One way to get a few “agrees” on the local aquarium forums would be to state that this article is slightly overdue, for more “agrees” I guess I should state it is significantly overdue. My apologies for the delay, this roller coaster called life has still not allowed me much time for researching, reading and writing, but one reaches a point where doing what you enjoy seems like the only viable option. So, without further ado… I think it is time for……
Keeping SPS, Part 3, Parameters: pH
]Having been in the hobby for a considerable length of time, and spending many years browsing and contributing to various on-line forums, I have often wondered how many hobbyists really understand what they are measuring when dealing with pH. I suspect that for most hobbyists it is simply just another number or range to chase by means of measuring, posting the result and receiving advice on products or procedures to be implemented to bring it to within the acceptable range. A few keyboard strokes in your online search engine will tell you that the acceptable range to maintain our tanks at is between 7.9 and 8.4, with 8.2-8.3 being ideal.
Another search will yield a plethora of products marketed at maintaining levels at this range. I could end this article at this point, assuming that for most hobbyists, simply knowing the range and products to use will suffice. However, I do believe that there are some of us who have a more thorough interest in understanding what we are doing, how our actions affect our livestock and perhaps even gain a better understanding of what is really going on in the very complicated soup we keep in our glass boxes. When dealing with sensitive charges, I believe my philosophy rings true, especially when keeping more sensitive species like small polyp scleractinians.
Although the exact origin of the term pH is debated, and its definition can get really intricate with words like decimal logarithms, hydrogen activity and electrode potential, simply put, pH is an indication of whether a solution is a base or an acid.
An acid can be defined as a substance which donates a Hydrogen (H) ion, whereas a base will do the same, but with a hydroxide ion (OH). When when measuring pH, we are dealing with Hydrogen ions, or, better put, their activity, which is what the term pH stands for: potential / power of Hydrogen. Precisely how pH is measured when using a test kit or an electronic device will however be discussed in another article.
As most should remember from elementary science at school, pH is measured and reflected on the traditional scale from 0 to 14, with 7 being neutral. This misconception has been perpetuated by most literature however, but for our intended purpose, this range will suffice and delving into the real intricacies of the theory and practice is way beyond the scope of this article. Pure water has a pH of 7 (at 25 °C), termed as neutral, which effectively means that it contains equal amounts of Hydrogen and Hydroxide ions, readily available to react with other ions. Exactly how these numbers are derived is beyond the scope of this article, as it involves the negative logarithm of the Hydrogen concentration in and of a liquid. If you’ve had exposure to science beyond grade 8 (unlike yours truly), you’ll be familiar with the aforementioned.
Even though quite complicated for those of us not mathematically inclined, it is important to note that when pH differs by one pH unit, it essentially means that there is a ten-fold difference in its acidity or basicity. For example, a liquid with a pH of 6 is ten times more acidic than a liquid with a pH of 7, as it has ten times more Hydrogen ions present. Consequently, a liquid with a pH of 5 is one hundred times more acidic than a liquid with a pH of 7. This illustrates just how drastic 1 point of pH change in our tanks actually is.
So what is going on in our tanks?
Fortunately for us, the pH of freshly added seawater, whether we are using NSW or ASW is mostly within the accepted range. However, there are various naturally occurring factors and processes which influence the pH of our systems, both internal and externally.
In most cases, the predominant naturally occurring processes and factors which affect pH tend to cumulatively drive the pH of an aquarium downward. If left unchecked, this downward drive can eventually cause the pH in your tank to drop outside the range in which our precious inhabitants thrive, with potentially devastating consequences.
There are also circumstances under which pH can be driven up outside the acceptable range, although these circumstances are usually directly attributed to actions / additions by the aquarist.
Considering that the most common trend in our reefs is a downward drive in pH, let’s take a closer look at the most common culprits and how they influence sps.
The biggest “culprit” in driving down pH in our aquaria is Carbon Dioxide (CO2):
1.)The mere presence of CO2 in the air, and the fact that gas exchange occurs at any air-water- interface in our tanks, result in CO2 dissolving in our water. Where there is an excess of CO2 present in the air surrounding a tank, maintaining pH within the correct range can be quite challenging for the aquarist. I have often encountered aquarists who struggle to maintain pH within the acceptable range, simply due to insufficient air exchange between their homes and the outside environment.
Respiration by animals in our systems releases CO2; into the water column, further contributing to the amount of dissolved CO2;. Fortunately, this is combatted by photosynthesis, the process whereby algae and plants utilize CO2; in the presence of light to create Carbohydrates and Oxygen (O2) as a by-product. The O2 in turn“combats” the CO2; to keep pH stable. However, during the diurnal cycle, this process is not countered, as respiration exceeds photosynthesis in the absence of light, with CO2; driving down pH. It is for this reason that pH should not be measured as a singular measurement at random times, but rather at predetermined times, or continuously monitored to document its high and low points in a 24 hour cycle, but more on this in another article.
