I am more curious to know how you are measuring your pH? Just as a reminder to everyone, it is not
alkalinity that will drive pH this low, rather high concentrations of CO2 accumulating in the tank.
Alkalinity is the measure of ions that neutralize protons in the ocean. It is composed of the measure of carbonate, bicarbonate, borate, and to some extent a few other low concentration ions in the ocean's water column. At the surface, it is determined by the amounts of these salts (HCO3- = 89%, CO3-- = 6.7%, B(OH)4 = 2.9%) and by silicon hydroxide (0.2%), Magnesium hydroxide (0.2%), free hydroxyl ion (OH- = 0.1%), and hypophosphate ion (HPO4- = < 0.1%). For all practical purposes, alkalinity is the result of the first three ions ability to either uptake or release protons whenever the oceanic pH is outside their narrow collective isoelectric point pH ( which just happens to be around 8.2 at the surface at normal surface conditions) and occurs at a rate associated with their ability to react with protons (which accounts for why pH changes in seawater rather than staying at a set level of 8.2). The actual pH of the ocean will drive the equilibrium of the carbonate/bicarbonate system to either the left or right based on total alkalinity, pCO2, total CO2 and actual current pH. These are all so closely interrelated that knowing any three of these values will allow you to calculate the fourth value.* Very, VERY low alkalinity may show up as chronically low pH, but not at 7.7 or lower: if your tank pH is this low, you have other issues.
See this thread and start reading about post 25 for more specific details about how Mg++, etc. play a role in how this system works:
http://thereeftank.com/forums/showthread.php?t=30385
For more details about
alkalinity, look at the thread at this post, and check out the links in the thread, see:
http://thereeftank.com/forums/showthread.php?t=13878
One caveat here if you decide to use a buffer builder high in sodium carbonate content (baked baking soda): the pKa of sodium carbonate (Na2CO3) is around 11.4, and continued use of this product will tend to push your pH in the system up, but I think the skew of sodium content would be a bigger issue, more some other time. Rather than add a lot of high-pH additives, correcting how you measure pH, as well as providing good gas exchange, should resolve what might not be a problem to start with (CLEAN AND CALIBRATE YER PROBE!).
Right now, I would almost bet that this issue here is a measuring error, although I have seen systems that have no means of good gas exchange accumulate enough CO2 to drop pH this low, but not go there and stay. I would suggest that you re-evaluate your pH with a new test kit, or better yet, get a good pH meter (Pinpoint preferably) and test your values. There is a borate test you can use starting with 20 mule team borax (sodium metaborate) to see if your meter is reading correctly to verify the meter's readings. Before you go any farther, or make any changes to your tank, (EXPECIALLY IF THE TANK LOOKS GOOD), get a better means of testing your pH than what you're using now.
pH in marine aquaria is primarily a reflection of the CO2 content of the water column, and to some extent the
alkalinity (not a very big part though and usually short term), although it may reflect where your pH will go as the alk content buffers the water column. High
alkalinity tends to drive pH to
8.2 or so depending on CO2 content in the water column. However, upon addition of a buffer additive (usually sodium bicarbonate or a mix of sodium bicarbonate and sodium carbonate), the immediate effect will most likely be a drive up in pH, followed by a gradual decline back to 8.2 as the
alkalinity dissociates into its constituents and whatever results in equibria of the different carbonate species are demonstrated in the tank (ie., carbonate may precipitate Calcium as it is formed from bicarbonate and bicarbonate may also precipitate high levels of calcium, but it is more soluble than the carbonate species of this reaction. At surface conditions, bicarbonate usually will be the predominant inorganic carbon ion specie, resulting in isoelectric values of 8.2 for pH even when bicarbonate forms to excess (high
alkalinity). This means that there will usually be a gradual drop in both Ca++ and
alkalinity as equilibrium is attained for the supersaturation quotient for that particular set of conditions (excessive alk results in precipitation of Calcium Carbonate solids if the supersaturation quotient is exceeded by the level of bicarbonate). For instance in your display, if your pH stays at 8.0 in the presence of levels of
alkalinity WNL, and there is no issue with your probe or meter, then high CO2 is most likely the cause.
If you have a Ca reactor, excess CO2 adjustment will result in high CO2 and moderately high
Alkalinity if the system is allowed to come to equilibrium, but pH in the range of 8.0 to 8.05. This requires an adjustment in your delivery rate for the CO2 to the reactor, a simple but sometimes tedious task.
