raising Ph question

you need to figure out why the tank water is so different from your tap water. do the following: get a glass of water from the tap and immediately take the pH. then let the water stand in the glass for 24 hours and take the pH again. what are the results? also, either test yourself or take a sample to your LFS, and determine the gH and kH of your water ... these will have a significant influence on your pH.

generally, CO2 in your tap water will have a carbonic acid equillibrium. this acid causes an acid pH (like yours). when the carbon dioxide gasses off, the equillibrium shifts in an upward direction and your pH will shift from acid to more basic. also, calcarious rocks and/or gravel can elevate your pH as carbonate salts leach out of these materials. pH swings in the opposite direction (like yours) are unusual or at least, less typical.

driftwood, certain types of ornaments can have a pH decreasing effect but generally, it takes alot of drift wood for this to happen. do you have any?

to answer your question specifically, your best bet would be to add some crushed coral to your substrate or in a nylon bag in your filter. the issue here will be how much to add and that can only be determined by experimentation. in the final analyses though, you want to know why your pH changes from tap to tank to insure that you find the cause and solve the problem rather than just putting a bandaid on the result.