinity additives should not result in skewing the
aquarium water's chemistry toward too much calcium and too
little alkalinity, or too much alkalinity and too little calcium.
On the other hand, independent additions of calcium and alkalinity,
even with careful and frequent measurement, often lead to
Figure 1. Coralline algae can be a significant user
of calcium and alkalinity in many aquaria. It also incorporates
a lot of magnesium (1-5% by weight in the skeleton), compared
to many corals, and so can skew the demand toward less calcium.
This example was taken by Chris Holmes in his reef aquarium.
There exist a variety of such balanced
additives, and many have been described and compared in detail
articles. They include calcium carbonate/carbon dioxide
reactors, limewater (kalkwasser), the two part additive systems,
and some one-part systems. As a class, I strongly recommend
them over any other unbalanced additive method for most reef
There are, however, several reasons that calcium and alkalinity
balance is not always perfect. In many reef aquaria using
only balanced additive systems, the levels will slowly drift
away from perfect balance, and will require occasional correction.
Whether this correction is needed monthly or yearly, and in
what direction, will depend on the system's details. Before
discussing these real calcium and alkalinity demand imbalance
issues, I will also describe one "mechanism" that
confounds many aquarists by appearing to represent a drift
in the balance, but that really does not. The "mechanism"
arises in the simple fact that alkalinity rises and falls
much faster than does calcium because seawater has a much
bigger reservoir of calcium than it does alkalinity.
This article will describe the various
mechanisms that cause such drift, and will quantify the magnitude
of each effect. These mechanisms include:
Incorporation of magnesium and strontium in place of
calcium in deposited calcium carbonate.
Reduction in alkalinity through partial completion
of the nitrogen cycle.
Changes in the calcium and alkalinity balance through
Additions of calcium or alkalinity via top-off water.
Understanding these various factors will take some of the
mystery out of reef aquarium chemistry, and will allow aquarists
to become masters of their aquarium chemistry, rather than
slaves to "unpredictable" changes.
Calcium and Alkalinity Demand: Calcium Carbonate
formation consumes its two components in an exact 1:1 ratio.
In the units used by aquarists, this ratio corresponds to
one meq/L (2.8 dKH; 50 ppm CaCO3 equivalents)
for every 20 ppm of calcium. Not surprisingly, this is also
the ratio of alkalinity to calcium that is supplied when calcium
carbonate is dissolved, as in a CaCO3/CO2
reactor. Fortuitously for the aquarist, this is also the ratio
supplied when calcium hydroxide is dissolved, as with the
use of limewater (kalkwasser).
Apparent Excess Demand for Alkalinity
of the most common complaints of new aquarists is that their
aquaria seem to need more alkalinity than their balanced additive
system, such as limewater, is supplying. While there are reasons
this may actually be the case over the long term (these will
be detailed later in this article), frequently these aquarists
are seeing a "chemical mirage" rather than a real
excess demand for alkalinity.
One of the interesting features of seawater is that it contains
a lot more calcium than alkalinity. By this I mean that if
all of the calcium in seawater (420 ppm; 10.5 meq/L) were
to be precipitated as calcium carbonate, it would consume
21 meq/L of alkalinity (nearly 10 times as much as is present
in natural seawater). In a less drastic scenario, let's say
that calcium carbonate is formed from aquarium water starting
with an alkalinity of 3 meq/L that it is allowed to drop to
2 meq/L (a 33% drop). How much has the calcium declined? It
is a surprise to many people to learn that the calcium would
drop by only 20 ppm (5%). Consequently, many aquarists observe
that their calcium levels are relatively stable (within their
ability to reproducibly test it), but alkalinity can vary
up and down substantially. This is exactly what would be expected,
given that the aquarium already has such a large reservoir
Figure 2. Pocillopora sp.
Photo courtesy of www.ReeferMadness.us.
So the first "deviation" from the rule of calcium
and alkalinity balance really isn't a deviation at all. If
an aquarist is supplying a balanced additive to his aquarium,
and calcium seems stable but alkalinity is declining, it may
very well be that what is needed is more of the balanced additive,
not just alkalinity. This scenario should be assumed as
the most likely explanation for most aquarists who should
look for more esoteric explanations for alkalinity decline
only if calcium RISES substantially while alkalinity falls.
