Reef Alchemy by Randy Holmes-Farley

High pH: Causes and Cures

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:

  1. 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. Low 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.

  2. 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 on equipment).

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 in Seawater

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:

1.  CO2  +  H2  H2CO3

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. This 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:

2.  H2CO3  H+  +   HCO3-     2H+  +  CO3--

Equation 2 demonstrates that when an aquarium has a deficiency of H2CO3, 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 is:

3.     6CO2  +  6H2O  +  light     C6H12O6 (carbohydrate) + 6O2

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:

4.  C6H12O6 (carbohydrate) +  6O2   6CO2  +  6H2O  +  energy

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:

5. Ca(OH)2    Ca++  +  2OH-

6. OH-  +  H2CO3   HCO3-

Figure 3. The effect of limewater addition on alkalinity and pH.

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:

7.  CO3--  +  H2CO3    2HCO3-

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 and pH.

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 Work

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 pH.

Figure 6. The effect of aeration with air containing excessive carbon dioxide
(or otherwise adding excessive carbon dioxide) on alkalinity and pH.

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):

8. CH3COOH    CH3COO-   +  H+

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:

9. CH3COO-   +  O2    2CO2  +  H2O  +  OH-    CO2  +  H2O  +  HCO3-

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 alkalinity 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 and pH.

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 and pH.

Considerations Prior to Solving pH Problems

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:

Measuring pH with a Meter

A Comparison of pH Calibration Buffers

A pH calibration or verification fluid using grocery store borax.

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 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, 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 limewater.


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 own tanks.

Happy Reefing!

If you have any questions about this article, please visit my author forum on Reef Central.


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High pH: Causes and Cures by Randy Holmes-Farley -