The pH of a reef
aquarium significantly impacts the health and welfare of the
organisms that call it home. Unfortunately, many factors tend
to pull the pH out of many commonly kept organisms' optimal
range. Low pH is the most common problem, and its causes and
necessary corrective actions have been discussed in a previous
article. Excessively high pH, however, can also be a significant
problem in some aquaria. In addition to potentially impacting
the aquarium inhabitants' health, high pH can lead to other
problems, including the precipitation of calcium carbonate
on objects such as heaters and pump impellers. Such precipitation
can also artificially cap the attainable levels of calcium
and alkalinity. For these reasons, pH is a parameter that
aquarists should monitor.
This article details the steps necessary to understand why
an aquarium may have excessively high pH, and how best to
correct that situation.
What Is pH?
In a previous article I discussed
in detail what pH means in the context of a reef aquarium.
In short, all that most aquarists need to know is that pH
is a measure of the concentration of hydrogen ions (H+)
in solution, and that the scale is logarithmic. That is, at
pH 7 there is 10 times as much H+
as at pH 8, and that at pH 7 there is 100 times as much H+
as at pH 9. Consequently, a small change in pH can mean a
big change in the concentration of H+
in the water.
Another interesting and important fact is that the pH resulting
from two solutions being mixed together is not just an average
of the two solutions' pH values, but is also determined by
the buffering power of the solutions, and to a lesser extent
by more esoteric factors. Sometimes the pH that results when
two solutions are combined is not even in between the two
starting values. For example, combining a baking soda solution
at pH 8.3 with artificial seawater at pH 8.2 can result in
a pH that is actually below pH 8.2 (in this case, the pH drops
because the bicarbonate in baking soda is a stronger acid
in seawater than it is in freshwater). Consequently, interpreting
pH problems and solutions requires knowledge of more than
just the pH of the solutions involved. This fact is important
for reef aquarists when considering, for example, whether
the pH of pure water impacts the pH of artificial seawater.
In this case, the effect of the pure water is almost negligible
regardless of its measured pH value.
Figure 1. A scene from one of my reef aquaria. In this
system, the pH
typically varies between pH 8.3 and 8.5.
Why Monitor pH?
There are several
reasons that aquarists would want to monitor pH in marine
aquaria. One is that aquatic organisms thrive only within
a particular pH range. This range certainly varies from organism
to organism, and it is not easy to justify a claim that any
particular range is "optimal" for an aquarium containing
many species. Even natural seawater (pH = 8.0 to 8.3) isn't
likely to be optimal for every creature living in it, but
it was recognized more than eighty years ago that moving away
from the pH of natural seawater (down to pH 7.3, for example)
is stressful to fish.1 We
now have additional information about many organisms' optimal
pH ranges, but the data are inadequate to allow aquarists
to optimize the pH for most organisms in which they are interested.2-6
Additionally, the effect of pH on organisms can be direct
or indirect. For example, the toxicity to some organisms present
in our aquaria (such as mysids and amphipods)7
of metals such as copper and nickel is known to depend on
pH. Consequently, the pH ranges that are acceptable in one
aquarium may be different from those ranges in other aquaria,
even for the same organisms.
Nevertheless, some fundamental processes taking place in
many marine organisms are substantially impacted by pH changes.
One of these is calcification, and it is known that calcification
in corals depends on pH, and that calcification falls as pH
falls.8-9 Using these types
of facts, along with the integrated experience of many hobbyists,
we can develop some general guidelines about what is an acceptable
pH range for reef aquaria, and make some determination as
to what values are pushing the limits of acceptability.
What is the Acceptable pH Range
for Reef Aquaria?
The acceptable pH range for reef aquaria
is an opinion rather than a clearly defined fact, and will
certainly vary based on who is providing the opinion. This
range may also be quite different from the "optimal"
range. Justifying what is optimal, however, is much more problematic
than justifying that which is simply acceptable. As a goal,
I'd suggest that the pH of natural seawater, about 8.2, is
appropriate, but reef aquaria can clearly operate in a wider
range of pH values with varying degrees of success. 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. This statement
is based partly on the fact that many reef aquaria operate
acceptably in the pH 7.8 to 8.0 range, but that most of
the best examples of these types of aquaria incorporate
calcium carbonate/carbon dioxide reactors that, while
tending to lower the pH, also tend to keep the carbonate
alkalinity fairly high (at or above 3 meq/L.). In this
case, any problems associated with calcification at these
lower pH values may be offset by the higher alkalinity.
pH stresses calcifying organisms primarily by making
it harder for them to obtain sufficient carbonate to deposit
skeletons. Raising the alkalinity mitigates this difficulty
by supplying extra bicarbonate.