Likewise, if alkalinity is rising and calcium seems stable
when using a balanced calcium and alkalinity additive system,
the most likely explanation is that too much of the additive
system is being used.
The real imbalance effects described later in this article
take effect slowly, and are manifested over weeks, months
and years. This short term "chemical mirage" caused
simply by the mathematics of calcium and alkalinity additions
can be seen in a single addition. Any effect that develops
rapidly over the course of a few days is almost certainly
not a true imbalance.
The following scenarios show what can happen to a reef aquarium
whose dosage with a balanced additive system does not match
its demand. Table 1 shows what can happen when the dosing
is inadequate. Alkalinity drops fairly rapidly. After two
days, many aquarists might conclude that they need additional
alkalinity, when in reality, they need more of both calcium
and alkalinity to stabilize the system.
1. Calcium and alkalinity
declines in a reef aquarium where balanced additions
are not meeting demand.
Table 2 shows what happens when too much of a balanced additive
is added. After a few days, many aquarists would conclude
that alkalinity is rising too much, but that calcium is fairly
stable. Again, what is needed is less of the balanced additive,
not just less alkalinity.
2. Calcium and alkalinity
increases in a reef aquarium where balanced additions
are greater than demand.
Real Excess Demand for Alkalinity: Magnesium and
Many sharp aquarists
will correctly dispute the notion I professed in the introduction,
that calcium and alkalinity are exactly balanced, because
coral skeletons are not pure calcium carbonate. In fact, they
contain significant amounts of magnesium and strontium. Abiotically
precipitated calcium carbonate also contains such ions. In
and strontium enter the calcium carbonate structure in place
of calcium, reducing the amount of calcium required for a
given amount of carbonate. Consequently, the aquarium is skewed
toward less calcium demand and more towards alkalinity demand
for this reason.
How big is this effect? In terms of magnesium, it is hard
to say exactly how big the effect will be because the amount
of magnesium deposited depends on the species involved, and
ranges from less than 1% magnesium
by weight in the skeleton, to more than 4%. Consequently,
the magnesium demand in one aquarium may be very different
from the magnesium demand in a second aquarium whose calcium
demand is exactly the same.
Figure 3. This photograph of the underside
of coralline algae was taken by Bob Bottini (aquababy)
owner of Tanks alot!
Nevertheless, we can roughly calculate the magnitude of the
effect. Depositing pure calcium carbonate requires 20 ppm
of calcium for every 1 meq/L of alkalinity. Substituting magnesium
to the extent of 1% by weight in the skeleton decreases the
calcium content by 4.1%. So the demand is then 19.2 ppm calcium
for every 1 meq/L of alkalinity. Substituting magnesium to
the extent of 4% by weight in the skeleton decreases the calcium
content by 16.5%. So the demand is then 16.7 ppm calcium for
every 1 meq/L of alkalinity. The change in the balance
of the demand caused by magnesium incorporation into corals
will depend on the exact species driving the demand, but can
be larger than the other causes described in this article.
has a rather smaller effect. Corals, coralline algae, and
abiotically precipitated calcium carbonate in natural seawater
typically have roughly one strontium ion for every 100 calcium
ions (whether these are dispersed within calcium carbonate,
or as a separate strontium carbonate phase). In a reef aquarium,
where the strontium level can be twice the natural level,
this strontium incorporation can be higher, on the order of
one strontium ion for every 50 calcium ions. The replacement
of calcium by strontium in the carbonate crystals has the
effect of reducing the calcium demand from 20 ppm per one
meq/L of alkalinity to 19.8 meq/l for natural levels of strontium,
and to 19.6 ppm at double the natural level. This strontium
effect is smaller than the magnesium effect, but can be comparable
to the other effects described in this article. In addition,
the amount of substitution by ions other than calcium in forming
carbonates may depend on other factors, including temperature
and (as with the strontium example above) the relative concentrations
of the ions present.
