PH And The Reef Aquarium

Posted by on November 1, 2015 - zero

by Randy Holmes-Farley


For many aquarists, pH is not something that they have much experience with aside from their aquarium.  For many, pH is almost a black box measurement: something to be considered, but whose physical meaning makes little sense to them.  This article will describe pH in an intuitive way (as opposed to a more rigorous, mathematical way that I have used in previous articles).  While plenty of chemical mathematics can be used to determine some of the interrelationships between various water parameters (such as atmospheric carbon dioxide, alkalinity, and pH), this article assumes that most aquarists are better off focusing on the answers, rather than how they are found.

Aquarists frequently debate which foods, lights, water flow regimes, temperatures and filtration methods are most appropriate for a coral reef aquarium. The effect of pH on tank organisms can also be debated to some extent, but it is often a more minor contributor to tank health than the bigger issues of alkalinity, calcium, lighting, etc.  Many questions relating to pH leave little room for debate, however.  It is well understood, for example, what effect most chemicals have on seawater’s pH, how carbon dioxide in the air impacts aquarium pH, how aeration impacts the daily pH swing of an aquarium and what buffers do and how.  Consequently, the ways of dealing with various sorts of pH problems are very well understood on a scientific basis. It also turns out that the answers may surprise some aquarists. For example, water changes almost never solve low pH problems, even 100% water changes.

This article focuses on what pH ranges are appropriate in coral reef aquaria, what factors tend to drive pH away from normal, what happens to a coral reef aquarium if its pH deviates from natural levels, and how to best control an aquarium’s pH.


Some 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 probably justified, as true pH problems can lead to poor animal health, or even a tank crash (such as a due to a limewater overdose).  In many cases, however, the only problem is with the pH measurement or its interpretation.  A different set of aquarists think that pH doesn’t matter.  They’ve never measured pH, and have never had a problem, so why worry?  For many of these aquarists, they are right: the pH conditions in their aquarium are obviously conducive to maintaining a healthy aquarium.  However, I do not consider pH measurement a wasted effort, particularly in cases where very high or low pH additives are being used because not everyone’s pH will naturally fall into an acceptable range.

Several factors make monitoring a marine aquarium’s pH potentially useful. 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 (typically 8.0 to 8.3) may be suboptimal for some of its creatures.  It was recognized more than 80 years ago, however, that pH levels different from those of natural seawater can be stressful to fish.  Additional information is now available about optimal pH ranges for many organisms, but the data are inadequate to allow aquarists to optimize pH for most organisms which interest them.

Changes in pH do substantially impact some fundamental processes taking place in marine aquaria. One of the most important of these processes is calcification, both biological (formation of coral skeletons, for example) and abiotic (precipitation on pumps, for example).  Higher pH will accelerate precipitation, with a rise of 0.3 pH units having about the same increase in potential for precipitation as doubling the alkalinity or calcium level.  Consequently, high pH is a big driver of this type of precipitation.  Interestingly, higher temperatures also drive such precipitation, which is why pumps eventually get fouled with calcium carbonate deposits in many reef aquaria, and why dissolving a salt mix at cooler water temperatures can be desirable.

Calcification by many organisms is known to depend on pH, at least in laboratory tests, often dropping as pH falls below normal levels.  If the pH is low enough, coral skeletons actually dissolve.  That dissolution begins somewhere below pH 7.7, with the exact value depending on the alkalinity, calcium, and how long one is interested in waiting for it to happen.  Interestingly, and despite the previous tests showing substantial concern for calcification at low pH, a recent test in the open ocean gave different results.  These scientists locally acidified the water by adding extra CO2 and did not find that the growth of a particular coral (Porites cylindrica) was reduced by the reduction in pH of between 0.05 and 0.25 pH units below the value in the surrounding ocean.  The conclusion is that corals in such an environment can adapt to lower pH values than lab studies might suggest.  Folks who want to read the full original article can find it here.

At present, however, it is not clear whether these open ocean results extend to other species, or whether a coral reef aquarium responds more like this open ocean test, or tests in more controlled environments.   I suggest that the effect of low pH on calcification by corals in reef aquaria is somewhat unclear, but there’s more evidence that it would be a concern in reef aquaria than that it is not, assuming that growth rate is a primary goal.

Using the various types of information described in many studies like those mentioned above, 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. These recommended ranges are detailed in subsequent sections.

