Chasing the tail of pH
In this article, I will attempt to cover one of the most frequently aggravating and perplexing subjects we come across in reefkeeping, and hopefully it will put a few wrongs to rest. If nothing else it give you a few ideas that you may wish to try out in an effort to solve any recurring issues you have with your pH then its served a valuable purpose. pH in its entirety is a very complex subject where reef tanks are concerned so I’ve tried as much as possible to avoid any complex equations and chemistry, Alas though, there are some inescapable links between what goes on at a chemical level, your stock and your hardware so please bare with me. Hopefully I’ve put things across in a logical manner… So without further ado…
What is pH and pH drift ?
The pH scale as we know it represents a logarithmic translation of the differences in hydrogen ion concentration in a given aqueous solution (in our case sea water). Too complicated ? Ok, so to put it another way. We have a scale of 1 to 14 that represents whether a solution is acidic or alkaline in nature, with 1 representing the most acidic, 14 representing the most alkaline, and 7 representing a neutral level, i.e. pure water. The key to understanding the pH scale is in realising that it is a logarithmic scale i.e. a pH of 5.0 is 10x more acidic than a pH of 6.0 and a pH of 8.0 is 10x more alkaline than a pH of 7.0. as we move the margin further, the difference becomes cumulatively greater i.e. a pH of 4.0 is 100x (10x10) more acidic than a pH of 6.0, and so on and so forth.
Greedy fish place a big load on your pH. Too many, and your system struggles to deal with the waste generated.
In relation to what we see in our aquariums, we are usually hovering between the 7.7 to 8.5 mark with 8.5 being 70x more alkaline than 7.7. As a general rule we use NSW (natural sea water) as a basis for our needs, with a guide parameter of 8.3 to 8.4 considered as an ideal. The drift we commonly encounter over the course of either 24 hrs or long term, represents the shift in hydrogen ion concentration within our water. Although in its entirety the chemistry involved can be quite complex, the largest factors we commonly encounter, are directly attributed to the balance between o2 / co2 concentrations, and the waters ability to buffer against pH suppression through increasing Co2 levels and lowered o2 concentration. So what is a good pH and what’s a bad pH? The general consensus among experts is that we should try and avoid running pH outside the comfortable extremes of 7.7 to 8.5 in a reef aquarium, with the higher end being considered the more ideal. What is commonly missed out and frequently witnessed by those keeping more sensitive species such as SPS corals, is that the overall reading seems to be less of a concern than the overall stability or drift. Many now attribute more stable growth, better colouration, and lower mortality with SPS corals under stable conditions with minimal 24 hr drift (below 0.10) than a higher pH but with a wider drift. So it’s up to the individual to decide on which side of the fence they fall and where they want to focus attention. In effect though, the results of the following, will hopefully herald results in both quarters, i.e. a higher overall pH reading, and a more stable 24hr drift.
So, to get control of pH issues we first need to know what the major factors are affecting it within any individual system. By and large we can break these factors down into five distinct areas.
Firstly we have the largest factor of all. Bio-load. All organisms within the reef aquarium require oxygen to a greater or lesser degree from fish to bacteria. Even algae which are known for their ability to utilise Co2 and give off o2 during the lit period, switch to using o2 at night, and give off Co2 if not lit, hence the common reason for reverse lighting refugium’s in an effort to offset the 02 demand going on in the tank and stabilise day/night pH drift..
Bioload is important, as it links two key processes within the aquarium. Namely waste generation and waste breakdown. When we feed our fish or corals, a percentage is utilised for biological processes and growth ‘both of which require o2’. The rest is expelled as waste which feeds smaller and smaller organisms until we get to the final stage of bacterial breakdown to nitrate. In effect, each and every gram of food we feed, may take a multitude of paths before ending its journey. Each one of these paths steals from the 02 reserves of the system affecting the balance between 02 and Co2 causing a drop in overall pH. The more we feed, the greater this effect, as bacterial populations grow to take advantage of the larger nutrient reserves.