3.) CO2; is also a by-product of the de-nitrification process, whereby bacteria feed off decomposing organic matter.
All the abovementioned processes drive down pH as a function of CO2, with the following chemical reaction: CO2+ H2O H2CO3. In layman’s terms, the dissolved CO2 reacts with water to form carbonic acid (H2CO3). Where there is an excess of carbonic acid, Hydrogen ions will break off, essentially adding more Hydrogen to the solution, driving down pH.
Other than CO2; produced in and incorporated into our tanks, both strong and weak mineral acids are produced, which add to the acidification pool.
As protein is broken down into nitrite (NO2) and nitrate (NO3), it reacts with water to form nitrous (3HNO2) and nitirc acids (HNO3) respectively.
There are various ways to counteract the effect of CO2 & other acids driving down pH, mostly incorporating the effect of Oxygen driving off Carbon Dioxide:
Opening windows or installing air vents in proximity to the aquarium which facilitate fresh O2 rich air coming into contact with the aquarium.
Ensuring decent gas exchange between the aquarium and the surrounding air by increasing the surface area of water coming into contact with air by means of surface agitation. The greater the amount of water breaking the surface, the more Oxygen can enter water to drive off CO2. Think high turnover of water going through your overflow weir to your sump with lots of broken air-water surface area. (Of course this will only help if the air surrounding the tank does not have an excess of CO2).
Running your skimmers air intake to a well vented area / outside.
Growing algae in a well-lit sump / connected tank / refugium to increase potosynthesis. To combat the diurnal pH drop as respiration overtakes photosynthesis, the algae can be lit on a cycle opposite to the display tanks lighting schedule or an a 24/7 basis.
Ensuring that your t3op-up water is well aerated, or buffered with a pH buffering compound.
Increasing the alkalinity of your aquarium water. Although this will not drive off CO2, it will help to raise the pH if below acceptable levels. Details of Alkalinity and its relation with pH and its role in our tanks are however for another article.
Addition of Calcium Hydroxide (Ca (OH)2) (Kalkwasser) either via top up water or as a direct “one-shot” addition via Calcium Hydroxide dissolved in Reverse Osmosis Water. Addition of Kalkwasser gets rid of CO2 due to the OH (Hydroxide) combining with CO2
You might be asking why pH is important?
The animals and plants we keep live and thrive in a very narrow pH range. A too drastic deviation from their accepted range will be disruptive to their cellular processes, with potentially dire consequences. As more often than not, pH is driven down in our aquaria, the next logical point of interest would be to take a closer look at how pH outside the usual range potentially affects specifically sps corals in our systems.
One of the unfortunate effects of the burning of fossil fuel is the release of excessive CO2 into the atmosphere. Due to gas exchange between the ocean and the atmosphere, more CO2 has dissolved in the oceans and scientists have shown that on average, the pH of our oceans surface is 0.1 units lower than they were prior to the industrial age, and speculate that by the turn of this century, the pH value of our oceans can drop by 0.4 units. This process, known as ocean acidification, has prompted various studies testing its effects on scleractinians, of which our beloved “sticks” are the most abundant.
Interesting, but not relevant to us until we start actively inducing coral spawning in our systems, is that pH values as low as 7.7 had no effect on fertilized eggs, larval settlement and development of Acropora palmata larvae.Invertebrate Reproduction & Development, Volume 57, Issue 2, 2013, Pedro Medina-Rosas, Alina M. Szmant & Robert F. Whitehead.
What is more relevant to us aquarists is how depressed pH affects effects calcification (growth) of corals.
A study by K. Schneider and J. Erez in 2006, showed quite alarmingly to what degree a small change in pH can negatively influence coral calcification rates, specifically as a result of increased CO2.
A pH drop of only 0.2 units can cause a reduction in calcification by up to 50%.http://epic.awi.de/29614/1/Sch2006bz.pdf. In layman's terms, a coral grows at half the speed it can with only a 0.2 drop in pH!! Scary. Yes this can be counteracted by raising alkalinity, but simply keeping pH up and increasing alk will have a synergisitic effect on growth.
Fortunately, decreased pH levels expected within our tanks, don’t negatively influence skeletal structure, as found by Isani Chan1, Shao-Hung Peng, Ching-Fong Chang, Jia-Jang Hung, and Jiang-Shiou Hwang from the Institute of Marine Biology, National Taiwan Ocean University.
