Aquarium Chemistry: Solutions To pH Problems

by | Jun 15, 2002 | 0 comments

Aquarists spend a considerable amount of time and effort worrying about, and attempting to solve, apparent problems with the pH of the water in their tanks. Some of this effort is certainly justified, as true pH problems can lead to poor animal health. Some of it, however, isn’t justified because the only problem is with the measurement itself, or with the aquarists interpretation of what is a problem. This article endeavors to spell out what situations constitute true pH problems in marine aquaria, and how to solve them.

Before stepping right into solutions to pH problems, the first few sections of this article define pH and discuss what pH values are desirable for reef tanks. If you are comfortable with pH as a measurement, or have an immediate pH emergency, you can skip down a few sections to the heading “Solving pH Problems” without compromising the remainder of the article.

What is pH?

The concept of pH in a seawater application has a variety of different definitions. In the system used by most aquarists (the NBS system, with NBS standing for the old National Bureau of Standards), the pH is defined in equation 1

1. pH = -log aH

where aH is the “activity” of hydrogen ions in the solution. Activity is the way that chemists measure “free” concentrations. So pH is simply a measure of the hydrogen ions (H+; aka protons) in solution. Hydrogen ions in seawater are partly free (well, not really free but attached only to water molecules in complexes such as H3O+) and partly complexed to other ions. This effect is why chemists use activity instead of concentration. In particular, H+ ions in normal seawater are present as free H+(about 73% of the total), as HSO4-(about 25% of the total), and as HF (a small fraction). These activity issues also impact calibration buffers, and that is part of the reason that there are different pH scales and calibration buffers for use in seawater.

In order to understand most pH problems in marine aquaria, however, these issues can be ignored, and pH can be simply be thought of as relating directly to the concentration of H+:

2. pH = -yHlog[H+]

where yH is simply a constant (the activity coefficient) that we can ignore for this purpose (yH = 1 in pure fresh water and ~0.72 in seawater). In a sense, all that most aquarists need to know is that pH is a measure of the hydrogen ions in solution, and that the scale is logarithmic. That is, at pH 6 there is 10 times as much H+as at pH 7, and that at pH 6 there is 100 times as much H+ as at pH 8. Consequently, a small change in pH can mean a big change in the concentration of H+ in the water.

Why Monitor pH?

There are several reasons why one would want to monitor pH in marine aquaria. One is that aquatic organisms only thrive in 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 with many species. Even natural seawater (pH = 8.0 to 8.3) isn’t going 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 7.3, for example) is stressful to fish.1 There is now additional information about optimal pH ranges for many organisms, but the data is woefully inadequate to allow aquarists to optimize 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 of metals such as copper and nickel is known to depend on pH for some of the organisms present in our tanks (such as mysids and amphipods).7 Consequently, the ranges of pH that are acceptable in one tank may be different in other tanks, even for the same organisms.

Nevertheless, there are some fundamental processes taking place in many marine organisms that are substantially impacted by changes in pH. One of these is calcification, and it is known that calcification in corals is dependent on pH, with it dropping as the pH is lowered.8-9 Using these types of information, along with the integrated experience of many hobbyists, we can develop some guidelines about what is an acceptable range for reef tanks, and what values are pushing the limits.

Acceptable pH Range

The acceptable pH range for reef tanks is an opinion rather than a clearly delineated fact, and will certainly vary based on who is providing the opinion. This range may also be quite different than the “optimal” range. Justifying what is optimal, however, is much more problematic than that which is simply acceptable, and we will focus on the latter. As a goal, I’d suggest that the pH of natural seawater, about 8.2, is appropriate, but tanks can clearly operate 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 tanks, with several caveats. These are:

  1. That the alkalinity is at least 2.5 meq/L, and preferably higher at the lower pH end of this range. In part, this statement is based on the fact that many reef tanks operate quite effectively in the pH 7.8 to 8.0 range, but that most of the best examples of these types of tanks incorporate calcium carbonate/carbon dioxide reactors that, 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 calcification at these lower pH values may be offset by the higher alkalinity.
  2. That the calcium level is at least 400 ppm. Calcification becomes more difficult as the pH is lowered, and it also becomes more difficult as the calcium level is lowered. 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 a tank with a CaCO3/CO2 reactor), at least make sure that the calcium level is acceptable (~400-450 ppm).
  3. Likewise, one of the problems at higher pH (above 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 clogging of heaters and pump impellers). If you are going to push the pH to 8.4 or higher (as often happens in a tank 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 on equipment).

Solving pH Problems

The following sections provide specific advice about how to go about solving a pH problem. The advice can also be used to tug the pH levels closer to natural values even if they are already within the “acceptable” range described above. Before embarking on a pH altering strategy, however, here are some general concerns:

  1. Make sure that you really do have a pH problem. Many apparent problems are really measurement problems rather than tank problems. This problem seems to be especially common using pH test kits, rather than electronic measurement using a pH meter, but all methods can and do go wrong, and you would not want to make a good situation into a bad one simply because a pH meter was not properly calibrated. Consequently, be sure to verify the pH reading before acting in any but the most benign ways. Here are two articles worth reading on pH measurement to help ensure that the readings are accurate:
  2. Try to determine why you have a pH problem before enacting a band-aid solution. For example, if the problem is high pH top off water, then adding an acid to bring down the tank pH will only be a temporary solution. Changing the root of the problem may be a much more satisfactory solution than masking it with a pH-altering additive.