What is pH?

In previous articles I have discussed in detail what pH means in the context of a coral reef aquarium. In short, all that many aquarists need to know is that pH is a measure of the hydrogen ion (H+) concentration in solution, and that its scale is logarithmic. Regardless of the pH, some H+ ions are always present, and the lower the pH, the more there are. A difference of 0.3 pH units between any two pH values implies about a twofold difference in the H+ concentration. A difference of one pH unit between any two pH values means a difference of a factor of 10 in the H+ concentration.  For example, at pH 8 there are 10 times fewer H+ ions than at pH 7. At pH 9 there are 100 times fewer H+ ions than at pH 7. Consequently, a small change in pH indicates a big change in the concentration of H+ in the water.  There are very few chemical parameters that vary as much from reef tank to reef tank or in a single reef tank during the course of a day as the hydrogen ion concentration.  Many fine reef aquaria vary by more than a factor of two in H+ concentration during the course of 24 hours, and the range from the reef tank with the lowest to that with the highest hydrogen ion concentration is more than a factor of four.

Figure 1 may help some aquarists understand the relationship between pH and the concentration of H+ more readily than do numerical comparisons. It shows a color representation of the concentration of H+ as a function of pH, with the intensity of the green directly representing the concentration. The darker the green, the more H+ in solution.


Figure 1. The relationship between pH and the concentration of H+ ions. The intensity of the green color is directly related to the H+ concentration.

Another interesting and important fact is that the pH resulting from equal volumes of two solutions being mixed together is not just an average of the two solutions’ pH values (which the measurement scale’s logarithmic nature alone implies), but is also determined by each of the solutions’ buffering power and, to a lesser extent, by more esoteric factors.  Sometimes the pH that results when two solutions are combined is not even between the two starting values.  Consequently, evaluating pH problems and potential solutions requires some 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 freshwater impacts the pH of seawater when added to it. In this case, the pure water’s effect is almost negligible, regardless of its measured pH value.

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 certainly varies based on who is providing the opinion.  This range also may be quite different from the “optimal” range.  Justifying what is optimal, however, is much more problematic than justifying what is simply acceptable.  I suggest that the pH of natural seawater, about 8.1 to 8.2, is an appropriate goal, but reef aquaria can clearly operate in a wide range of pH values with varying degrees of success. The pH of highly successful coral reef aquaria often deviates substantially from pH 8.1-8.2 for at least part of the day.  In my opinion, the pH range from 7.8 to 8.5 is an acceptable one for reef aquaria.  Reefs falling outside of that range may also thrive, but others may not.

How is pH Measured?

After having helped with the pH problems of literally thousands of aquarists, it is apparent to me that pH measurement is a problem more often than is a real deviation from acceptable pH ranges. So whenever pH seems to be outside the desirable range, it is important to first verify that the pH measurement is accurate.

The measurement of pH can be made in a variety of ways. Most common for aquarists are pH test kits and electronic pH electrode/meter combinations. Test kits contain one or more dyes that change color as a function of the pH, so they may be blue at one pH, purple at a slightly higher pH and red at a still higher pH. Users just add the dye mixture to a test water sample and match the color to a reference chart to determine the pH. Such test kits are usually specific for a small pH range of interest, so a pH test kit for use in freshwater systems at pH 5-7 may not be useful in seawater at pH 8.1.

Several drawbacks to pH test kits make them less desirable than pH meters. Such kits may be hard to read precisely enough if their color change is slight over the pH range of interest. This concern may be particularly important to aquarists with certain types of color blindness. A generally larger concern with pH test kits, however, is that it is usually not easy to determine whether they are accurate. Even if they are made correctly, the organic dyes they contain degrade over time, and some kits are known to vary in accuracy when their dyes are old. If you choose a pH test kit, I recommend a high quality brand that you trust, and to not rely on it when it gets old. How old is too old is hard to judge, because aquarists may not know when their kit was actually made, and different brands use different dyes and can be packaged in ways that make them more or less susceptible to degradation over time. I would not rely on a pH test kit that was known to be more than two years old. If I were using a pH kit, and got a result that seemed odd (unusually high or low, for example), I would try to verify it with a different brand of kit or a pH meter before taking any action to solve a problem that may just be a testing error.

