Aquarists often ask what water parameter
levels make for a successful reef aquarium. This article gathers
these recommendations in one place, showing them in tables,
as well as the corresponding levels in natural seawater.
Many of the recommendations are my own
opinions, and other aquarists may recommend slightly different
levels. To make clear the basis for each recommendation, a
brief description of each particular parameter's importance
follows the tables, along with links to other online articles
that go into much greater depth on each subject (click on
any blue text for the linked article).
Table 1 shows important water parameters
for reef aquarists to control for various reasons. Table 2
shows less important parameters, or those too complicated
to carefully control, but about which many aquarists have
concerns or questions.
Table 1. Parameters
critical to control in
Surface Ocean Value:1
125-200 ppm CaCO3 equivalents
125 ppm CaCO3 equivalents
sg = 1.026
sg = 1.025-1.027
8.1-8.3 is better
(can be lower or higher in lagoons)
(typically <0.1 ppm)
Table 2. Other
parameters in reef aquaria.
2 ppm, much lower if diatoms are a problem
- 2.7 ppm
ppm total of all forms
(typically below 0.1 ppm)
0.2 ppm typically
(typically below 0.0001 ppm)
Kit Detection Limits (additions OK)
Many corals use calcium to form their skeletons,
which are composed primarily of calcium carbonate. The corals
get most of the calcium for this process from the water surrounding
them. Consequently, calcium often becomes depleted in aquaria
housing rapidly growing corals, calcareous red algae, Tridacnids
and Halimeda. As the calcium level drops below 360
ppm, it becomes progressively more difficult for the corals
to collect enough calcium, thus stunting their growth.
Maintaining the calcium level is one
of the most important aspects of coral reef aquarium husbandry.
Most reef aquarists try to maintain approximately natural
levels of calcium in their aquaria (~420 ppm). It does
not appear that boosting the calcium concentration above natural
levels enhances calcification (i.e., skeletal growth) in most
corals. Experiments on Stylophora pistillata, for example,
show that low calcium levels limit calcification, but that
levels above about 360 ppm do not increase calcification.3
Exactly why this happens was detailed in a previous article
on the molecular
mechanisms of calcification in corals.
For these reasons, I suggest that aquarists
maintain a calcium level between about 380 and 450 ppm.
I also suggest using a balanced
calcium and alkalinity additive system for routine maintenance.
The most popular of these balanced methods include limewater
(kalkwasser), calcium carbonate/carbon dioxide reactors, and
the two-part additive systems.
If calcium is depleted and needs to be
raised significantly, however, such a balanced additive is
not a good choice since it will raise alkalinity too much.
In that case, adding
calcium chloride is a good method for raising calcium.
Like calcium, many corals also use "alkalinity"
to form their skeletons, which are composed primarily of calcium
carbonate. It is generally believed that corals
take up bicarbonate, convert it into carbonate, and then
use that carbonate to form calcium carbonate skeletons. That
conversion process is shown as:
Carbonate + acid
To ensure that corals have an adequate
supply of bicarbonate for calcification, aquarists could very
well just measure bicarbonate directly. Designing a test kit
for bicarbonate, however, is somewhat more complicated than
for alkalinity. Consequently, the use of alkalinity as a surrogate
measure for bicarbonate is deeply entrenched in the reef aquarium
is alkalinity? Alkalinity in a marine aquarium is simply
a measure of the amount of acid (H+)
required to reduce the pH to about 4.5, where all bicarbonate
is converted into carbonic acid as follows:
In normal seawater or marine aquarium water,
the bicarbonate greatly dominates all other
ions that contribute to alkalinity, so knowing the amount
of H+ needed to reduce the
pH to 4.5 is akin to knowing how much bicarbonate is present.
Aquarists have therefore found it convenient to use alkalinity
as a surrogate measure for bicarbonate.
One important caveat to this surrogate
measure is that some artificial seawater mixes, such as Seachem
salt, contain elevated
concentrations of borate. While borate is natural at low
levels, and does contribute to pH
stability, too much interferes with the normal relationship
between bicarbonate and alkalinity, and aquaria using those
mixes must take this difference into account when determining
the appropriate alkalinity level.
Unlike the calcium concentration, it is
widely believed that certain organisms calcify more quickly
at alkalinity levels higher than those in normal seawater.