However, what is of concern is that significant skeletal structural damage occurs once pH levels start dropping below 7. Unlikely for our aquariums to experience such low levels, what is interesting to note is that the onset of skeletal damage occurs after only a short time period. This would be of concern to aquarists utilising Calcium reactors, whereby a solenoid failure could drive down pH to unacceptable levels in a short space of time.
Another serious concern to sps keepers is the influence depressed pH has on the microbial community found on corals:
The relationship between the coral holobiont and its microbial populations has been studied intensively, showing a definite correlation between microbial populations and coral health. The above study showed that
In a study published in the ISME journal in January 2011 (The impact of reduced pH on the microbial community of the coral Acropora eurystoma), the effect of lowered pH on holobiont microbial communities is clearly shown:
Corals were exposed to pH values of 8.2 (current NSW values) and 7.3 for a period of 10 weeks.
It was found that there was a significant difference between microbial populations on the coral, not influenced by the difference in microbial populations in the surrounding water.
Alarming to me is that for corals kept at a pH of 7.3, there was a significant increase in bacteria generally associated with diseased and stressed corals, such as Vibrionaceae, Alteromonadaceae (associated with bleached corals)and Rhodobacteraceae (associated with white band disease), Bacteroidetes (associated with Black Band Disease).
From the article “However, the shift to opportunistic bacterial species that are known to be associated with diseased or stressed corals is in our opinion an important biomarker that may suggest this is indeed the case.”
Whether there is an exact number at which the microbial population shift occurs, and whether it is as a function of the lowered pH in the surrounding water, or opportunism by microbes detecting a stressed host is not known at this stage, but the mere fact that a downward trend in pH does have a potentially negative effect on our precious sticks is crystal clear.
What also came to light in this study is that anti-microbial activity by the coral holobiont increased at lower pH, which, to me at least, is an expenditure of energy by the coral, something I would much rather have the coral employ in production of colour pigments or growth .
And you are still asking why pH is important when keeping sps?
I must be honest, when starting this article, I was not sure which direction to take with it, whether to keep it simple or get into the real nitty gritty. As with any article involving chemistry, it really is a huge subject, and I hope I have managed to give some of you a slightly better insight into this often misunderstood “parameter,” I know I have learnt something.
I hope my follow up article will come shortly, wherin I plan to elaborate a bit more on this subject and investigate measuring / testing techniques and the importance of when such measurements are done.
Lastly, a special thank you to Liaquat Sain (@Obi-Wan) from Aquality for assisting my non-mathematical mind to come to terms with the more technical intricacies of the subject and to Keith Baxter (@Nemos Janitor)from 555 Aquariums for proof reading and ensuring I don’t make a fool of myself. And of course, thanks to all fellow hobbyists who actually read what I write and encourage me to keep doing so, and yes, to the admin staff on Marine Aquarium SA for putting up with my rambling J.
For further hobbyist material on the subject, please follow the following links:
http://reefkeeping.com/issues/2007-05/rhf/ The “How To” Guide to Reef Aquarium Chemistry for Beginners, Part 3: pH
http://reefkeeping.com/issues/2004-09/rhf/index.php Low pH: Causes and Cures
- Wikipedia http://en.wikipedia.org/wiki/PH
- The “How To” Guide to Reef Aquarium Chemistry for Beginners,Part 3: pH, Randy Holmes Farley
- Invertebrate Reproduction & Development, Volume 57, Issue 2, 2013, Pedro Medina-Rosas, Alina M. Szmant & Robert F. Whitehead.
- The Reef Aquarium, Vol. 3: Science, Art, and Technology ,by Julian Sprung & J. Charles Delbeek.
- The effect of carbonate chemistry on calcification and photosynthesis in the hermatypic coral Acropora eurystom,Kenneth Schneider and Jonathan Erez
The Hebrew University of Jerusalem, The Institute of Earth Sciences.
- Environmental Toxicology, Volume 14, Issue 2, pages 235–240, May 1999, pH dependent toxicity of five metals to three marine organisms, Kay T. Ho, Anne Kuhn, Marguerite C. Pelletier, Tracey L. Hendricks and Andrea Helmstetter
- The impact of reduced pH on the microbial community of the coral Acropora eurystoma ISME J. Jan 2011, Dalit Meron, Elinor Atias, Lilach Iasur Kruh, Hila Elifantz, Dror Minz, Maoz Fine, and Ehud Banin
- Effects of Acidified Seawater on the Skeletal Structure of a Scleractinian, Coral from Evidence Identified by SEM, Isani Chan1, Shao-Hung Peng, Ching-Fong Chang, Jia-Jang Hung, and Jiang-Shiou Hwang
- Effects of lowered pH and elevated nitrate on coral calcification, F. Marubini & M.J.Atkinson.