Most of the sites I've seen recommend neutral or slightly acidic ph for a turtle like a slider. Here's the relevant excerpt from this site - read about half way down the page:
http://www.redearslider.com/water_quality_issues.html
Where I am, tap water is alkaline - 8.5 or higher. I was having fungus problems with my turtle's shell so I lowered his ph and he's doing ever so much better.
Lowering the ph was NOT the only thing I did to help his shell - I changed up just about everything else too, but ph was one of them and he's definitely doing better. I keep the ph now between 6.8 and 7.2.
One caution - I learned the hard way that if you lower the ph too much (like in the low 6's) you'll kill off the good bacteria in your bio filter.
Here's another excerpt from Austin's Turtle Page that pretty much confirms what's on this site:
"Why is pH so important?
The first major concern for pH with turtles is that some turtles require a certain pH setting. For example, Diamondback Terrapins require a pH rating of around 7.5, whereas Mata Matas require a pH setting of about 5.. For a happy, healthy turtle, the proper pH is a must. Without the proper pH, there could be complications that occur.
Secondly, a more acidic pH is desired because of its ability to prevent certain strains of bacteria and fungal outbreaks. The lower the pH, the less likely certain types of these nasty intruders will have in surviving. The higher the pH rating, the more these nasty things can develop and thrive in the turtles' habitat.
Most people's tap water has a rating of about 7.5 - 8.5. This will work for most North American species, but when get into the more exotic species, then you will have to pay close attention to the pH levels. With the North American species, you can get by with lowering the pH levels to about 6.2 to assist in preventing shell infections, but if you own a softshell, I would not go any lower than 6.5 for fear of burning the softy."
