New article: my updated pH article

[Randy Holmes-Farley]

It has published in tghe November issue of Reef Edition:

http://www.reefedition.com/

specifically here:

http://www.reefedition.com/ph-and-the-reef-aquarium/

Here's the intro:

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.

Introduction

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.