Found this on the net..... seems like more about freshwater, but still interesting...
The term pH stands for the Power of Hydrogen & is defined as the negative logarithm of the hydrogen ion concentration. In other words, pH is a measure of the amount of hydrogen ions in your water.
The pH scale runs from 0.0 to 14.0. Values less then 7.0 are acidic, 7.0 is neutral, & values greater than 7.0 are basic. ( basic has formerly been referred to as alkalinity. Alkalinity is a measure of the buffering capacity of water.) Because pH is a logarithmic function, a change in pH from 6.0 to 7.0 would represent a ten-fold decrease in the hydrogen ion concentration; 6.0 to 8.0 would be a 100-fold decrease. In other terms a pH of 6.0 is 100 times more acidic than a pH of 8.0.
pH in the aquarium is an important factor. Not only do fish require a certain "safe" range to survive, but pH is an important controlling factor for many chemical balances, including the ratio of nontoxic ammonium ion (NH4 +) to toxic ammonia (NH3), & between the toxic nitrite ion (NO2 ¯ ) & very toxic nitrous acid (HNO2).
For most species of fish, the pH in a freshwater aquarium should be between 6.5 & 7.8. At pH levels below 6.5, the growth & survival of nitrifying bacteria becomes reduced & possibly ceased. If pH levels below 6.5 are to be used (specialized cases only, i.e. breeding dwarf cichlids) frequent water changes are vital to prevent the accumulation of nitrogenous wastes (ammonia).
Correcting low pH The following methods may be used to correct low pH.
A water change will replenish the natural buffers, from your water supply, unless they are normally low to start, in which case the pH may revert to low levels in a few days. If low pH is a recurring problem, the water can be buffered using commercially available buffers preset to various pH values, usually between 6.5 & 7.5. If such buffers are used, they must be used in sufficient amounts to hold the pH at the desired level until the next water change. The amount of buffer to add can be determined by trial & error. Adding more buffer does not significantly change the pH, but only extends the time that the pH of the water will be held at the desired level. Generally, in soft-water areas where acid rain is a problem, buffers should be added with every water change.
Dolomitic limestone, coral gravel, or oyster shell can be incorporated into an outside filter or an inside corner filter. Ammonia absorbers also buffer freshwater to approximately pH of 7.5. The amount used must be by trial & error. With time, the natural carbonates from these natural elements will become exhausted, & fresh materials may be required. Be sure to monitor the results, otherwise the pH may become too high.
Sodium bicarbonate is another pH adjuster, but not a buffer. If added in excess to the water, the pH can be elevated to unacceptable high levels. Gradually add small amounts with frequent testing of the pH. Because it's not a buffer, the level may not hold for long.
Correcting high pH
High pH water which is usually hard, high in alkalinity & well buffered, usually requires removing of some of that natural hardness & alkalinity. Mixing softened with unsoftened water could result in water with a desired pH. This must be done by trial & error.
Softening the water with commercial water softeners, which remove calcium & magnesium ions by an exchange process with sodium ions, will also help to reduce pH.
Commercially available buffers are very effective in soft-water areas but not effective for decreasing pH in very hard water. However, after the water is softened, buffers can be added to adjust pH.
