From this article
http://reefkeeping.com/issues/2005-01/rhf/index.php
Randy Holmes-Farley states.
"In such cases of acute overdose, here is my advice:
1. If the pH is 8.5 or lower (as it often is since a precipitation event itself reduces pH even if it was much higher to start with), there is little that can or needs to be done. Just wait a few days for the white calcium carbonate to slowly disappear. A water change is not necessary, although once the water is clear, testing calcium and especially alkalinity is in order (don’t bother to test the cloudy water as it will give false high readings as these tests detect solids even though they are not truly in solution). Few aquarists suffer the loss of organisms from such events. I’ve had several such events without any apparent losses.
2. If the pH is above 8.5, some action to reduce the pH is warranted. The higher it is, the faster and greater the needed action. Since such events may happen when few tools are available to solve them (e.g., New Year's morning when few stores are open), I’ll provide a number of options, although some are better than others. In all cases, reduce the pH only to 8.5 to avoid overshooting.
The best option is to add carbon dioxide, either by bubbling the gas directly, or by adding soda water/seltzer (or blowing into a skimmer inlet if it is your only option). At least in the normal aquarium pH range, a teaspoon of soda water per gallon of tank water will lower pH by a couple of tenths of a pH unit. Overshooting with carbon dioxide, while undesirable, is less of a concern than is overshooting with any other option.
A second option is to add vinegar. Be especially careful to not overshoot pH 8.5 or so, because when bacteria begin to metabolize the acetate, the resulting CO2 will further lower the pH, and oxygen will be consumed (equation (14)). For this reason, it is especially important to maintain aeration when using vinegar in such a fashion. I’ve added vinegar to my aquarium in similar situations without difficulty, although the pH was only marginally high and I did not need to add much.
A third rung of options involves adding a mineral acid such as muriatic acid (HCl or hydrochloric acid) or sulfuric acid. I’ve added HCl to my aquarium in similar situations without difficulty. When performing such a mineral acid treatment, be very careful not to overshoot, and to monitor the pH during any acid additions. I would intervene in this fashion only if I could monitor the pH in real time, and could add the acid to a high flow area far from any organism. Diluting the acid in water (say, 20:1 or 100:1) prior to adding it to the tank is highly recommended for the safety of both the aquarist and the tank’s inhabitants (diluting vinegar, which is already dilute, isn’t necessary). One other drawback to adding a mineral acid is that it reduces the alkalinity. In such a case, the result may be elevated calcium and reduced alkalinity that will require significant correction."