This result has also been demonstrated in the scientific literature,
which has shown that adding bicarbonate to seawater increases
the rate of calcification in Porites porites.4
In this case, doubling the bicarbonate concentration resulted
in a doubling of the calcification rate. Uptake of bicarbonate
can apparently become rate limiting in many corals.5
This may be partly due to the fact that both photosynthesis
and calcification are competing for bicarbonate, and that
the external bicarbonate concentration is not large to begin
with (relative to, for example, the calcium concentration).
For these reasons, alkalinity maintenance
is a critical aspect of coral reef aquarium husbandry.
In the absence of supplementation, alkalinity will rapidly
drop as corals use up much of what is present in seawater.
Most reef aquarists try to maintain alkalinity at levels at
or slightly above those of normal seawater, although exactly
what levels different aquarists target depend a bit on the
goals of their aquaria. Those wanting the most rapid skeletal
growth, for example, often push alkalinity to higher levels.
I suggest that aquarists maintain alkalinity between about
2.5 and 4 meq/L (7-11 dKH, 125-200 ppm CaCO3
equivalents), although higher levels are acceptable as
long as they do not depress the calcium level.
Alkalinity levels above those in natural
seawater increase the abiotic
(nonbiological) precipitation of calcium carbonate on
objects such as heaters and pump impellers. This precipitation
not only wastes calcium and alkalinity that aquarists are
carefully adding, but it also increases equipment maintenance
requirements. When elevated alkalinity is driving this precipitation,
it can also depress the calcium level. A raised alkalinity
level can therefore create undesirable consequences.
I suggest that aquarists use a balanced
calcium and alkalinity additive system of some sort for
routine maintenance. The most popular of these balanced methods
include limewater (kalkwasser), calcium carbonate/carbon dioxide
reactors, and the two-part additive systems.
alkalinity corrections, aquarists can simply use baking
soda or washing soda to good effect.
There are a variety of different ways to
measure and report salinity, including conductivity probes,
refractometers, and hydrometers. They typically report values
for specific gravity (which is unitless) or salinity (in units
of ppt or parts per thousand, roughly corresponding to the
number of grams of dry salt in 1 kg of the water), although
conductivity (in units of mS/cm, milliSiemens per centimeter)
is sometimes used.
Somewhat surprisingly, aquarists do not
always use units that naturally follow from their measurement
technique (specific gravity for hydrometers, refractive index
for refractometers, and conductivity for conductivity probes)
but rather use the units interchangeably.
For reference, natural
ocean water has a salinity of about 35 ppt, corresponding
to a specific gravity of about 1.0264 and a conductivity of
As far as I know, there is little real
evidence that keeping a coral reef aquarium at anything other
than natural levels is preferable. It appears to be common
practice to keep marine fish, and in many cases reef aquaria,
at somewhat lower than natural salinity levels. This practice
stems, at least in part, from the belief that fish are less
stressed at reduced salinity. Substantial misunderstandings
also arise among aquarists as to how
specific gravity really relates to salinity, especially
considering temperature effects.
Ron Shimek has discussed salinity on natural
reefs in a previous
article. His recommendation, and mine as well, is to maintain
salinity at a natural level. If the organisms in the aquarium
are from brackish environments with lower salinity, or from
the Red Sea with higher salinity, selecting something other
than 35 ppt may make good sense. Otherwise, I suggest targeting
a salinity of 35 ppt (specific gravity = 1.0264; conductivity
= 53 mS/cm).
Temperature impacts reef aquarium inhabitants
in a variety of ways. First and foremost, the animals' metabolic
rates rise as temperature rises. They may consequently use
more oxygen, carbon dioxide, nutrients, calcium and alkalinity
at higher temperatures. This higher metabolic rate can also
increase both their growth rate and waste production at higher
Another important impact of temperature
is on the chemical aspects of the aquarium. The solubility
of dissolved gases such as oxygen and carbon dioxide, for
example, changes with temperature. Oxygen, in particular,
can be a concern because it is less soluble at higher temperature.
So what does this imply for aquarists?
In most instances, trying to match the
natural environment in a reef aquarium is a worthy goal. Temperature
may, however, be a parameter that requires accounting for
the practical considerations of a small closed system. Looking
to the ocean as a guide for setting temperatures in reef aquaria
may present complications, because corals grow in such a wide
range of temperatures. Nevertheless, Ron Shimek has shown
in a previous
article that the greatest variety of corals are found
in water whose average temperature is about 83-86° F.
