Ask the WWM Crew
|Please visit our Sponsors
One of the most frustrating aspects of reef
keeping for aquarists is undoubtedly water chemistry. Aquarists without
a formal education in the sciences may find the challenge to understand
the relationship between calcium and alkalinity in reef aquariums
formidable. Adequate and consistent levels of calcium and alkalinity
not only insure continued growth of invertebrates, but also are
required for their very survival and sustenance in the shadow of system
stability at large. Saltwater chemistry is indeed a complex matter and
there are admittedly numerous factors that can influence the tide and
turn of various equations in balance. Nevertheless, none are so
significant or challenging that fundamentally good husbandry with
regular partial water changes and basic supplementation won't allow
you to run a simple and successful aquarium without resorting to rocket
science, an abacus and a crystal ball! If you will indulge some
generalizations about the dynamics of invertebrate growth
(calcification and skeletogenesis), I can proffer a very simple
explanation of the application of calcium and alkalinity that will
spare you the need to earn a degree chemistry just to keep a healthy
The first thing that we need to recognize is that
calcareous animals need adequate supplies of both calcium and
alkalinity to grow. Corals, coralline algae, and shelled animals alike
are comprised of more than just calcium… they are made of calcium
carbonate. It is possible to also form skeletons with strontium and
silica components, but most aquarists need only focus on the fact that
skeletogenesis (skeletal growth) is essentially founded on the
composition of both calcium and carbonate elements from the environment
(water and/or food) by the process we call calcification.
Calcium is easily learned and applied by most
aquarists (simple dosing and test kits), but alkalinity is rather less
clear. Some folks mistake "Alkalinity as a measure of the
buffering ability of seawater" (expressed in degrees of hardness)
with "Alkalinity as an expression of the pH scale" (in
contrast to acidity). Alkalinity and pH, however, are distinctly
different from each other, although their definitions and functions can
be easily confused.
For those of you as uninformed about water
chemistry as I was when I first entered the hobby, know that Alkalinity
is a measure of water's ability to neutralize acids and resist
change with a reserve of "hard" minerals (pool of dissolved
buffers). On the contrary, pH is a measurement of the concentration of
hydrogen ions in water in terms of acidity or alkalinity. So, the
alkalinity of water in terms of pH merely refers to the basic end of a
pH scale (alkaline) in contrast to the acidic end of the scale. It is a
separate matter, however, from Alkalinity as a measure of water
There are several minerals that commonly
contribute to the overall hardness of water, but calcium and magnesium
are the primary elements. A higher measure of water hardness
(Alkalinity) translates to a higher buffering ability, and subsequently
a lower chance of a pH change in the system. Alkalinity can be measured
as GH (total or General hardness) or KH (Carbonate hardness). I would
recommend a carbonate hardness test kit as your primary measure of
It is easy to believe that water with
"alkaline" or basic pH is likely to be high in
alkalinity (buffering capacity). However, this is not always
true. Water with a high pH but a low alkalinity, albeit uncommon, is
possible and regarded as unstable. Aerated RO water (purified by
reverse osmosis) or DI water (demineralized by de-ionization) is
commonly in this category. Such unbuffered water will quickly decline
in pH with the natural accumulation of organic acids in the aquarium.
Reef aquarists that are surprised to see a falling or depressed pH in
their systems despite high calcium levels may have neglected to measure
and maintain alkalinity.
For most marine aquariums, calcium is recommended
within a range of 350-450ppm. Alkalinity is recommended with a range of
8 to 12 dKH. Reef aquarists with very large populations of stony corals
or with fast growth as a priority, however, may entertain levels
somewhat higher. These are special circumstances that indeed have
benefits, limitations and inherent dangers and require specific address
beyond the scope of this article as a primer. Casual aquarists instead
can rest comfortably within the outlined ranges and enjoy healthy
corals and good growth of calcifying organisms in display.
Ironically, within the recommended ranges
of 350 and 450 ppm Ca (calcium) and 8-12 dKH Alk (alkalinity) it is not
necessary to maintain both parameters at the higher end concurrently.
In fact, it is not practical or easily attained in most systems. In
gross terms, high calcium and high alkalinity are mutually exclusive.
