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This suggests to me that dosing less alkalinity at night (and more during the day), say 1/3 as much or some such, in order to decrease swings in dKH that are the result of different rates of consumption might actually increase the instability of carbonate availability. Consequently, this practice could actually be counterproductive. It might also be easier to maintain carbonate stability by allowing Alkalinity to swing up at night/early morning like it will in a typical automatic dosing setup to compensate for the increased pH later in the day, rather than trying to hold dKH and pH steady all day and night.It is current the dogma of the reef aquarium hobby to say that alkalinity stability is very important for SPS corals, and that pH is not. Often, the very idea of someone trying to optimize pH is called “chasing numbers” as if that statement by itself shows it to be foolish.
Since I have never seen anyone in the hobby actually provide experimental data on the sorts of pH and alkalinity issues that I am about to discuss, let’s back up and think through what these issues really mean and what they may imply for reefing.
Before going any further, let me pose a thought question:
Which of the scenarios below is more “stable” in the sense of the concentration of bicarbonate in the water through the course of a day in a reef aquarium?
A. Alkalinity held at 8.00 dKH, pH varies from 7.9 to 8.3
B. Alkalinity varies from 7.5 to 8.4 dKH, pH fixed at 8.2
As you might have guessed, if not actually calculated, those have identical variation in bicarbonate over the time period of interest.
Let’s start this discussion with some background on total alkalinity, and why we use such a weird, theoretical measurement. It is certainly not because it is the exact thing corals “care” about. Total alkalinity is the sum of a bunch of different things in the water, some of which are counted once (bicarbonate, silicate, hydrogen phosphate, borate, magnesium monohydroxide, hydroxide), some twice (phosphate and carbonate) and one (hydrogen ion) is subtracted back out to get the final answer. Certainly, it is mostly bicarbonate, and it is far easier to measure than is bicarbonate alone. So if we care about bicarbonate and cannot readily measure it, total alkalinity is a fall back measurement that may have value.
In fact, the reason we measure total alkalinity is because corals use bicarbonate (at least that is the current consensus in the scientific community) to gain the carbonate they need for formation of calcium carbonate skeletons (and potentially for the CO2 they need for photosynthesis as well). Let me just back up that assertion with a recent reference to justify the importance of bicarbonate. Using “current” references is important, as this very complex field has evolved a lot in the past 20 years (and still has a lot to learn).
Figure 1 in this paper from 2019 shows bicarbonate as the ion taken up from the bulk water:
Electrophysiological evidence for light-activated cation transport in calcifying corals https://royalsocietypublishing.org/doi/full/10.1098/rspb.2018.2444
However, the point I’m going to make about stability isn’t changed by whether corals take up bicarbonate or carbonate (or even CO2) as their source of dissolved inorganic carbon for photosynthesis.
The point that I will make is that NONE of these (bicarbonate, carbonate, or CO2) are kept stable with stable “total alkalinity”, if one ignores pH. If none of these are stable, how could stability of total alkalinity be important? Or, thought of a little differently, maybe it is only part of a larger “stability” issue that reefers have yet to really understand.
Let’s now delve into what it means for total alkalinity to be stable. What, if anything, is really stable about water with stable alkalinity if the pH is allowed to vary?
The table below shows the amount of bicarbonate and carbonate present as a function of pH, where the total alkalinity is held constant at 8 dKH (for ease of calculation, I’m ignoring all the other minor contributors to total alkalinity). I know my scientist friends will laugh at using dKH as a unit of measure of bicarbonate or carbonate, but I think it makes it easier for ordinary reefers to not get distracted by conversion into other units, such as meq/L.
Table 1. Bicarbonate and Carbonate as a function of pH in seawater with total alkalinity of 8 dKH. Bicarbonate delta is the percentage difference in bicarbonate for each 0.1 pH unit change.
The last column of Table 1 is a value I am calling bicarbonate delta. It is the percent change in bicarbonate for a 0.1 pH unit change in the pH. The way to interpret it is as follows. If you have a tank with an average pH of 8.15 with a daily pH swing of 0.1 pH unit, the bicarbonate concentration is swinging by 5% daily even if you hold alkalinity steady at exactly 8.00 dKH. Obviously, if the pH swing is zero (which is quite unusual), the bicarbonate will be fixed at 6.92 “dKH”. On the other hand, some hobbyists have much higher pH changes day to night (or during the day from morning to evening) and might see a change of 0.3 or 0.4 pH units and might see a change in bicarbonate of 13% over that time, even with alkalinity constant.
pH HCO3- (dKH) CO3— (dKH) Bicarbonate Delta
Now we can answer the question I posed above:
Which of the scenarios below is more “stable” in the sense of the concentration of bicarbonate in the water through the course of a day in a reef aquarium?
A. Alkalinity held at 8.00 dKH, pH varies from 7.9 to 8.3
B. Alkalinity varies from 7.5 to 8.4 dKH, pH fixed at 8.2
Each, it turns out, has about a 0.9 “dKH” change in the bicarbonate concentration over time, or about 10%. Curiously, the carbonate concentration changes far more (on a percentage basis) in scenario A ( 128%) than in scenario B (10%). Finally, if you think CO2 is important, it is changing a lot in scenario A (more than 100%) and only changing about 10% in scenario B.
I won’t try to claim what is and isn’t optimum for growing corals, or even how to define a test that evaluates it.
But I will suggest that there is more to a coral's environmental stability than total alkalinity, even assuming that total alkalinity is a big part of it.
Perhaps folks with an experimental bent might consider “chasing pH” or “chasing stable pH” while also maintaining stable alkalinity to see if it has benefits.
For folks who are interested in details of these sorts of calculations, they can be found in calculations for Bjerrum plots of carbonate in seawater, which is easy to find online and not that hard to put into excel.
