Since you had no replies I googled it. This is what I found.
http://www.reefkeeping.com/issues/2005-03/rhf/index.php
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Adding vinegar is another option for reducing pH. It has two actions that lower the pH. The first happens instantly, as the acetic acid releases H+ to the aquarium water (a process called ionization):
8. CH3COOH àCH3COO- + H+
This effect is shown graphically in Figure 8 (step 1). Then, over a period of time (perhaps hours), the acetate is metabolized by bacteria and other organisms, using up the available oxygen and producing carbon dioxide:
9. CH3COO- + O2 à2CO2 + H2O + OH- àCO2 + H2O + HCO3-
This effect is shown in step 2 of Figure 8. The net result of both reactions is that the acetic acid is converted into carbon dioxide, lowering pH (Figure 8). The real and measured alkalinity is reduced a bit by the initial vinegar addition (equation 8), but that loss is exactly replaced when the acetate is metabolized (equation 9). The only concerns with using vinegar are overshooting the pH target by adding too much, and the consumption of oxygen by bacteria metabolizing the acetate. With sufficient aeration or photosynthesis, that O2 loss is not necessarily a problem, but in some aquaria, adding too much vinegar might cause a significant drop in O2.
Figure 8. The two-step effect of vinegar on alkalinity and pH. The first step (simple ionization)
reduces carbonate alkalinity and pH, while the second step (bacterial metabolism) raises
carbonate alkalinity and reduces pH.
In another recent test, I bought a commercial bottle of distilled white vinegar (Heinz; Figure 9), and added it to my sump. The sump was stirring well with a large skimmer, but was not circulating through the main display tank during this test. The ~38 gallons of sump water's pH was initially measured at 8.53. After 25 mL of vinegar were added and allowed to mix in for a few minutes, the pH dropped to 8.41. Another 25 mL of vinegar dropped the pH to 8.15. A third 25 mL dose dropped the pH to 7.88. These data serve as the basis for the recommendation that I make later in this article of using 1 mL of distilled white vinegar per gallon of aquarium water to achieve an initial pH drop of about 0.3 pH units.
Figure 9. Heinz Distilled White Vinegar,
used to lower pH.
Adding a mineral acid, such as hydrochloric acid (muriatic acid) or sulfuric acid, will reduce pH, but will also reduce the alkalinity (Figure 10). This alkalinity loss is not returned as in the case of vinegar, when the vinegar is metabolized. These acids also are usually very concentrated, so it is very easy to grossly overshoot the pH target (Figure 11). For these two reasons, I seldom recommend using such acids to reduce pH (although they can be used effectively under certain circumstances when the aquarist is acutely aware of their drawbacks). I showed experimentally in a previous article that adding enough hydrochloric acid to reduce alkalinity by 0.5 meq/L (1.4 dKH) instantly dropped the pH from 8.10 to 6.91. Then, as the water released excess carbon dioxide into the air, the pH rose back up to 7.91 after 24 hours, and finally to 8.15 after 48 hours (the same water without acid treatment rose from pH 8.10 to 8.11 to 8.21 over the same time frame).
Figure 10. The effect of hydrochloric acid on alkalinity and pH.
Figure 11. The effect of excessive hydrochloric acid on alkalinity and pH.
Adding a buffer is a very poor way to control high pH. The best option in this regard is to add straight baking soda, which only slightly lowers pH and provides a large boost to alkalinity (Figure 12). I showed experimentally in a previous article that adding enough baking soda to lower pH in artificial seawater by 0.04 pH units raised alkalinity by 0.5 meq/L (1.4 dKH).
Figure 12. The effect of baking soda on alkalinity and pH.
Considerations Prior to Solving pH Problems
The following sections provide specific advice on how to go about solving a high pH problem. The advice can also be used to adjust the pH levels closer to natural values even if they are already within the "acceptable" range described above, but are still not as low as desired. Before embarking on a pH altering strategy, however, here are some general concerns.
Make sure that there really is a pH problem. Many apparent pH problems are really measurement problems rather than real aquarium problems. This issue seems to be especially common when the aquarist is using pH test kits, rather than electronic measurement with a pH meter, but all methods can and do go wrong. Avoid turning a good situation into a bad one simply because a pH meter was not properly calibrated. Also, when not adding limewater or other high-pH additives, a pH reading above pH 8.5 is most likely an error.
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Hope this helps!
Charity