Electronic pH meters usually use a glass electrode to directly sense the concentration of H+ in the solution.  In previous articles I have detailed how they work, but having such information is not always necessary.  An important aspect of electronic pH meters is that they must always be calibrated before use.  That does not mean that they must be calibrated with every use. Depending on the required accuracy and other aspects of how they are used, useful calibration may last from a few hours to a few months.  Nevertheless, the calibration is important both to get an accurate reading, and as an assurance that the meter is actually working. Such assurances are not readily obtained with test kits, and that is a primary reason that I strongly prefer pH meters to pH kits, even if the pH meters are fairly inexpensive. Many of the attributes that make for good pH meter selection have been discussed in previous articles.

Carbon Dioxide and pH

The pH in a marine aquarium is intimately tied to the amount of carbon dioxide dissolved in the water and to its alkalinity. The reason that carbon dioxide impacts pH is because when it enters the water, it rapidly turns into carbonic acid. Acids lower pH, so more carbon dioxide means more carbonic acid, which means lower pH.

If seawater is fully aerated with normal air (that is, it is in full equilibrium with the air), then its pH is exactly determined by its 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) or when the carbon dioxide is deficient (as may happen in aquaria using limewater, also known as kalkwasser).  Understanding the overall relationship between carbon dioxide, alkalinity and pH (Figure 2) is a key principle in solving most pH problems encountered in coral reef aquaria.


Figure 2. The relationship between alkalinity and pH in seawater with normal carbon dioxide levels (black), excess carbon dioxide (purple) and deficient carbon dioxide (blue). The green area represents normal seawater.

The Daily pH Swing

One of the first things that aquarists who measure pH notice is that the pH changes from day to night in coral reef aquaria. This 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. So there is a net consumption of carbon dioxide during the day. This causes many aquaria to become deficient in CO2 during the day, raising their pH.

Likewise, all organisms also carry out the process of respiration, which converts carbohydrates back into energy. In the net sense, it is the opposite of photosynthesis, producing carbon dioxide and reducing pH. This process is happening continuously in reef aquaria, but is most evident at night when photosynthesis is not pushing pH upward.

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’s water will entirely prevent this diurnal pH swing, by driving out any excess carbon dioxide or absorbing carbon dioxide when deficient (assuming the carbon dioxide levels in the home air are steady).  In practice, equilibration of carbon dioxide by aeration is difficult, and this goal is not often attained.  Consequently, the pH does change between day and night.

Higher alkalinity implies more bicarbonate and carbonate in the water, and together these serve to buffer the water against pH changes (that is, they resist the change in pH as additional acids or bases are added). So the higher the alkalinity, the lower the diurnal pH swing. Also, the higher the pH, the more effective is the buffering provided by bicarbonate and carbonate in seawater (up to about pH 9), so the higher the average pH, the smaller the diurnal swing. Additional chemicals in the water also help to reduce the pH swing; borate, for example, buffers against pH changes.

With that all said, however, I do not believe that the actual change in pH each day is particularly important. I won’t go into the reasoning behind this claim here, other than stating that it is my opinion, based on my understanding of how most organisms control their internal pH, but I do not believe that diurnal pH changes that stay within the range of pH 7.8 to 8.5 are particularly stressful to most reef organisms. That is, these changes are no more stressful than being at the same pH all day. A constant pH of 7.9 may be worse for many organisms than a pH that varies from 8.0 to 8.5 each day. Of course, if the diurnal swing takes the pH outside of this range, i.e., below 7.8 or above 8.5, then certain processes take place that should be corrected, as detailed below.

Why Does pH Become Elevated?

Under normal circumstances, an aquarium’s pH is nearly always 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 (kalkwasser), other alkalinity additions, and even the entry of carbon dioxide from the room’s air, in which the carbon dioxide level may vary as human activities around the aquarium change throughout the day). The diurnal pH swing alone is not typically strong enough to drive reef aquaria’s pH to excessive levels (i.e., > 8.5). If it does, the aeration is clearly inadequate (assuming normal alkalinity), so more aeration likely will solve the problem.

The most common way for reef aquaria to reach excessively high pH is through high-pH additives, most notably the use of alkalinity additives that contain hydroxide (limewater, also known as kalkwasser) or carbonate (some two-part calcium and alkalinity additives, for example). In a previous article, I showed that adding sufficient hydroxide to increase the alkalinity by 1.4 dKH (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, its pH subsided to 8.33. Likewise, I showed that adding sufficient carbonate to increase alkalinity by the same amount resulted in an immediate pH increase from 8.10 to 8.44. This rise is smaller than with limewater, but is still potentially enough to merit caution.