That the calcium level is at least 400 ppm. Calcification
becomes more difficult as the pH falls, and it also becomes
more difficult as the calcium level falls. It would not
be desirable to push all of the extremes of pH, alkalinity,
and calcium at the same time. So, if the pH is on the
low side and cannot be easily changed (such as in an aquarium
with a CaCO3/CO2
reactor), at least make sure that the calcium level is
acceptable (~400-450 ppm). Likewise, one of the problems
at higher pH (above say, 8.2, but getting progressively
more problematic with each incremental rise) is the abiotic
precipitation of calcium carbonate, resulting in a drop
in calcium and alkalinity, and the resultant clogging
of heaters and pump impellers. If the aquarium's pH is
8.4 or higher (as often happens in an aquarium using limewater),
then it is especially important 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
Carbon Dioxide and pH
The pH of marine aquarium water is
intimately tied to the amount of carbon dioxide dissolved
in the water and to its alkalinity. In fact, if water is fully
aerated (that is, it is in full equilibrium with normal air),
then the pH is exactly determined by the carbonate alkalinity.
The higher the alkalinity, the higher the pH. There is, in
fact, a simple
mathematical relationship between alkalinity, pH, and
carbon dioxide that I have discussed previously. Figure 2
shows this relationship graphically for seawater equilibrated
with normal air (350 ppm carbon dioxide), and equilibrated
with air having extra carbon dioxide as might be present in
certain homes (1000 ppm). Figure 2 also shows the pH/alkalinity
relationship in water that is deficient in carbon dioxide.
Nearly all high pH situations encountered in reef aquaria
are caused by a carbon dioxide deficiency.
Only rarely would excessively high pH be caused by high alkalinity
alone, because in order for the pH to rise above pH 8.5 with
a "normal" amount of carbon dioxide present, the
alkalinity would have to be above 5 meq/L (Figure 2). At these
high levels of both pH and alkalinity, calcium carbonate would
very likely begin to precipitate abiotically, and such precipitation
itself reduces pH and alkalinity. So if such a situation arose,
it would not typically last long on its own in a reef aquarium.
Figure 2. The relationship between alkalinity and pH
for seawater with normal carbon dioxide
levels (black), excess carbon dioxide (purple) or deficient
carbon dioxide (blue). The green
area represents normal seawater.
Detailed Chemistry of CO2
A simple way to think of the relationship
between carbon dioxide and pH is as follows. Carbon dioxide
in the air is present as CO2. When
it dissolves into water, it becomes carbonic acid, H2CO3:
CO2 + H2O à
The amount of H2CO3
in the water (when fully aerated) is dependent not on pH,
but only on the amount of carbon dioxide in the air (and somewhat
on other factors, such as temperature and salinity). Systems
not at equilibrium with the air around them, which includes
many reef aquaria, may have too much or too little CO2
in them, which is effectively defined by the amount of H2CO3
in the water. Consequently, if an aquarium is "deficient
in CO2," that means that it has
a deficiency of H2CO3.
deficiency, in turn, means that pH will tend to be on the
high side, and the more H2CO3
deficient it is, the higher the pH will be.
Seawater contains a mixture of carbonic acid, bicarbonate,
and carbonate that are always in equilibrium with each other:
H+ + HCO3- ßà
2H+ + CO3--
Equation 2 demonstrates that when an aquarium has a deficiency
some of the HCO3-
can combine with H+, to
form more H2CO3
(moving to the left in equation 2). Since H+
is used up, the pH (which is simply a measure of H+)
rises. If seawater has a big enough deficiency of CO2,
the pH can be as high as pH 9 or more.
Why Does pH Become Elevated?
As discussed above, a reef aquarium's
pH rises when its water becomes deficient in carbon dioxide.
In practice, this deficiency can be caused in several ways.
The diurnal (daily) change in pH in reef aquaria occurs because
of the biological processes of photosynthesis and respiration.