Many consider pH stability crucial to success with SPS corals
Secondly, we have gaseous exchange. The air/ water interface, is a focal point where C02 escapes and 02 enters. Too smooth with little or no surface agitation, and gas exchange is restricted, so ideally we should have a very active surface layer to break up surface tension and allow gases to pass in both directions freely. Protein skimmers can also play an integral roll in this process as well. Delivering large volumes of air through the reaction chamber. This helps drive off excess Co2.as it is generated by the organisms present in the system. The more powerful the skimmer, the greater this beneficial effect. In fact, even weirs can play a roll, where the rushing air/water mix helps in the same manner the skimmer did.
Next we have the chemistry of the water itself, and how the various concentrations of elements affect how pH responds to these changes in 02 or Co2 levels. So without getting overly complicated, there are three main components to sea water that are linked in such a manner that they have a direct influence on the pH drift we encounter over any 24 hour period. Namely, Magnesium (Mg), Calcium (Ca) and Alkalinity (DkH / Meq/l). At first glance we may think that Magnesium has little to do with this relationship, but this couldn’t be further from the truth. At levels below NSW concentrations of around 1350ppm, we see an increasing loss of ability to maintain both Ca levels and Alkalinity regardless of the methods used to maintain or boost them. This is crucial to our goal, because Alkalinity ‘is’ directly linked to pH fluctuation. As levels drop below NSW levels of around 8.0 – 8.5 dkH, we see an ever increasing swing between our daytime high pH reading, and our lowest pH reading at the end of the unlit period. pH swing is important, because most marine organisms are extremely sensitive to pH changes. Many SPS and soft corals as well as crustaceans can suffer quite badly from excessive and repetitive daily changes. Keeping these three main chemical constituents at NSW levels of around 1350ppm Mg, 420ppm Ca and 8.0 DkH will minimise pH swings to more natural levels. Increasing dkH into the 10-12 dkH bracket has been seen to further stabilise pH drift, however the full consequences of excessively elevated dkH are still not fully understood beyond the possibility of brittle skeletal formation in some SPS. Even then it is recommended that Ca and Mg are brought up to balanced levels with this higher reading. A common mistake made by people trying to raise pH is to use pH buffer (usually Sodium bicarbonate) It should be noted that pH buffers derived of Sodium Bicarbonate will not ‘raise’ pH, even though a minor increase may be noticed for the first few hours after dosing. What Sodium bicarb does do, is to increase the dkH level, thereby buffering the pH against suppression below its normal and existing position. In effect, all buffers do, is bolster up the ‘lowest’ reading that’s encountered at the end of the unlit period. They won’t ‘raise’ the overall pH value to any degree. Whilst this may seem beneficial, the long term addition of these buffers will see a gradual increase in the number of sodium ions in the water, and a gradually increasing imbalance between Alkalinity, calcium, and Magnesium, so regular testing is essential where these parameters are concerned. If you affect one, you have to consider the others.
Next we have system maturity. Most systems when new will go through various stages of maturation and settling as bacterial and algal populations become established. In many cases we will see various blooms occurring during the first few months. These blooms will have an affect on the systems pH and should be expected. This is one reason that we should always stock new systems slowly to avoid putting extra pressure on the systems ability to stay stable, accepting that any animals we place in this environment, will have to put up with these changes as everything settles down. Equally, it is not unusual with new substrates to find some initial demand placed upon our Alkalinity, Magnesium and Calcium reserves as they are drawn from solution to bind to these surfaces and balance out the chemical differences between the water and the media itself. In these initial phases we may need to monitor these levels and boost accordingly through manual supplementation until everything has settled. This can commonly take a few months from the initial start up, during which time we will see initially wide swings in day / night time pH, settling over time to more consistent and stable readings as bacterial and algal populations even out, and chemistry has settled. As such we should avoid panicking unduly about pH stability in the early stages, and avoid chucking chemicals at the situation until we know whether we actually have a long term issue or not. Sometimes its just better to ride it out for the first few months, being sensible with our stocking regime and wait to see what happens a few months down the line.