Reef aquaria do, however, have limitations
that may make their optimal temperature somewhat lower. During
normal functioning of a reef aquarium, the oxygen level and
the metabolic rate of the aquarium inhabitants are not often
important issues. During a crisis such as a power failure,
however, the dissolved oxygen can be rapidly used up. Lower
temperatures not only allow a higher oxygen level before an
emergency, but will also slow the consumption of that oxygen
by slowing the metabolism of the aquarium's inhabitants. The
production of ammonia as organisms begin to die may also be
slower at lower temperatures. For reasons such as this, one
may choose to strike a practical balance between temperatures
that are too high (even if corals normally thrive in the ocean
at those temperatures), and those that are too low. Although
average reef temperatures in maximal diversity areas (i.e.
coral triangle centered Indonesia,) these areas are also often
subject to significant mixing. In fact, the cooler reefs,
( i..e. open Pacific reefs) are often more stable at lower
temperatures due to oceanic exchange but are less tolerant
to bleaching and other temperature related perturbations.
All things considered, those natural guidelines
leave a fairly wide range of acceptable temperatures. I keep
my aquarium at about 80-81° F year-round. I am actually
more inclined to keep the aquarium cooler in the summer, when
a power failure would most likely warm the aquarium, and higher
in winter, when a power failure would most likely cool it.
All things considered, I recommend
temperatures in the range of 76-83° F unless there is
a very clear reason to keep it outside that range.
Aquarists spend a considerable amount
of time and effort worrying about, and attempting to solve,
apparent problems with the pH of their aquaria. Some of this
effort is certainly justified, as true pH problems can lead
to poor animal health. In many cases, however, the only problem
is with the pH measurement or its interpretation.
Several factors make monitoring a marine
aquarium's pH level important. One is that aquatic organisms
thrive only in a particular pH range, which varies from organism
to organism. It is therefore difficult to justify a claim
that a particular pH range is "optimal" in an aquarium
housing many species. Even natural seawater's pH (8.0 to 8.3)
may be suboptimal for some of its creatures, but it was recognized
more than eighty years ago that pH levels different from natural
seawater (down to 7.3, for example) are stressful to fish.6
Additional information now exists about optimal pH ranges
for many organisms, but the data are woefully inadequate to
allow aquarists to optimize pH for most organisms which interest
Additionally, pH's effect on organisms
can be direct, or indirect. The toxicity of metals such as
copper and nickel to some aquarium organisms, such
as mysids and amphipods,12
is known to vary with pH Consequently the acceptable
pH range of one aquarium may differ from another aquarium's,
even if they contain the same organisms, but have different
concentrations of metals.
Changes in pH nevertheless do substantially
impact some fundamental processes taking place in many marine
organisms. One of these fundamental processes is calcification,
or deposition of calcium carbonate skeletons, which is known
to depend on pH, dropping as pH falls.13,14
Using this type of information, along with the integrated
experience of many hobbyists, we can develop some guidelines
about what is an acceptable pH range for reef aquaria, and
what values push the limits.
The acceptable pH range for reef aquaria
is an opinion rather than a clearly delineated fact, and will
certainly vary with the opinion's provider. This range may
also be quite different from the "optimal" range.
Justifying what is optimal, however, is much more problematic
than is justifying that which is simply acceptable, so we
will focus on the latter. As a goal, I'd suggest that the
pH of natural seawater, about 8.2, is appropriate, but coral
reef aquaria can clearly succeed in a wider range of pH values.
In my opinion, the pH range from 7.8 to 8.5 is an acceptable
range for reef aquaria, with several caveats. These are:
That the alkalinity is at least 2.5
meq/L, and preferably higher at the lower end of this
pH range. I base this statement partly on the fact that
many reef aquaria operate quite effectively in the pH
7.8 to 8.0 range, and that most of the best examples of
these types of aquaria incorporate calcium carbonate/carbon
dioxide reactors which, while tending to lower the pH,
keep the carbonate alkalinity fairly high (at or above
3 meq/L.). In this case, any problems associated with
at these lower pH values may be offset by the higher
That the calcium level is at least
400 ppm. Calcification becomes more difficult as the pH
and calcium levels fall. It is not desirable to push all
of the extremes of pH, alkalinity, and calcium at the
same time, so if the pH is low and cannot be easily changed
(as may be the case in an aquarium with a CaCO3/CO2
reactor), at least make sure that the calcium level is
normal to high (~400-450 ppm).