Solutions to High pH Problems

Some solutions to pH problems are peculiar to a specific cause, such as adding vinegar directly to limewater, or using less limewater than normal.  Some general solutions, however, are frequently effective. Water changes are generally not an effective long-term solution to any pH problems.  My recommendations on how to deal with high pH problems are detailed below.

Adding a buffer is a very poor way to control high pH. The best option in this regard is to add straight baking soda, but it lowers pH only slightly and provides a large boost to alkalinity. I showed experimentally in a previous article that adding enough baking soda to lower artificial seawater’s pH by 0.04 pH units raises alkalinity by 1.4 dKH (0.5 meq/L).

The most benign way to reduce high pH is to aerate the water more. Whether the aquarium looks well-aerated or not, and regardless of its oxygen level, if its pH is above 8.5 and its alkalinity is below 11 dKH (4 meq/L), then the aquarium is not fully equilibrated with carbon dioxide in the air (if its alkalinity is much higher than 11 dKH, then that may also require correction). 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 accomplished, and other methods can be useful. These other methods are:

  1. Direct addition of carbon dioxide: Bottled soda water (seltzer) can be used to instantly reduce an aquarium’s pH. Be sure to select unflavored soda water, and check its ingredients to be sure it doesn’t contain anything that should be avoided (phosphate, etc.). Many manufacturers list water and carbon dioxide as the only ingredients.

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 than others, and the lower the aquarium’s alkalinity, the more the pH will drop. Also, the higher the pH, the less the pH will drop, because seawater’s buffering capability declines steadily as the pH drops from about 9 to 7.5.


Figure 3. Adirondack Seltzer, used to add carbon dioxide and lower pH.

  1. Direct addition of vinegar: Commercial distilled white vinegar (typically 5% acetic acid or “5% acidity”) can be used to instantly reduce an aquarium’s pH. Do not use wine vinegars because 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 more the pH will drop.  Also, the higher the pH, the less the pH will drop, because seawater’s buffering capability declines steadily as the pH drops from about 9 to 7.5. Remember, there may be an additional later drop in pH as the aquarium’s organisms metabolize the vinegar to carbon dioxide.


Figure 4. Heinz Distilled White Vinegar, used to lower pH.

  1. Addition of vinegar via limewater: Commercial distilled white vinegar can be used to reduce a tank’s pH by adding it to limewater that is subsequently added to the aquarium (instead of using limewater alone). Do not use wine vinegars because 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 limewater.

Why Does pH Fall?

As mentioned above, low pH becomes a problem when it falls below about 7.8; that is, when the pH drops below 7.8 for any portion of the day.  Of course, if the pH reaches a low value of pH 7.9, aquarists may still want to raise it, but the need is not so immediate. Several things commonly reduce pH, and each has its own unique solution. Finally, there’s nothing to prevent a tank from having all of these problems simultaneously!

The first step toward solving a low pH problem is to determine why it exists in the first place. Some possibilities include:

  1. A calcium carbonate/carbon dioxide reactor (CaCO3/CO2 reactor) is in use on the aquarium.
  2. The aquarium has low alkalinity (substantially below 7 dKH (2.5 meq/L)).
  3. The aquarium contains more CO2 than the surrounding air due to inadequate aeration. Don’t be fooled into thinking that an aquarium must have adequate aeration because its water is very turbulent.  Equilibrating carbon dioxide is MUCH harder than simply providing adequate oxygen. There would be NO diurnal pH swing if carbon dioxide were perfectly equilibrated.  Because most aquaria’s pH is lower during the night than during the day, they are demonstrating incomplete aeration.
  4. The aquarium contains excess CO2 because the air that it is being equilibrated with contains excess CO2. This is the most common cause in cases that I have discussed that involve more than a thousand aquarists mentioning pH problems.
  5. The aquarium is still cycling and excess acid is being produced by the nitrogen cycle and the degradation of organics to CO2.
  6. Organic carbon addition lowers aquarium pH by producing carbon dioxide.

