Here is what I found on the web...It kinda hints at what you were talking about.
This link is a pretty informative article...I'm sure you have read it though...
http://saltaquarium.about.com/gi/dy...bts=1&zu=http://www.amdareef.com/ho_chem1.htm
The normal trend for pH in a tank is downward, or more acidic. The additions of acids into a tank will lower the pH in the tank water. These acids come from several sources, the primary ones being: (1) excess carbon dioxide (CO2) from respiration caused by lack of sufficient gas exchange, (2) nitric acid from biological filtration (nitrification), and (3) organic acids from metabolic wastes. Respiration and metabolic wastes are a natural part of the ocean. The reason that sea water pH does not change is that sea water contains a number of chemicals, such as bicarbonate, calcium, carbonate, borate and hydroxide, which act as natural "buffers" that retard the drop in pH.
When the pH in a tank starts to drop, it is an indication that the buffers are getting worn out. There are a few things that you can do to remedy this increase in acidity. You can use "quick fix" methods by adding bicarbonate of soda (baking soda)
The generally accepted "tried and true" method for stabilizing pH is still performing regular partial water changes in your tank. This not only refreshes the natural buffers, but also restores the trace minerals in your tank water. Of course, reducing the causes of the drop in pH is always wise. Removing all uneaten foods and fish detritus from the tank on a regular basis will go a long way toward retarding the pH drop.