Likewise, one of the problems at higher
pH (anywhere above 8.2, but progressively more problematic
with each incremental rise) is the abiotic precipitation
of calcium carbonate, resulting in a drop in calcium and
alkalinity, and the clogging of heaters and pump impellers.
If you push the pH to 8.4 or higher (as often happens
when using limewater), make sure that both the calcium
and alkalinity levels are suitably maintained (that is,
neither too low, inhibiting biological calcification,
nor too high, causing excessive abiotic precipitation
Transient upward spikes are less deleterious
than transient downward spikes in pH.
Magnesium's primary importance is its interaction
with the calcium and alkalinity balance in reef aquaria. Seawater
and reef aquarium water are always supersaturated with calcium
carbonate. That is, the solution's calcium and carbonate levels
exceed the amount that the water can hold at equilibrium.
How can that be? Magnesium is a big part of the answer. Whenever
calcium carbonate begins to precipitate, magnesium binds to
the growing surface of the calcium carbonate crystals. The
magnesium effectively clogs the crystals' surface so that
they no longer look like calcium carbonate, making them unable
to attract more calcium and carbonate, so the precipitation
stops. Without the magnesium, the abiotic (nonbiological)
precipitation of calcium carbonate would likely increase enough
to prohibit the maintenance of calcium and alkalinity at natural
For this reason, I suggest targeting
seawater concentration of magnesium: ~1285 ppm. For practical
purposes, 1250-1350 ppm is fine, and levels slightly outside
that range (1200-1400 ppm) are also likely acceptable.
I would not suggest raising magnesium by more than 100 ppm
per day, in case the magnesium supplement contains impurities.
If you need to raise it by several hundred ppm, spreading
the addition over several days will allow you to more accurately
reach the target concentration, and might possibly allow the
aquarium to handle any impurities that the supplement contains.
An aquarium's corals and coralline algae
can deplete magnesium by incorporating it into their growing
calcium carbonate skeletons. Many methods of supplementing
calcium and alkalinity may not deliver enough magnesium to
maintain it at a normal level. Settled
limewater (kalkwasser), in particular, is quite deficient
in magnesium. Consequently, magnesium should be measured occasionally,
particularly if the aquarium's calcium and alkalinity levels
seem difficult to maintain. Aquaria with excessive abiotic
precipitation of calcium carbonate on objects such as heaters
and pumps might suffer from low magnesium levels (along with
high pH, calcium, and alkalinity).
The "simplest" form of phosphorus
in reef aquaria is inorganic orthophosphate (H3PO4,
HPO4--, and PO4---
are all forms of orthophosphate). Orthophosphate is the form
of phosphorus that most test kits measure. It is also present
in natural seawater, although other forms do exist there as
well. Its concentration in seawater varies greatly from place
to place, and also with depth and with the time of day. Surface
waters are greatly depleted in phosphate relative to deeper
waters, due to biological activities in the surface waters
that sequester phosphate in organisms. Typical ocean surface
phosphate concentrations are very low by reefkeeping standards,
sometimes as low as 0.005 ppm.
Absent of specific efforts to minimize
the phosphate level, it will typically accumulate and rise
in reef aquaria. It is introduced mostly with foods, but can
also enter with top-off water and in some methods of calcium
and alkalinity supplementation.
If allowed to rise above natural levels,
phosphate can cause two undesirable results. One is inhibition
of calcification. That is, it can reduce the rate at which
corals and coralline algae can build calcium carbonate skeletons,
potentially stunting their growth.
Phosphate can also be a limiting nutrient
for algae growth. If phosphate is allowed to accumulate, algae
growth may become problematic. At concentrations below about
0.03 ppm, the growth rate of many species of phytoplankton
depends on the phosphate concentration (assuming that something
else is not limiting growth, such as nitrogen or iron). Above
this level, the growth rate of many of the ocean's organisms
is independent of phosphate concentration (although this relationship
is more complicated in a reef aquarium containing iron and/or
nitrogen sources such as nitrate above natural levels). So
deterring algae growth by controlling phosphate requires keeping
phosphate levels quite low.