The Aeration Test

Some of the possible causes of low pH listed above require an effort to diagnose. Problems 3 and 4 are quite common, and here is a way to distinguish them.  Remove a cup of tank water and measure its pH. Then aerate it for an hour with an airstone using outside air.  Its pH should rise if it is unusually low for the measured alkalinity (Figure 2). Then repeat the same experiment on a new cup of water using inside air. If its pH also rises, then the aquarium’s pH will rise simply with more aeration because it is only the aquarium that contains excess carbon dioxide. If the pH does not rise in the cup (or rises very little) when aerating with indoor air, then that air likely contains excess CO2, and more aeration with that same air will not solve the low pH problem (although aeration with fresher air should). Be careful implementing this test if the outside aeration test results in a large temperature change (more than 5°C or 10°F), because such changes alone impact pH measurements.

Solutions to Low pH Problems

Some solutions to low pH problems are peculiar to each cause, and these are detailed is subsequent sections.  Some general solutions, however, are frequently effective.  Water changes are generally not an effective long-term solution to any pH problems.  Effective solutions for low pH problems include regularly using high pH additives and providing more aeration with fresh (low CO2) air.

The more common cause pf low aquarium pH is elevated indoor carbon dioxide levels.  As an aside, it has nothing to do with oxygen.

People can reduce the carbon dioxide in the air that a reef tank is exposed to in several ways.  These include:

  1. Opening windows near the aquarium or sump. This option is cheap and effective if the weather is suitable.
  2. Bringing an airline from outside to a skimmer air inlet. Make sure the line has an adequate diameter to not excessively restrict the flow (or use a pump).
  3. Using a whole house air exchanger. Expensive, but effective.
  4. Plumbing a CO2 scrubber into an airline bringing air to a skimmer. These scrubbers are filled with a CO2 absorbing material that needs to be replenished periodically.  This method works well but can be expensive.
  5. Growing macroalgae, or any other rapidly photosynthesizing organism, in a refugium. This effect can also be timed to happen at night when the aquarium most needs a pH (and, fortunately, O2) boost.

Note that any chemical additive that raises pH in an aquarium will necessarily add alkalinity as well.  There is no way around this issue.  These products are detailed below.

  1. Limewater (kalkwasser) is the best choice as an additive for raising pH.  Since it is a hydroxide additive, it has the largest possible pH rise per unit of alkalinity. It also ads a balanced amount of calcium so alkalinity does not rise relative to calcium.
  2. Sodium and potassium hydroxides (such as Aquavitro Balance) have a similar boost to pH per unit of alkalinity as limewater, but do not provide calcium.
  3. High pH two-part calcium and alkalinity additives generally contain carbonate and possibly some bicarbonate. These products have the advantage of raising pH without undesirably raising alkalinity relative to calcium, but the pH boost per unit of alkalinity added is about half that of limewater.

Do not worry too much about calcium or alkalinity rising from the addition of limewater to solve low pH problems. The demand for calcium and alkalinity rises in a reef aquarium as pH rises, so the net effect may be little change in calcium and alkalinity. Most often, the pH rises and no other problems are encountered. In any case, try these methods first and worry about such rises only if they happen and if they seem excessive.

Buffers alone are not generally a good method for raising (or lowering) pH because they raise (or lower) pH relatively little, and often result in excessive alkalinity. Unfortunately, the labels on many commercial buffers are written in ways that convince aquarists that their pH will be fine if they just add some buffer. More often than not, the pH is not improved for more than a day, and the alkalinity rises above desired limits.

A final method that is generally useful to increase pH involves growing macroalgae that absorb CO2 from the water as it grows.  The growth is often in a sump that’s lit on a reverse light cycle to the main tank to provide the maximum pH increase when the main tank is at its minimum pH.  The effectiveness of this effect depends on the amount of macroalgae and how fast it is growing.

Low pH Due to CaCO3/CO2 Reactors

A common cause of low pH in a reef aquarium is the use of a calcium carbonate/carbon dioxide reactor. These reactors use acidic carbon dioxide to dissolve calcium carbonate and their effect is to deliver a substantial, but transient, amount of acid to the tank. Ideally, the carbon dioxide is blown back out of the tank after it has been used to dissolve the CaCO3. In reality, however, this process does not go to completion, and aquaria using CaCO3/CO2 reactors typically run at the low end of the pH spectrum.

The solutions that follow assume that the reactor is properly adjusted. A maladjusted reactor can drive the pH down even lower than usual and, in that case, proper adjustment is the first step. How to set a reactor’s various parameters is beyond the scope of this article, but from this standpoint, the effluent’s pH or alkalinity must not be too low.