Photosynthesis is the process whereby organisms convert carbon
dioxide and water into carbohydrate and oxygen. The net reaction
6CO2 + 6H2O +
So there is net consumption of carbon dioxide during the
day. This leads to many aquaria becoming deficient in CO2
during the day, raising their pH.
Likewise, all organisms also carry out the process of respiration,
which converts carbohydrate back into energy for other processes.
In the net sense, it is the opposite of photosynthesis:
+ 6O2 à
6CO2 + 6H2O +
This process is happening continuously in reef aquaria, and
it tends to reduce the pH due to the carbon dioxide produced.
The net effect of these processes is that pH rises during
the day and drops at night in most reef aquaria. This change
varies from less than a tenth of a pH unit, to more than 0.5
pH units in typical aquaria. Complete aeration of the aquarium
water will prevent the diurnal pH swing entirely, by driving
out any excess carbon dioxide or absorbing excess carbon dioxide
when deficient. In practice this goal is not often attained,
and the pH does change between day and night.
Consequently, the pH will nearly always be highest at the
end of the light cycle. The only time that this is not the
case is when there are timed additions of other things that
impact pH (e.g., limewater, other alkalinity additions, and
even the entry of carbon dioxide from the room air, in which
the level of carbon dioxide may vary as human activities around
the aquarium change throughout the day). The diurnal pH swing
alone is not typically strong enough to drive the pH of reef
aquaria to excessive levels (i.e., pH > 8.5). If it does,
the aeration is clearly inadequate, and more aeration will
likely solve the problem.
The more common way for reef aquaria to attain excessive
pH is through high pH additives, most notably the use
of alkalinity additives that contain hydroxide (limewater)
or carbonate (some two part additives, for example). Figure
3 shows how the pH and alkalinity change as limewater is added
to a reef aquarium. The limewater
converts some of the carbonic acid into bicarbonate, effectively
making the water deficient in carbon dioxide (H2CO3)
until the aquarium can absorb more carbon dioxide from the
air to replace the lost carbonic acid:
à Ca++ + 2OH-
6. OH- + H2CO3
Figure 3. The effect of limewater addition on alkalinity
In a previous
article, I showed that adding sufficient hydroxide to
increase the alkalinity by 0.5 meq/L (a 10 ppm calcium rise,
if using limewater) immediately boosted pH from pH 8.10 to
8.76. After the system had a chance to recover by pulling
in more carbon dioxide from the air, the pH subsided to 8.33.
Additives containing carbonate (such as many two
part calcium and alkalinity additive systems) also deplete
carbon dioxide by a similar process:
The effect of added carbonate on alkalinity and pH is shown
in Figure 4. The effect of this on pH is smaller than the
pH change caused by limewater, but these additive systems
can still drive the pH excessively high if sufficient quantities
are added to a marine aquarium.
Figure 4. The effect of carbonate addition on alkalinity
In a previous
article, I showed that adding sufficient carbonate to
increase alkalinity by 0.5 meq/L resulted in an immediate
pH rise from pH 8.10 to 8.44. After the system had a chance
to recover by pulling in more carbon dioxide from the air,
the pH subsided to 8.34, matching that produced by limewater
and bicarbonate (after equivalent alkalinity additions followed
by complete aeration).
Methods of Lowering pH: Why They
Figures 5-12 show graphically some
methods of lowering pH in marine aquaria.
Aerating the water, driving in carbon dioxide, is
shown graphically in Figure 5. As carbon dioxide is added,
the data point representing the aquarium's pH and alkalinity
begins to shift horizontally from the "CO2
Deficient" curve to the normal CO2
curve (green line in Figure 5). Aerating with normal air cannot
overshoot, and perfect aeration will land the aquarium on
the normal CO2 line. Aeration with
interior air that may contain excessive carbon dioxide can
overshoot the pH target, and drive the aquarium's pH even
lower (Figure 6).
Figure 5. The effect of aeration on alkalinity and
Figure 6. The effect of aeration with air containing
excessive carbon dioxide
(or otherwise adding excessive carbon dioxide) on alkalinity
Adding soda water (seltzer = carbon dioxide dissolved
in fresh water) or otherwise directly adding carbon dioxide
(from a cylinder, by breathing into a skimmer inlet, etc.),
will reduce the pH as shown in Figures 5 and 6. In these cases,
overshooting is a possibility. In a later section of this
article I recommend how much soda water to add. All other
methods should be done only with real time pH monitoring to
prevent overshooting the target pH.
