Tap water, aged 48 hours:
pH 9.3
GH 14
KH 10
Tap water, aged 48 hours, filtered by peat moss 48 hours:
pH 5.2
GH 3
KH 1
I mixed the two to get the pH I wanted (5.2 would crash your biofilter).
I keep one of my tanks with a pH of 6.2-6.3. Works well.
Make sure you make any changes slowly to your tank though. A sudden change up or down is very harmful and can be deadly to some fish. I changed from 9.3 to 6.3 over 4 weeks time.