Alas, too many aquarists get caught up in the roller coaster
application of excessive amounts of supplements (randomly applied or
not) and skew the balance of Ca and Alk in the system. Maintaining
stable and consistent levels should not be difficult at all. To
illuminate this paradox I offer you a simple analogy with
The Marble Analogy
Fact: it is only possible to dissolve so many
solids into a given volume of water (calcium, carbonates, and
everything else). At the risk of oversimplifying the dynamic, imagine a
bowl that holds one hundred marbles representing the total dissolved
solids in seawater in a given system. If red marbles represented
calcium, and blue marbles represented carbonates (alkalinity), the bowl
can still only hold one hundred marbles no matter what mix of color
they are. Now, if seventy marbles were the equivalent of 400-ppm
calcium and the remaining marbles were blue, the only way to increase
calcium would be to displace alkalinity (to remove blue marbles). In
troubled systems, the misapplication of calcium supplements (dosing
suddenly or to excess) is known to cause a sudden precipitation of
carbonates (the alkalinity falls/crashes) that is commonly referred to
as a "snowstorm". It is instigated by the influx of a large
or rapid amount of calcium entering the system that spikes the pH
immediately surrounding carbonate molecules and causes a crystalline
precipitation (fallout). In keeping with our analogy, a
"snowstorm" would be like taking another bowl of one hundred
red marbles (calcium) and trying to pour it into the original bowl of
mixed, colored marbles (balanced calcium and alkalinity). The result is
the displacement of all blue marbles (carbonates/alkalinity) and the
overflow of excess red marbles beyond the one hundred marble limit. The
ramifications of this in an aquarium is a crash in water chemistry and
water quality that cannot be corrected while the chemical reaction
occurs. Dosing more supplements to try to correct the imbalance (or
even doing a concurrent water change with hopes of dilution) will only
serve to feed the chain reaction. Tragically, the "snowstorm"
must be allowed to finish and an aquarium system is traumatized in the
To safely avoid dangerous imbalances in the Ca-Alk
dynamic, aquarists simply need to avoid pushing either component to an
extreme end or both simultaneously high. Instead, think of the
relationship as a Hi-Lo situation within the safe ranges. Within the
accepted ranges (350-450 ppm Ca and 8-12 dKH Alk), one parameter can be
pushed to a high end while the other is allowed to stray toward the
middle or lower end. Any reasonably mix of the two will still provide
more than enough of both elements for successful calcification. More
importantly, consistent levels of both are far more supportive of
growth in calcareous organisms than the inconsistent but high average
of either component otherwise. Many aquarists enjoy phenomenal growth
in their reef creatures with rather modest Ca and Alk levels. Indeed,
consistency with all aspects of aquatic husbandry is more conducive to
success than random high points.
Options for maintaining Calcium and Alkalinity
My ultimate advice for aquarists regarding
supplementation here is to use kalkwasser (calcium hydroxide) in
combination with a deep sand bed (fine aragonite) and/or a calcium
reactor. These combinations are time-tested (20 years) and reliable
with the most benefits and least trouble when properly applied.
There are many calcium and alkalinity boosting
products on the market but most are easily abused with little
explanation or support from the labeling, instruction or even popular
literature in the hobby. The following is a brief summary of the merits
and demerits of each categorically:
Dry Calcium (Calcium
hydroxide: AKA "Kalkwasser"): In summation, one of the
absolute best supplements to dose calcium in aquaria with a long list
of benefits and few disadvantages. Easy to use, but easy to abuse.
Follow dosing protocol strictly.
A potent and direct method of providing calcium Caustic
nature increases pH and indirectly supports Alkalinity by tempering
acids that would otherwise burden the buffer pool is self-purifying by virtue of its highly
caustic nature. Most impurities commonly found in the raw reagent
precipitate out. Aids the neutralizing precipitation of nuisance
phosphate in aquaria. This is a tremendous advantage over all other
methods of dosing.
Improves skimmer performance
Dosing at night tempers pH swings from
inevitable drop from respiration by photosynthetic dynamics.
No negative, residual by-product of its use
(like accumulating Chloride ions with "Liquid calcium"
A long-term calcium supplement
Disadvantage: High pH limits supplementation to dosing at night or in increments
that will not raise pH suddenly by more than
Dosing by the commonly recommended
supersaturated solutions is tedious and messy (instead, see
"Slurry method" in the Book of Coral Propagation, Volume 1
for a tidy and succinct dosing method).
Can be dangerous or stressful to system if
Liquid Calcium (Calcium
Chloride): Perhaps the most commonly used and abused methods for
dosing calcium. Not recommended for long-term use.
A potent and fast method for dosing
Calcium without any impact on pH.
Clean, easy and brief application.
Very safe and not easily
overdosed in the short-term.
Potentially dangerous accumulation of by-product
Chloride ions can skew water chemistry and plague Ca/Alk balance with
long-term use (1-2 years).
Requires aquarist to conduct more
frequent and larger water changes on the system to dilute and delay
Chloride ion accumulation.
No ancillary benefits like
Calcium hydroxide (support of pH, Alk, skimmer, etc.)
A temporary calcium supplement only to be used
for "quick fixes" of strayed Ca.
Deep Sand Bed (Aragonite sand): Be sure to purchase media comprised of
aragonite (oolitic) material and not calcite. Both calcite and
aragonite are forms of calcium carbonate, but only aragonite will
dissolve easily at the necessarily high pH of a healthy marine system.
Grain size should be sugar fine at 10 cm or more depth. Courser grains
require even greater sand bed depth and stronger water flow to
A very natural, significant and
passive way of supporting both Calcium and Alkalinity through the
dissolution of aragonite in balanced quantity.