In a recent test, I bought a commercial bottle of soda water
(Adirondack Seltzer; Figure 7), and added it to my sump. The
sump was stirring well with a large skimmer, but was not circulating
through the main display tank during this test. The ~38 gallons
of sump water's pH was initially measured at 8.48. After 255
mL of soda water were mixed in, the pH dropped to 8.15. After
adding an additional 65 mL, the pH dropped to 8.04. These
data serve as the basis for the recommendation that I make
later in this article of using 6 mL of soda water per gallon
of aquarium water to achieve a pH drop of about 0.3 pH units.
Figure 7. Adirondack Seltzer, used to add carbon dioxide
and lower pH.
Adding vinegar is another option for reducing pH.
It has two actions that lower the pH. The first happens instantly,
as the acetic acid releases H+
to the aquarium water (a process called ionization):
This effect is shown graphically in Figure 8 (step 1). Then,
over a period of time (perhaps hours), the acetate is metabolized
by bacteria and other organisms, using up the available oxygen
and producing carbon dioxide:
+ O2 à
2CO2 + H2O +
CO2 + H2O +
This effect is shown in step 2 of Figure 8. The net result
of both reactions is that the acetic acid is converted into
carbon dioxide, lowering pH (Figure 8). The real and measured
is reduced a bit by the initial vinegar addition (equation
8), but that loss is exactly replaced when the acetate is
metabolized (equation 9). The only concerns with using vinegar
are overshooting the pH target by adding too much, and the
consumption of oxygen by bacteria metabolizing the acetate.
With sufficient aeration or photosynthesis, that O2
loss is not necessarily a problem, but in some aquaria, adding
too much vinegar might cause a significant drop in O2.
Figure 8. The two-step effect of vinegar on alkalinity
and pH. The first step (simple ionization)
reduces carbonate alkalinity and pH, while the second step
(bacterial metabolism) raises
carbonate alkalinity and reduces pH.
In another recent test, I bought a commercial bottle of
distilled white vinegar (Heinz; Figure 9), and added it to
my sump. The sump was stirring well with a large skimmer,
but was not circulating through the main display tank during
this test. The ~38 gallons of sump water's pH was initially
measured at 8.53. After 25 mL of vinegar were added and allowed
to mix in for a few minutes, the pH dropped to 8.41. Another
25 mL of vinegar dropped the pH to 8.15. A third 25 mL dose
dropped the pH to 7.88. These data serve as the basis for
the recommendation that I make later
in this article of using 1 mL of distilled white vinegar per
gallon of aquarium water to achieve an initial pH drop of
about 0.3 pH units.
Figure 9. Heinz Distilled White Vinegar,
used to lower pH.
Adding a mineral acid, such as hydrochloric acid (muriatic
acid) or sulfuric acid, will reduce pH, but will also
reduce the alkalinity (Figure 10). This alkalinity loss is
not returned as in the case of vinegar, when the vinegar is
metabolized. These acids also are usually very concentrated,
so it is very easy to grossly overshoot the pH target (Figure
11). For these two reasons, I seldom recommend using such
acids to reduce pH (although they can be used effectively
under certain circumstances when the aquarist is acutely aware
of their drawbacks). I showed experimentally in a previous
article that adding enough hydrochloric acid to reduce
alkalinity by 0.5 meq/L (1.4 dKH) instantly dropped the pH
from 8.10 to 6.91. Then, as the water released excess carbon
dioxide into the air, the pH rose back up to 7.91 after 24
hours, and finally to 8.15 after 48 hours (the same water
without acid treatment rose from pH 8.10 to 8.11 to 8.21 over
the same time frame).
Figure 10. The effect of hydrochloric acid on alkalinity
Figure 11. The effect of excessive hydrochloric acid
on alkalinity and pH.
Adding a buffer is a very poor way to control high pH.
The best option in this regard is to add straight baking soda,
which only slightly lowers pH and provides a large boost to
alkalinity (Figure 12). I showed experimentally in a previous
article that adding enough baking soda to lower pH in
artificial seawater by 0.04 pH units raised alkalinity by
0.5 meq/L (1.4 dKH).
Figure 12. The effect of baking soda on alkalinity
Considerations Prior to Solving
The following sections provide specific
advice on how to go about solving a high pH problem. The advice
can also be used to adjust the pH levels closer to natural
values even if they are already within the "acceptable"
range described above, but are still not as low as desired.