Alkalinity Decline in the Nitrogen Cycle
of the best known chemical cycles in aquaria is the nitrogen
cycle. In it, ammonia excreted by fish and other organisms
is converted into nitrate. This conversion produces acid,
H+ (or uses alkalinity depending
on how one chooses to look at it), as shown in equation 1:
NH3 + 2O2 à
NO3- + H+
For each ammonia molecule converted into
nitrate, one hydrogen ion (H+)
is produced. If nitrate is allowed to accumulate to 50 ppm,
the addition of this acid will deplete 0.8 meq/L (2.3 dKH)
However, the news is not all bad. When
this nitrate proceeds further along the nitrogen cycle, depleted
alkalinity is returned in exactly the amount lost. For example,
if the nitrate is allowed to be converted into N2
in a sand bed, one of the products is bicarbonate, as shown
in equation 2 (below) for the breakdown of glucose and nitrate
under typical anoxic conditions as might happen in a deep
4NO3- + 5/6 C6H12O6
(glucose) + 4H2O
2 N2 + 7H2O + 4HCO3-
In equation 2 we see that exactly one bicarbonate ion is
produced for each nitrate ion consumed. Consequently, the
alkalinity gain is 0.8 meq/L (2.3 dKH) for every 50 ppm of
Likewise, equation 3 (below) shows the uptake of nitrate
and CO2 into macroalgae to form typical
122 CO2 + 122 H2O + 16 NO3-
+ 138 O2 + 16 HCO3-
Again, one bicarbonate ion is produced for each nitrate ion
It turns out that as long as the nitrate concentration is
stable, regardless of its actual value, there is no ongoing
net depletion of alkalinity. Of course, alkalinity was depleted
to reach that value, but once it stabilizes, there is no continuing
alkalinity depletion because the export processes described
above are exactly balancing the depletion from nitrification
(the conversion of ammonia to nitrate).
There are, however, circumstances where the alkalinity is
lost in the conversion of ammonia to nitrate, and is never
returned. The most likely scenario to be important in reef
aquaria is when nitrate is removed through water changes.
In that case, each water change takes out some nitrate, and
if the system produces nitrate to get back to some stable
level, the alkalinity again becomes depleted.
Figure 4. Porites species vary with
respect to the amount of magnesium incorporated,
from less than 0.1% to over 1% magnesium in the
skeleton. Photo courtesy of Skip Attix.
If, for example, nitrate averages 50 ppm at each water change,
then over the course of a year with 10 water changes of 20%
each, the alkalinity will be depleted by 1.6 meq/L (4.5 dKH)
over the course of that entire time period. This process
is one of the primary reasons that fish-only aquaria that
often export nitrate in water changes need occasional buffer
additions to replace that depleted alkalinity.
While the magnitude of the depletion described in the paragraph
above is fairly easy to understand, it also can be converted
into units that clarify the imbalance. The impact of alkalinity
depletion on the calcium and alkalinity demand balance depends,
of course, on the amount of calcium and alkalinity added (and
consumed) over the course of that same year.
For a typical reef aquarium (assuming a daily addition of
saturated limewater equal to 2% of the tank's volume), the
amount of alkalinity added during the course of a year is
297.8 meq/L. Likewise, the amount of calcium added is 5,957
ppm Ca++, given the ratio
of 1 meq/L of alkalinity for every 20 ppm of calcium that
has been discussed above. If that 1.6 meq/L of alkalinity
is added to create a larger demand of 299.4 meq/L over the
course of a year, the new ratio for the total demand becomes
19.90 ppm Ca++ per 1 meq/L
of alkalinity. Consequently, while this effect of nitrate
production on alkalinity is enough to be noticed over the
course of a year, it is substantially smaller than the other
effects discussed in this article, and is unimportant for
aquaria that maintain low nitrate levels.
Effects Due to Water Changes
reason that calcium and alkalinity demand is not exactly balanced
in many aquaria has to do with water changes. Many aquarists
(including myself) do not attempt to match the calcium and
alkalinity levels in water change water to the aquarium water.