For these reasons, phosphate should
be kept below 0.03 ppm. Whether keeping it below 0.01
ppm will yield substantial additional benefits remains to
be established, but that is a goal that some aquarists are
pursuing with various ways of exporting phosphate. The best
ways to maintain low levels of phosphate in normal aquaria
are to incorporate some combination of phosphate export mechanisms,
such as growing and harvesting macroalgae or other rapidly
growing organisms, using foods without excessive phosphate,
skimming, using limewater, and using phosphate binding media,
especially those that are iron-based (which are always brown
or black). Some aquarists have also tried to reduce phosphate
by inducing blooms of microorganisms such as bacteria. This
last method should, in my opinion, be left to experienced
Ammonia (NH3) is
excreted by all animals and some other aquarium inhabitants.
Unfortunately, it is very toxic to all animals, although it
is not toxic to certain other organisms, such as some species
of macroalgae that readily consume it. Fish are not, however,
the only animals that ammonia harms, and even some algae,
such as the phytoplankton Nephroselmis pyriformis,
are harmed by less than 0.1 ppm ammonia.15
In an established reef aquarium, the ammonia
produced is usually taken up rapidly. Macroalgae use it to
make proteins, DNA, and other biochemicals that contain nitrogen.
Bacteria also take it up and convert it to nitrite, nitrate,
and nitrogen gas (the famous "nitrogen cycle").
All of these compounds are much less toxic than ammonia (at
least to fish), so the ammonia waste is rapidly "detoxified"
under normal conditions.
Under some conditions, however, ammonia
may be a concern. During the initial setup of a reef aquarium,
or when new live rock or sand is added, an abundance of ammonia
may be produced that the available mechanisms cannot detoxify
quickly enough. In these circumstances, fish are at great
risk. Ammonia levels as low as 0.2 ppm can be dangerous
to fish.16 In such instances,
the fish and invertebrates should be removed to cleaner water,
or the aquarium treated with an ammonia-binding product such
Many aquarists are confused by the difference
between ammonia and a form of it that is believed to be less
toxic: ammonium. These two forms interconvert very rapidly
(many times per second), so for many purposes they are not
distinct chemicals. They are related by the acid base reaction
NH3 + H+
Ammonia + hydrogen ion (acid) ßà
The only reason that ammonium is thought
to be less toxic than ammonia is that, being a charged molecule,
it crosses the fishes' gills and enters their bloodstream
with more difficulty than does ammonia, which readily passes
across the gill membranes and rapidly enters the blood.
In aquaria with higher pH levels, which
contain less H+, more of
the total ammonia will be in the NH3
form. Consequently, the toxicity of a solution with a fixed
total ammonia concentration rises as pH rises. This is important
in such areas as fish transport, where ammonia can build to
I will discuss issues concerning ammonia
in greater detail in a future column.
Silica raises two issues. If diatoms are
a problem in an established reef aquarium, they may indicate
a substantial source of soluble silica, especially tap water.
In that case, purifying the tap water will likely solve the
problem. In such a situation, testing may not reveal elevated
silica levels because the diatoms may use it as quickly as
it enters the aquarium.
If diatoms are not a problem, then I suggest
that many aquarists should consider dosing soluble silica.
Why would I recommend dosing silica? Largely because creatures
in our aquaria use it, the concentrations in many aquaria
are below natural levels, and consequently the sponges, mollusks,
and diatoms living in these aquaria may not be getting enough
silica to thrive.
I suggest dosing sodium silicate solution,
as it is a readily soluble form of silica. I dose a bulk grade
of sodium silicate solution (water glass), which is very inexpensive.
You may find "water glass' in stores because consumers
use it for such activities as preserving eggs. Finding chemicals
to buy can be difficult for many people, however, and this
chemistry store sells to individuals. Ten dollars plus
shipping buys enough to last for 150 years of dosing a 100-gallon
aquarium, so cost is not an issue.
Based on my dosing experience, aquarists
are probably safe dosing to 1 ppm SiO2
once every 1-2 weeks. This is based on the fact that my aquarium
uses that much in less than four days without any sort of
"bad" reaction. Of course, there's nothing wrong
with starting at a tenth of that dosage and gradually ramping
it up. If you do get too many diatoms, just back off on the
dosing. I presume that all of the SiO2
I have added to my aquarium has been used by various organisms
(sponges, diatoms, etc), but perhaps I have more sponges than
other aquarists. Consequently, diatoms may be more of a concern
in some aquaria than in mine.
I would also advise occasionally measuring
the soluble silica concentration in the water, in case the
demand in your aquarium is substantially less than mine. If
the concentration started to rise above 3 ppm SiO2,
even in the absence of diatoms, I would probably reduce the
dosing rate because that is close to the maximum concentration
that surface seawater ever contains. Additional details on
dosing amounts and methods are described in this previous
I do not presently dose iodine to my aquarium,
and do not recommend that others necessarily do so either.