Many approaches have been suggested, with varying success, to minimize the low pH problem encountered with CaCO3/CO2 reactors. One is to use a two-stage reactor that passes the fluid through a second chamber of CaCO3/CO2 before releasing it into the tank. Dissolving additional CaCO3 raises the pH, and also raises both the calcium and alkalinity levels in the effluent. This approach seems to successfully raise the effluent’s pH, but it cannot raise it all the way to the tank’s pH, so the low pH problem may not completely disappear.

Another approach is to aerate the effluent before it is delivered to the tank. In this case, the goal is to blow off the excess CO2 before it gets to the tank. This approach can work in theory, but typically does not in practice because not enough degassing time is permitted before the effluent enters the tank. Another concern with this approach is that if it really could raise the effluent’s pH, the supersaturation of CaCO3 in the effluent might rise high enough to precipitate CaCO3 in the reactor, fouling it and reducing its effectiveness.

A final approach, and probably the most successful, is to combine the CaCO3/CO2 reactor with another alkalinity supplementation scheme that raises pH. The most useful method for this is limewater. In this situation, the limewater is not used to provide large amounts of calcium or alkalinity, but to soak up some of the excess CO2, and thereby raise the pH. The amount of limewater needed is not as large as for full maintenance of calcium and alkalinity. The limewater addition can also be put on a timer to add it only at night and early morning, when the daily pH lows are most likely to be problematic. The limewater addition could also be put on a pH controller, so that it is added only when the pH gets unusually low (such as below pH 7.8 or so).

Low pH Due to Low Alkalinity

Low alkalinity can also lead to low pH, although this is rare as aquarists are generally quite concerned with maintaining appropriate alkalinity. If alkalinity is not supplemented as fast as it is removed by calcification, the pH will likely drop. This drop will occur with all alkalinity supplementation schemes, but will be most observable when using schemes that do not themselves raise pH (such as CaCO3/CO2 reactors or additives using sodium bicarbonate or similar buffers. In this situation, the obvious solution is to somehow add more alkalinity. The higher the combined respiration of the aquarium, the larger the diurnal swings will be as alkalinity falls.

Low pH Due to Organic Carbon Dosing

Dosing organic carbon for purposes of driving bacterial growth and reducing free nutrient levels in the aquarium will tend to reduce pH.  All organic carbon sources end up mostly as carbon dioxide, reducing pH.  Some types, such as vinegar, will impart much of their pH lowering immediately on addition.  Others, such as the ethanol in vodka, will have more of their pH lowering effect later when the CO2 is produced by the bacteria.  Interestingly, vinegar or vodka added slowly enough will have the same net effect on pH.  It is counter-intuitive that this is so, but vinegar just puts its pH lowering effect up front, while other carbon sources have more later, balancing out to the same per carbon atom added (since each carbon atom ends up as one carbon dioxide molecule).  If you want to dose vinegar and cannot do so slowly (such as with a dosing pump), one option is to saturate the undiluted vinegar with calcium hydroxide (kalk powder).  The residual clear liquid will have an elevated pH and can be added all at once without lowering the pH of the aquarium (and the rise is small enough to not worry about).


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 in an acceptable range. Measuring pH is not complicated, and generally need not be expensive. Even inexpensive pH meters can be a good choice if properly selected and used. Test kits can also be effective, but run the risk of being inaccurate without easy ways to verify their accuracy (aside from testing them against a pH meter).

In many reef aquaria, aquarists find that the pH is naturally in a range that they are comfortable with (say, 8.0 to 8.5), and they may need to do nothing to actively “control” pH. However, some aquarists do find pH to be too high (usually only when using limewater) or too low (most often from too much carbon dioxide in their home’s air). The best pH control methods are generally inexpensive, although some of the best options may be limited for aquarists in climates where bringing in a lot of fresh (outside) air is problematic. Nevertheless, there are useful ways to control pH (aeration and using limewater), and there are others that almost always have limited success or undesirable consequences (such as excessive alkalinity when using buffers). Hopefully, this article has provided the information necessary for aquarists to control pH in their aquaria in appropriate and effective ways that avoid these undesirable consequences.

Happy Reefing!


If you have any questions about this article, please visit my forum at Reef2Reef.



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