Consequently, each water change will alter these levels in
the aquarium, and will alter the observed balance between
calcium and alkalinity demand. What direction the change takes,
however, will depend on the salt mix chosen and the aquarium
water parameters. Commercial salt mixes vary from high calcium
and normal alkalinity to high alkalinity and low calcium.
For example, if the aquarium is maintained at 420 ppm calcium
and 4 meq/L of alkalinity, and the water change has 500 ppm
calcium and 2.5 meq/L of alkalinity, each 20% water change
will increase calcium by 16 ppm, and will drop alkalinity
by 0.3 meq/L. Using the same water change scenario used in
the nitrate calculations above (10 changes of 20% each over
the course of a year), these water changes will increase calcium
by 160 ppm and drop alkalinity by 3 meq/L.
Figure 5. While many soft corals do use calcium
and alkalinity to form internal structures made from
calcium carbonate, Xenia seems to have few if
any such structures. Consequently, it does not significantly
impact the demand for calcium or alkalinity in reef
aquaria. Photo courtesy of Gregory (www.ximinasphotography.com).
For a typical aquarium (assuming a daily
addition of saturated limewater equal to 2% of the tank's
volume), the amount of alkalinity added during the course
of a year is 297.8 meq/L. Likewise, the amount of calcium
added is 5,957 ppm Ca++,
given the ratio of 1 meq/L of alkalinity for every 20 ppm
of calcium that has been discussed above. If that amount of
alkalinity demand is increased by 3 meq/L to 300.8 meq/L over
the course of a year, and the calcium demand is decreased
by 160 ppm to 5797 ppm, the new ratio for the total demand
becomes 19.30 ppm Ca++ per
1 meq/L of alkalinity. Consequently, the effect of water
changes can be significant, but will depend entirely on how
much the aquarium water deviates from the water change water,
and on the amount of water changed.
The Effect of Top-Off Water
final factor that can impact the apparent calcium and alkalinity
demand is the possibility of delivering calcium or alkalinity
or both in top-off water. Water that is purified by reverse
osmosis (RO) followed by deionization (DI), and water that
is purified by distillation will not deliver any significant
amount of calcium or alkalinity (regardless of the apparent
pH when such a measurement is taken). The same is true for
water purified by DI only. Water purified by only RO may have
a small amount of calcium or alkalinity in it, depending on
the nature of the source water.
The greatest chance for effects to tankwater calcium and
alkalinity levels comes from the use of tap water or spring
water (neither of which I recommend for reef aquaria). In
a recent article describing concerns with the use of tap
water in reef aquaria, I showed that water from municipal
water supplies can range from 0 to 93 ppm calcium and 0 to
5.5 meq/L alkalinity. Obviously, tap water with close to zero
calcium and alkalinity will not appreciably impact the calcium
and alkalinity balance. At the extremes, however, these values
can have a large impact.
If we assume that an aquarium receives 2% of its tank volume
daily to replace evaporated water, then one extreme is a case
where over the course of a year, 679 ppm of calcium is added,
and no alkalinity. At the other extreme, 40 meq/l of alkalinity
is added, and no calcium.
For a typical aquarium (using 2% of the
tank volume daily in saturated limewater), the amount of alkalinity
added during the course of a year is 297.8 meq/L. Likewise,
the amount of calcium added is 5,957 ppm Ca++,
given the demand ratio of 1 meq/L of alkalinity for every
20 ppm of calcium discussed above. If that amount of alkalinity
demand is decreased by 40 meq/L, due to alkalinity in tap
water, to 257.4 meq/L over the course of a year, and the calcium
demand is unchanged, the new ratio for the total apparent
demand becomes 23.1 ppm Ca++
per 1 meq/L of alkalinity. Likewise, if the calcium demand
is decreased by 679 ppm of calcium, to 5278 ppm, the new apparent
demand ratio becomes 17.7 ppm Ca++
per 1 meq/L of alkalinity. These extreme cases may not actually
happen anywhere, since the extreme case for calcium and the
extreme case for alkalinity occur in the same city (Kansas
City in 2003), so they partially offset each other. Nevertheless,
the effect easily could be half as large in many cities, and
it is apparent that this effect of tap water can be significant,
and may even dominate the other effects.
effects may also skew the demand for calcium and alkalinity
in aquaria. These include foods that contain calcium or, rarely,
alkalinity, and various additives that aquarists use. Most
additives do not contain alkalinity (except, of course, buffers
and anything claiming to control pH or supply alkalinity),
although sodium silicate and borax (borate) do provide alkalinity.