Iodine dosing is much more complicated than dosing other ions
due to its substantial number of different naturally existing
forms, the number of different forms that aquarists actually
dose, the fact that all of these forms can interconvert in
reef aquaria, and the fact that the available test kits detect
only a subset of the total forms present. This complexity,
coupled with the fact that no commonly kept reef aquarium
species are known to require significant iodine, suggests
that dosing is unnecessary and problematic.
For these reasons, I advise aquarists
to NOT try to maintain a specific iodine concentration using
supplementation and test kits.
Iodine in the ocean exists in a wide
variety of forms, both organic and inorganic, and the
iodine cycles between these various compounds are very complex
and are still an area of active research. The nature of inorganic
iodine in the oceans has been generally known for decades.
The two predominate forms are iodate (IO3-)
and iodide (I-). Together
these two iodine species usually add up to about 0.06 ppm
total iodine, but the reported values vary by a factor of
about two. In surface seawater, iodate usually dominates,
with typical values in the range of 0.04 to 0.06 ppm iodine.
Likewise, iodide is usually present at lower concentrations,
typically 0.01 to 0.02 ppm iodine.
forms of iodine are any in which the iodine atom is covalently
attached to a carbon atom, such as methyl iodide, CH3I.
The concentrations of these organic forms (of which there
are many different molecules) are only now becoming recognized
by oceanographers. In some coastal areas, organic forms can
comprise up to 40% of the total iodine, so many previous reports
of negligible levels of organoiodine compounds may be incorrect.
The primary organisms in reef aquaria that
"use" iodine, at least as far as are known in the
scientific literature, are algae (both micro and macro). My
experiments with Caulerpa racemosa and Chaetomorpha
sp. suggest that iodide additions do not increase the growth
rate of these macroalgae, which are commonly used in refugia.
Finally, for those interested in dosing
iodine, I suggest that iodide
is the most appropriate form for dosing. Iodide is more readily
used by some organisms than is iodate, and it is detected
by both currently available iodine test kits (Seachem and
Nitrate is an ion that has long dogged
aquarists. The nitrogen that forms it comes in with foods,
and can, in many aquaria, raise nitrate enough to make it
difficult to maintain natural levels. A decade or two ago,
many aquarists performed water changes with nitrate reduction
as one of their primary goals. Fortunately, we now have a
large array of ways to keep nitrate in check, and modern aquaria
suffer far less from elevated nitrate than did those in the
Nitrate is often associated with algae,
and indeed the growth
of algae is often spurred by excess nutrients, including
nitrate. Other potential aquarium pests, such as dinoflagellates,
are also spurred by excess nitrate and other nutrients. Nitrate
itself is not particularly toxic at the levels usually found
in aquaria, at least as is so far known in the scientific
literature. Nevertheless, elevated nitrate levels can excessively
spur the growth
of zooxanthellae, which in turn can actually decrease
the growth rate of their host coral.
For these reasons, most reef aquarists
strive to keep nitrate levels down. A good target is less
than 0.2 ppm nitrate. Reef aquaria can function acceptably
at much higher nitrate levels (say, 20 ppm), but run greater
risks of the problems described above.
There are many ways to reduce nitrate,
including reducing the aquarium's nitrogen inputs, increasing
nitrogen export by skimming, increasing nitrogen export by
growing and harvesting macroalgae or turf algae (or any other
organism of your choice), using a deep
sand bed, removing existing filters designed to facilitate
the nitrogen cycle, using a carbon denitrator,
using a sulfur
denitrator, using AZ-NO3, using
nitrate absorbing solids, and using polymers and carbon that
bind organics. All of these methods are described in more
detail in a previous
Aquarists' concerns about nitrite are usually
imported from the freshwater hobby. Nitrite is far less toxic
in seawater than in freshwater. Fish are typically able to
survive in seawater with more than 100 ppm nitrite!17
Until future experiments show substantial nitrite toxicity
to reef aquarium inhabitants, nitrite is not an important
parameter for reef aquarists to monitor. Tracking nitrite
in a new reef aquarium can nevertheless be instructive by
showing the biochemical processes that are taking place. In
most cases, I do not recommend that aquarists bother to
measure nitrite in established aquaria.