Is your house air conditioned and kept closed all summer? Is the aquarium located in an area with a large number of people (dormitory or classroom, for example, or where you work if there is an office environment)? In the winter, use of gas logs, gas furnaces, or all year round with a gas stove and/or oven may drive up your indoor environment's pCO2, and as seawater is at almost instantaneous equilibrium with the atmosphere in the presence of good surface area with turbulence, will consequently drive up pCO2 of your ASW in the tank, driving down pH. In-home environments can easily reach 10000PPM (our outdoor environment is high, and it is only 382 PPM).
Systems with glass covers, tight hoods and stands, covered sumps, inadequate circulation, or deep tanks with a relatively small surface area in relationship to total volume are apt to have these same conditions arise. Sometimes the rest of the house will be fine, but location in a seldom used room or a basement may allow CO2 to accumulate as well. Systems with large populations of fishes may experience these same conditions, but this is most often remedied with good circulation without too much trouble.
A chart for the CO2 concentration determination based on what the actual pH of the water column is and the corresponding
alkalinity:
Ratio pCO2 For ASW compared to NSW
VALUE IN CHART IS COMPARED TO A VALUE OF 1 FOR nsw AT SEALEVEL, OR x:1
Total
Alkalinity (AT)
(mEq/l) . . . 2.0 .. 2.5 .. 2.6 .. 3.0 .. 3.5 .. 4.0 .. 4.5 .. 5.0 ... 5.5 ... 6.0
(dKH) . . . . 5.6 .. 7.0 .. 7.3 .. 8.4 .. 9.8 . 11.2 . 12.4 . 14.0 . 15.4 . 16.8
pH __________________________________________________ _________
7.7---------3.0 .. 3.7 .. 3.9 .. 4.5 .. 5.3 .. 6.0 .. 6.8 ... 7.5 ... 8.3 ... 9.1
7.8---------2.3 .. 2.9 .. 3.0 .. 3.5 .. 4.1 .. 4.7 .. 5.3 ... 5.8 ... 6.4 ... 7.O
7.9---------1.8 .. 2.2 .. 2.3 .. 2.7 .. 3.1 .. 3.6 .. 4.0 ... 4.5 ... 5.0 ... 5.4
8.0---------1.3 .. 1.7 .. 1.8 .. 2.0 .. 2.4 .. 2.7 .. 3.1 ... 3.4 ... 3.8 ... 4.1
8.1---------1.0 .. 1.3 .. 1.3 .. 1.6 .. 1.8 .. 2.1 .. 2.3 ... 2.6 ... 2.9 ... 3.1
8.2---------0.8 .. 1.0 .. 1.0 .. 1.2 .. 1.4 .. 1.7 .. 1.8 ... 2.0 ... 2.2 ... 2.4
8.3---------0.6 .. 0.7 .. 0.7 .. 0.7 .. 1.0 .. 1.2 .. 1.3 ... 1.5 ... 1.6 ... 1.8
8.4---------0.4 .. 0.5 .. 0.5 .. O.6 .. 0.7 .. 0.9 .. 1.0 ... 1.1 ... 1.2 ... 1.3
8.5---------0.3 .. O.4 .. 0.4 .. 0.5 .. 0.5 .. 0.6 .. 0.7 ... 0.8 ... 0.9 ... 0.9
8.6---------0.2 .. 0.3 .. 0.3 .. 0.3 .. 0.4 .. 0.4 .. 0.5 ... 0.6 ... 0.6 ... 0.7
__________________________________________________ ____________
Once the total
alkalinity and pH are known, the ratio between the partial pressure of CO2 due to the atmosphere on seawater and the actual pC02 of the system being evaluated can be determined utilizing the above chart. With the value from the chart known, you can decide whether you need to start ventilating the tank with outside air to your skimmer (or fans to the surface or remove glass tops) to degas the CO2 content or if you need to increase your
alkalinity.
Think of conditions that might contribute to either excess CO2 production, poor exchange of CO2, or conditions that allow for accumulation of CO2 as probable causes for your current issue if your measurements are corrected to the low side.
HTH
*
Somewhere I have a post that explains this in detail, I will look for a link over the next few days.
I play with water and electricty