Since many additives do not even say what they contain, it
is hard to say what effect they might have, but I'd expect
most of them to be inconsequential in this respect.
How Are Additive Systems Really Balanced?
for the reasons described above, the demand for calcium and
alkalinity may not be precisely balanced at 20 ppm calcium
per 1 meq/l of alkalinity (matching pure calcium carbonate
formation), the question arises, what ratio is used in balanced
According to the ESV web site, the two part system B-ionic
has a balance of 19.3 ppm calcium per 1 meq/L of alkalinity.
That value is probably a fine balance for the calcium and
alkalinity ions given the effects of magnesium and strontium
incorporation. Other brands that aquarists frequently use
do not give adequately detailed information about their products
to show what the exact ratio might be.
settled limewater has a ratio of approximately 20.0 ppm
Ca++ to 1 meq/L of alkalinity.
It has no significant magnesium in it, and its strontium level
is very low. For those dosing cloudy limewater, lime
solids that I have measured contain enough magnesium to
drop the ratio to about 19.9 ppm calcium per 1 meq/L of alkalinity.
media has slightly less magnesium and strontium than does
the lime that I tested, and would have a ratio of 19.9 ppm
calcium per 1 meq/L of alkalinity. A different brand of media,
Calc Gold, has more magnesium, with a resulting ratio
of about 19.8 ppm calcium per 1 meq/L of alkalinity. A third
Ocean crushed coral, has a similar level of magnesium,
resulting in a ratio of 19.8 ppm calcium per 1 meq/L of alkalinity.
All of these brands may fall short of the rate of incorporation
of magnesium in reef aquaria, as has been discussed in previous
articles. Some aquarists have taken to adding a small
amount of dolomite (a material containing both calcium and
magnesium carbonates) to their CaCO3/CO2
reactors to add an appropriate amount of magnesium.
Which Mechanisms Predominate in Reef Aquaria?
resulting in a deviation from an exact balance between calcium
and alkalinity will obviously vary between aquaria with different
calcifying species and with different husbandry practices.
Some of the mechanisms may have opposite effects on the balance,
partially canceling each other out in some aquaria (e.g.,
water changes with a high alkalinity/low calcium salt mix
vs. magnesium and strontium incorporation). Consequently,
it isn't possible to say which effect will dominate reef aquaria
Figure 6. Tridacna species of clams deposit
calcium carbonate in their shells, and can be a significant
source of calcium and alkalinity demand in reef aquaria
with many clams.
Photo courtesy of Gregory (www.ximinasphotography.com).
In my aquarium, using limewater, I do enough water changes
that over time my aquarium keeps a balance that is similar
to that found in the Instant Ocean salt mix that I use for
water changes. After running this aquarium for about 10 months
after I took the last calcium and alkalinity measurement,
the levels were about 3.6 meq/L for alkalinity and 360 ppm
for calcium. At that point, I raised the calcium to about
420 ppm with calcium chloride. As I have stated in previous
articles regarding magnesium
in my aquarium, I am not certain why the demand for magnesium
(and its effect on the calcium and alkalinity demand) is not
A variety of reasons
prevent reef aquarists from experiencing exactly balanced
demands for calcium and alkalinity. These include the effects
of incorporation of magnesium and strontium into coral skeletons,
the effects of water changes with new water that does not
match the aquarium water in terms of chemical ions present,
and top off water that contains calcium or alkalinity. Aquarists
also may be sometimes fooled into thinking they are seeing
imbalanced demand when in reality they are simply observing
the fact that on a percentage basis, alkalinity goes up and
down much faster than calcium. Understanding how and when
these differences arise will allow reef aquarists to better
deal with them, and not take inappropriate actions to "correct"