http://www.redearslider.com/water_quality_issues.html
"pH
pH is a measure of acidity or alkalinity of a solution. Through a scale of 0 - 14, pH = 7 is neutral; a pH below 7 is increasingly acidic and above 7 is increasingly alkaline. Pure water has a pH of 7. Slightly acidic water is desirable as it can prevent certain outbreaks of bacteria and fungus where as greater alkalinity may encourage outbreaks. Check your local aquarium supply store for materials that can test your water quality and adjust your pH levels. pH levels between 6-8 is normally considered safe for RES.
Some simple methods of lowering pH are adding peat (which will also reduce water hardness), bogwood to your tank or partially adding water that has been softened, distilled or been through reverse-osmosis (RO). Water pH can be raised by adding limestone (which will also increase the hardness), by aerating the water or by the use of a buffering agent."