Still, with these caveats in mind there will be instances when you need immediate corrective actions (such as accidental delivery of a large amount of limewater to your tank) and instances when you need long-term corrective actions (such as low pH caused by high indoor carbon dioxide levels). Solutions to both of these types of situations are given below.

Low pH Problems

As described above, low pH problems are those where the pH is below 7.8. That is, where the daily pH low drops below 7.8. Of course, if the pH hits 7.9 you may still want to raise it, but the need is not so immediate. There are three things that commonly result in low pH, and the solution to each of them is different. Of course, there’s nothing to prevent a tank from having all three!

Low pH due to CaCO3/CO2 Reactors

The most common cause of low pH in a reef tank is the use of a calcium carbonate/carbon dioxide reactor. These reactors use acidic carbon dioxide to dissolve calcium carbonate, and the effect is to deliver a substantial but transient amount of acid to the tank. Ideally, the carbon dioxide is blown back off of the tank after it has been used to dissolve the CaCO3. In reality, however, this process does not go to completion, and CaCO3/CO2 reactor tanks almost always run at the low pH end of the 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 the various parameters of a reactor is beyond the scope of this article, but from this standpoint, you do not want the pH or the alkalinity of the effluent to 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 before releasing it to the tank. Dissolving additional CaCO3 has the effect of raising the pH, and also raising both the calcium and alkalinity levels in the effluent. This approach seems to be successful at raising the pH of the effluent, but it cannot raise it all of the way to tank pH, and the low pH problem does 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 because not enough degassing time is permitted before the effluent enters the tank. Another concern of this approach is that if it really were successful at raising the pH, the supersaturation of CaCO3 in the effluent might get high enough to cause reprecipitation of CaCO3 in the reactor, fouling it and reducing it’s 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 in this application is limewater. In this situation, the limewater is not being 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. You can also put the limewater additions 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 on a pH controller, so that it is only added when the pH gets unusually low (such as below pH 7.8 or so).

Low pH Due to High Indoor Carbon Dioxide

High indoor carbon dioxide can also apparently lead to low pH problems in many tanks. Respiration by people and pets, the use of un-vented appliances burning natural gas (e.g., ovens and stoves) and the use of CaCO3/CO2 reactors can lead to high indoor carbon dioxide levels. The level of carbon dioxide can easily be more than twice that of exterior air, and this excess can substantially lower the pH. This problem is especially severe in newer, more airtight homes. It is unlikely to be a problem in homes like mine where you can feel the wind blowing around old window frames.

Many aquarists have found that opening a window near the tank can result in a significant rise in pH within a day or two. Unfortunately, those aquarists living in colder climates cannot readily open windows in the winter. Some have found it useful in these situations to run a pipe or tubing from the outside to the air input of a skimmer, where fresh, exterior air is rapidly mixed with the tank water.

Finally, the use of limewater in these situations is also appropriate. In fact, limewater may be more optimally used in this situation because the tank would be less likely to experience the undesirably high pH that sometimes accompanies limewater use. While limewater is the alkalinity supplement most potent at raising pH, other high pH additives would also suffice. Supplements based on carbonate, for example, would be very useful in this situation, while bicarbonate would not be. As a commercial example, the original B-ionic would be better than the newer version. For home brews, washing soda (sodium carbonate) would be better than baking soda (sodium bicarbonate).

Low pH Due to Low Alkalinity

Low alkalinity can also lead to low pH. For example, if alkalinity is not supplemented as fast as it is removed by calcification, the pH will likely drop. This drop will happen with all alkalinity supplementation schemes, but will be most observable using schemes that do not themselves raise pH (like CaCO3/CO2 reactors or bicarbonate). In this situation, the obvious solution is to add more alkalinity in some fashion.

Acute Downward pH Spikes

All of the situations described above involve chronically low pH. None of them involve acute, or transient, pH excursions. In certain situations these can occur, however, and knowing what to do may be of interest. Now you are not likely to do what I did, and add a chunk of dry ice to the sump just to see what happens. But if you do, you will notice the pH drop…and drop…and drop. Soon, you will be convinced that the pH of 5 is going to kill your entire tank (it didn’t in my case, but I don’t recommend this process for general entertainment).

A more likely scenario, however, involves some type of carbon dioxide accident that drives lots of CO2 into the tank from a malfunctioning reactor. In most cases, I’d advise doing nothing beyond substantial aeration to drive out the excess CO2. Maybe even open a window to ensure that the air being exchanged it not itself loaded with excess CO2. The tank ought to be back to normal in a day or so. If you did choose to add something to raise the pH, then you risk the pH rising too high in a day or so after the excess CO2 has blown off of the tank.