Long list of natural and organic
benefits to elaborate for the scope of this summary. Distinguished by
natural nitrate reduction with deep enough media (over 10cm), natural
plankton encouraged and cultured in situ, reflects light back up to
corals and plants thereby reducing or preventing bleaching undersides
in captivity, etc.
Very safe and not easily corrupted
without gross negligence or misapplication of standard aquatic
In some systems it can singly support Ca and Alk without any other
Requires monitoring and supplementation.
Falls prey easily to
misapplication by ill-advised or misinformed aquarists. Lack of
adequate water flow in the display or installation at an improper depth
(less than 7 cm) can contribute to nutrient accumulation (AKA:
"Nutrient sink") and algae.
After a long
time, calcium rectors are finally gaining due recognition for their
great worth and validity as a pivotal component in an optimal marine
A very potent method for dosing and
maintaining Calcium and Alkalinity with superb consistency and
reliability. The "ultimate" strategy for many
Low-maintenance in service,
inexpensive operating cost, ideal once properly tuned.
In some systems it can singly
support Ca and Alk without any other supplements.
initially complicated hardware to install and tune.
Performance is influenced by the quality and dissolution of reactor
media is not self-purifying (like Calcium hydroxide) by the
process/application and can impart contaminants
into the system.
misapplication can be dangerous or stressful to the system.
Two-Part Liquid Supplements: a popular
compromise for aquarists that find Calcium hydroxide
("kalkwasser") too tedious, but cannot afford a calcium
reactor. A reasonably good supplement with regular water changes and
Very clean, easy and reliable to dose if
application is followed strictly. Provides elements of calcium and
alkalinity in balance.
Very strict application protocol. Must be shaken/mixed
vigorously before every application as the components of the clear
supplement stratify and separate (the product does not stay uniformly
mixed in bottle)! Neglect otherwise can severely corrupt Ca/Alk
balance and make a problem system even worse.
Requires that a system be in balance
BEFORE supplementation begins. Large water changes are necessary on
problem systems (to restore balance) before two-part supplements can
carry on the job of metering Ca and Alk.
Categorically it is the most
expensive long-term supplement of Ca and Alk
Baking Soda: The common culinary
ingredient, sodium bicarbonate, is an often used and abused method of
increasing the pH/alkalinity of an aquarium system. Unsupported,
however, it is only temporarily effective and it is easily misapplied
(spiking and stressing the system). Although it is the single largest
component in commercial "seabuffers", it is not recommended
for casual aquarists as a sole supplement when the safer and more
effective options outlined above are available.
Although not a reliable source of calcium, commercial
"seabuffers" are traditional vehicles for dosing and
supporting adequate pH and alkalinity in the aquarium. Different brands
have variations in their recipes, but most are fundamentally comprised
of bicarbonate, carbonate and a little borate. Some embellished
supplements may include calcium oxide, magnesium sulfate and other
desirable ingredients. Categorically, they not unreasonably expensive
to dose long term and they are fairly reliable. Aquarists using such
supplements, however, will need to measure water quality as much or
more than any strategy of supplementation. Calcium hydroxide
("kalkwasser") employed with a deep sand bed or a calcium
reactor can often eliminate the need for the use of seabuffers. They
are an effective, but tedious and time-consuming way to supplement
Organic Calcium (Calcium
gluconate): This calcium supplement is an exception to the above
warnings about the dangers of "Liquid calcium" products.
Sugar-based calcium is an essentially safe source of calcium, but has
not clearly demonstrated its efficacy for coral growth. It has,
however, been shown to greatly accelerate the growth of desirable
coralline algae species. As such, it is recommended as a supplement to
a primary method for dosing calcium.
In summary, and for the
benefit of distracted speed-readers, maintain Calcium at levels between
350 and 450 ppm Ca, and alkalinity between 8-12 dKH without ever
resting both concurrently at the highest end of their range. Calcium
hydroxide ("kalkwasser") dosed properly in concert with a
deep sand bed (10cm + sugar fine aragonite) or a calcium reactor will
yield the most benefits when supplementing Calcium and Alkalinity.
Leading you to the next open door for advanced reef chemistry, I also
recommend keeping magnesium levels at approximately 3 times the Calcium
level (~1200 ppm Mg). For those not inclined to complicate their hobby
with advanced studies of the dynamics of seawater, rest assured that
regular partial water changes will serve your system well to dilute and
refresh water quality.
In shared admiration of the sea… Anthony
Calfo, Anthony Rosario. 2001. Book of Coral Propagation. A Reading Trees Publication
Delbeek, J. Charles and Sprung, Julian. 1994. The Reef Aquarium: Volume I. Ricordea Publishing.
* Parts of this report have been excerpted and revised from the Book of Coral Propagation, Volume 1 by Anthony Calfo. www.readingtrees.com