My recommendation is to maintain strontium
levels in reef aquaria in the range of 5-15 ppm. That level
roughly spans the level in natural seawater of 8 ppm. I
do not recommend that aquarists supplement strontium unless
they have measured strontium and found it to be depleted to
below 5 ppm. Measuring and supplementing strontium is not
a critical activity for most aquarists, and is not a trivial
exercise since the available test kits can be difficult to
use (see below).
recent tests, I found that in my reef aquarium, without
any recent strontium additions, strontium was already elevated
above natural levels (to 15 ppm due to elevated strontium
in the Instant Ocean salt mix that I was using). I would not
like to see it get any higher. Consequently, adding a supplement
without knowing the aquarium's current strontium level is
not advisable. Scientific evidence indicates that some
organisms need strontium, albeit not the organisms that
most reef keepers maintain. Certain gastropods, cephalopods,
and radiolaria, for example, require
strontium.18-34 It is,
however, clearly toxic at elevated concentrations. In one
reported case, 38 ppm was enough strontium to kill a particular
species of crab (Carcinus
maenas).34 No evidence
indicates that 5-15 ppm strontium is harmful to any marine
organism, although it is not known what strontium levels are
optimal. Finally, anecdotal evidence from a number of advanced
aquarists suggests that strontium that is substantially below
natural levels is detrimental to the growth of corals that
many aquarists maintain, but this effect has not been proven.
How can we maintain natural strontium
levels? Doing so, of course, necessitates a suitable strontium
test. Some test kits are perhaps suitable for this purpose.
If not, sending a sample out to a lab might be a reasonable
alternative for some aquarists. If the result comes back in
the 5-15 ppm range, no action likely need be taken. If the
level is higher than 15 ppm, the best reduction method may
simply be water changes with a suitable salt mix, without
abnormally high levels of strontium. If strontium levels are
below 5 ppm, adding a strontium supplement may be in order.
Overall, water changes with a salt mix
containing a suitable level of strontium may be the best way
to keep strontium at appropriate levels.
Photo courtesy of Mitchell Brown.
I do not recommend that aquarists try to
The oxidation reduction potential (ORP)
of a marine aquarium is a measure of its water's relative
oxidizing power. ORP has often been recommended to aquarists
as an important water parameter, and some companies sell products
(equipment and chemicals) designed to control ORP. Many who
recommended ORP control have convinced aquarists that it is
a measure of the aquarium water's relative "purity,"
despite this never having been clearly demonstrated.
ORP, at its heart, is very, very complicated.
It is perhaps the single most complicated chemical feature
of marine aquaria that aquarists will typically encounter.
ORP involves many chemical details that are simply unknown,
either for seawater or for aquaria. It involves processes
that are not at equilibrium, and so are difficult to understand
and predict. Even more daunting is the fact that the chemicals
that control ORP in one aquarium might not even be the same
chemicals that control ORP in another aquarium, or in natural
ORP is an interesting, if complicated,
measure of the properties of water in a marine aquarium. It
has uses for monitoring certain events in aquaria that impact
ORP but may be otherwise hard to detect. These events could
include immediate deaths of organisms, as well as long term
increases in the levels of organic materials. Aquarists who
monitor ORP, and who do other things that seem appropriate
for maintaining an aquarium (such as increasing aeration,
skimming, use of carbon, etc.) may find monitoring ORP to
be a useful way to see progress.
ORP measurements are very susceptible to
errors. Aquarists are strongly cautioned to not overemphasize
absolute ORP readings, especially if they have not recently
calibrated their ORP probe. Rather, ORP measurements are most
useful when looking at changes in measured ORP over time.
Some aquarists use oxidizers to raise
ORP. These additions may benefit some aquaria, and maybe in
ways that aren't demonstrated by changes in ORP alone. I've
never added such materials to my aquarium. In the absence
of convincing data otherwise, such additions seem to me to
be potentially riskier than is justified by their demonstrated
and hypothesized benefits.
Photo courtesy of Zak Klein.
Boron's importance in marine aquaria is
a subject not often discussed by hobbyists, despite the fact
that many people dose it daily with their alkalinity supplements.
Most commentary on boron, in fact, derives from manufacturers
who sell it in one fashion or another as a "buffering"
agent. These discussions, unfortunately, nearly always lack
any quantitative discussion of boron or its effects, both
positive and negative. In general, boron is not an important
element to control in aquaria.