If a mineral acid were the cause of a pH drop (like hydrochloric acid), then the carbonate alkalinity (and the total alkalinity as well) will have crashed. I’d advise measuring the alkalinity and using a carbonate alkalinity supplement (not one containing large amounts of borate) to raise the alkalinity back to where you normally maintain it (say, 2.5 to 4 meq/L). The end effect ought to be a rise in pH, though with some means of alkalinity supplementation (limewater or the original B-ionic) the pH rise will be fast, and with some schemes (like bicarbonate) the pH rise will be slower as the tank needs time to blow off the excess CO2 that results.

If excessive vinegar or other organic acid were the cause of a pH drop, then I’d advise the same treatment as for the hydrochloric acid above, except that over time (hours to days) the acetate that resulted from the vinegar (acetic acid) will be oxidized to CO2 and OH-. The net effect is that the pH and alkalinity will rise. So in this case, err on the side of less alkalinity supplementation (maybe even nothing) because it will resolve itself before too long. If you add a ton of alkalinity supplement to stabilize things, you may find that the pH and/or alkalinity later creep higher than you want.

High pH Problems

High pH problems in reef tanks are most frequently encountered when using limewater, but are also sometimes encountered when using other high pH alkalinity supplements, such as the original version of B-ionic. If the carbonate alkalinity is not more than 4 meq/L, then high pH is typically caused by a carbon dioxide deficiency. Additives like limewater generate a substantial deficiency of carbon dioxide in reef tanks, and the end effect is that the pH runs on the high side.

All of the discussion to follow presumes that you are using limewater (or other high pH supplement) properly. That assumption implies that you are adding an appropriate amount (i.e., balancing the rate of calcification, or less if combined with some other supplementation scheme) and that it is not being added too rapidly (overwhelming the ability of the tank to buffer against pH rises).

One way to decrease the pH in tanks using limewater is to drive more carbon dioxide into the water. One can drive more carbon dioxide into the water through better aeration, by adding carbon dioxide directly, or by adding an organic acid that eventually breaks down into carbon dioxide (like vinegar; aka acetic acid). The vinegar can be added directly to the limewater itself, or directly to the tank.

Another good option to lower chronically high pH is to switch to an alkalinity supplement that has less of a pH raising effect. Limewater is the worst of the lot, followed by sodium carbonate (washing soda). Sodium bicarbonate (baking soda) will actually have a very slight pH lowering effect on it’s own, and will make a big pH effect relative to adding limewater or washing soda. A CaCO3/CO2 reactor, of course, has the biggest pH lowering impact of any alkalinity supplement scheme (as described above).

Commercial alkalinity supplements track these suggestions, but there are too many to list. They cannot even be lumped together by class, though most solids sold as “buffers” will have a fairly small impact on pH. Some two-part additives are largely carbonate and some are largely bicarbonate. To tell the difference, just measure the pH of the alkalinity portion. If it is less than 9, then it is primarily bicarbonate, and would be a good choice to switch to if chronic high pH is the issue. If it is more than 11, it is primarily carbonate, and would not be as optimal in this application.

Switching to another supplement does not necessarily mean a complete switch. Switching partially from limewater to the new version of B-ionic, or one of the other two-part additives with a low pH impact can have a considerable long-term effect on pH.

Addition of Mineral Acids to Lower pH

You should not, in a chronic situation, add any sort of mineral acid (such as hydrochloric acid) to lower the pH. This caution applies equally well to adding it to the limewater or the tank. What you will end up doing is decreasing alkalinity and increasing the chloride concentration relative to the other anions (such as sulfate). In a sense, you convert some of your limewater to calcium chloride (or, if added to the tank, convert some of your carbonate/bicarbonate to chloride). What you should do in such a chronic situation is determine why the pH is high, and fix it, rather than just blast the pH down with acid. Acute pH spikes are another matter, and may require this type of intervention as described below.

Acute Upward pH Spikes

In an acute high pH situation (such as 5 gallons of limewater entering the tank and sending the pH over 9), adding an acid such as vinegar, muriatic acid (HCl or hydrochloric acid) or sulfuric acid are all acceptable ways to lower the pH. A number of aquarists have successfully treated such situations with vinegar, and have not ended up losing any animals, though the tank is loaded with white calcium carbonate precipitate. I’ve added HCl to my tank in similar situations without difficulty.

If you do such an acid treatment, be very careful to not overshoot, and monitor the pH during any acid additions. I would only intervene in this fashion if I could monitor the pH in real time, and could add the acid to a high flow situation far from any organism. Diluting the acid in water (say, 20:1 or 100:1) prior to adding it to the tank is highly recommended both for your safety and that of the tank inhabitants (dilution isn’t necessary for vinegar which is already dilute).


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 the inhabitants of our systems, and we owe it to them to do the best we can to keep it in an acceptable range. This article provides a series of solutions to common pH problems in aquaria, and should permit most aquarists to diagnose and solve the pH problems that may arise in their own tanks.


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