Before embarking on a pH altering strategy, however, here
are some general concerns.
Make sure that there really is a pH problem. Many apparent
pH problems are really measurement problems rather than
real aquarium problems. This issue seems to be especially
common when the aquarist is using pH test kits, rather than
electronic measurement with a pH meter, but all methods can
and do go wrong. Avoid turning a good situation into a bad
one simply because a pH meter was not properly calibrated.
Also, when not adding limewater
or other high-pH additives, a pH reading above pH 8.5 is most
likely an error.
Consequently, be sure to verify the pH reading before taking
any but the most benign measures. Here are several articles
that are worth reading on pH measurement to help ensure that
the readings you are seeing are accurate:
pH with a Meter
Comparison of pH Calibration Buffers
pH calibration or verification fluid using grocery store
Also, try to determine why there is a pH problem before
enacting a band-aid solution. For example, if the problem
of high pH is due to excessive use of limewater, then perhaps
using less limewater is the simplest solution.
Solutions to pH Problems
Some solutions to pH problems are
peculiar to a specific cause, such as adding vinegar
to limewater, or using less of it. Some general solutions,
however, are frequently effective. My recommendations on how
to deal with high pH problems are detailed below.
The most benign way to reduce high pH is to aerate the
water more. Whether the aquarium looks well-aerated or
not, if the pH is above 8.5 and the alkalinity is not above
4 meq/L, then the aquarium is not fully equilibrated with
carbon dioxide in the air. Equilibrating carbon dioxide can
be much more difficult than equilibrating oxygen. Air contains
very little carbon dioxide (about 350 ppm) relative to oxygen
(210,000 ppm). Consequently, a lot more air needs to be driven
through the water to introduce the same amount of carbon dioxide
as oxygen. Perfect aeration will solve nearly any high pH
problem, and will rarely cause any problem of its own.
That said, sufficient aeration is not always easily attained,
and other methods can be useful. These other methods are:
A. Direct addition of carbon
dioxide. Bottled soda water (seltzer) can be used to
instantly reduce aquarium pH. Be sure to select unflavored
soda water, and check the ingredients to be sure it doesn't
contain anything that should be avoided (phosphate, etc).
Many manufacturers list water and carbon dioxide as the
I recommend adding 6 mL of soda water per gallon of tank
water to reduce pH by about 0.3 units. Add it to a high
flow area away from organisms (such as in a sump). The local
pH where it first is added will be very low. Going about
this procedure slowly is better than proceeding too fast.
If you do not have a sump, add it especially slowly.
Some soda water may have more, or less, carbon dioxide in
it, and the lower the aquarium's alkalinity, the larger
will be the pH drop. Also, the higher the pH, the smaller
will be the pH drop, because the buffering of seawater declines
steadily as the pH drops from about 9 to 7.5.
B. Direct addition of vinegar. Commercial distilled
white vinegar (typically 5% acetic acid or "5% acidity")
can be used to instantly reduce aquarium pH. Do not use
wine vinegars as they may contain undesirable organics in
addition to the acetic acid.
I recommend adding 1 mL of distilled white vinegar per
gallon of tank water to initially reduce pH by about 0.3
units. Once again, add it to a high flow area away from
organisms (such as in a sump). The local pH where it first
is added will be very low. Going about this procedure slowly
is better than proceeding too fast. If you do not have a
sump, add it especially slowly. The lower the aquarium's
alkalinity, the larger will be the pH drop. Also, the higher
the pH, the smaller will be the pH drop, because the buffering
of seawater declines steadily as the pH drops from about
9 to 7.5. Remember, there may be an additional, later drop
in pH as the vinegar is metabolized to carbon dioxide.
C. Addition of vinegar via limewater. Commercial
distilled white vinegar can be used to reduce tank pH by
adding it to limewater
that is subsequently added to the aquarium. Do not use wine
vinegars as they may contain undesirable organics in addition
to the acetic acid. A reasonable
dose to start with is 45 ml of vinegar per gallon of
The pH of marine aquaria is an important
parameter with which most aquarists are familiar. It has important
effects on the health and well-being of our systems' inhabitants,
and we owe it to them to do the best we can to keep it within
an acceptable range. This article provides a series of solutions
to high pH problems in aquaria, and should enable most aquarists
to diagnose and solve such pH problems that may arise in their