Boron actually contributes only a minor
fraction of normal seawater's pH buffering capacity. It appears
to be a necessary or desirable nutrient
for certain organisms,35-37
but is also toxic
to others at levels not far above natural levels,38-40
and below amounts present in at least one artificial salt
For these reasons, my recommendation
is to maintain approximately natural levels of boron, about
4.4 ppm. Any value below 10 ppm is likely acceptable for most
aquaria. Values above 10 ppm should be avoided. The Salifert
boron kit is suitable for determining ballpark boron levels
in marine aquaria, while other
kits may not be.
Iron is limiting
to growth of phytoplankton in parts of the ocean, and
may be limiting to macroalgae growth in many reef aquaria.
Because of its short supply and critical importance, it is
also subject to aggressive sequestration
by bacteria and other marine organisms. Consequently,
aquarists might consider dosing iron if they grow macroalgae.
Iron is not easy to measure at levels normally
encountered in marine aquaria. It is also not easy to determine
which of its many forms are bioavailable in seawater, and
which are not. Consequently, aquarists should not target a
specific concentration, but rather should decide if they want
to dose any at all, and then use an appropriate dosage going
forward. The reason to dose iron is that macroalgae may benefit
from it. If you are not growing macroalgae, then you may not
need to monitor or dose iron at all.
Deciding how much iron to add is fairly
easy because, in my experience, it doesn't seem to matter
too much. Presumably, once you add enough to eliminate it
as a limiting nutrient, extra iron does not cause apparent
harm (at least that I've detected in my aquarium or have heard
of from others). I dose about 0.1 to 0.3 mL of a solution
containing 5 g of iron (as 25 g of ferrous sulfate heptahydrate)
in 250 mL of water containing 50.7 g of sodium citrate dihydrate.
I presently dose once per week to my system with a total water
volume of about 200 gallons. This iron(II) citrate turns brown
and cloudy over time, but I still use it.
I've noticed no negative effects from dosing
this iron, or of Kent's iron and manganese supplement that
I have also used, that were attributable to the iron, nor
have I heard of any negative effects from others doing similar
dosing. Still, I don't keep all organisms available to the
hobby, and if a negative reaction does appear, I advise backing
off the dose or stopping completely.
Since many hobbyists do not have access
to the chemicals required to make iron(II) citrate, I advise
most aquarists to obtain a commercial iron supplement. A number
of appropriate and inexpensive supplements are available.
Some commercial supplements, such as Kent's product, combine
manganese with iron, presumably because the scientific literature
has demonstrated that phytoplankton also scavenge manganese
from the water column. I've not experimented with manganese,
but it is likely acceptable to use if a pure iron supplement
cannot be found.
I'd also advise using only iron supplements
that contain iron chelated to an organic molecule. The iron
sold for freshwater applications is sometimes not chelated
because free iron is more soluble in the lower pH of freshwater
aquaria. I'd avoid those products for marine applications.
It will likely still work, as many of the studies in the scientific
literature use free iron in seawater, but probably not as
well because it may precipitate before it has fully fortified
the system with iron.
In many cases of iron products intended
for the marine hobby, the product may not state what the iron
is chelated with, in order to protect proprietary formulations.
I don't actually know if it matters much. Very strong chelation
by certain molecules will actually inhibit bioavailability
by prohibiting release of the iron unless the chelating molecule
is completely taken apart, but I expect that manufacturers
have avoided those molecules. EDTA, citrate, and some others
actually degrade photochemically, continually releasing small
amounts of free iron. It is believed to be the free iron that
many of the organisms actually take up. "Captive Seawater
Fishes" by Stephen Spotte includes a more detailed discussion
of this degradation and uptake.16
It should be noted that iron may be a limiting
factor for many organisms other than macroalgae. These might
include microalgae, bacteria (even pathogenic bacteria), and
diatoms. These possibilities were discussed in a previous
article. If such problems should arise, backing off or stopping
the iron additions may be warranted.
Chemical issues in reef aquaria are often
daunting to aquarists. There are many chemical parameters
that aquarists monitor, some of which are critical for success,
and some of which are much less important. Of those critical
for success, only calcium and alkalinity require regular supplementation
in all reef aquaria, although the others in Table 1 may require
monitoring. Successfully keeping the parameters in Table 1
at appropriate levels should go a long way toward allowing
aquarists to more fully enjoy their aquaria while at the same
time ensuring that the inhabitants are well cared for.