Well, the ph of your tap water is 7.6 and the ph of your tank is 8.0. If your using tap water to fill your tank and there was nothing in the tank to raise the ph, the tank water would also be 7.6. Save for maybe some outgassing which might affect the ph slightly.
What all this means is that there is probably something in your tank raising the ph. It probably should be removed if you can figure out what it is. The point being that even if you add the bottled water with a ph of 6.4 it will probably rise back to 8.0 from the substance in your tank which is alkaline. Whatever that may be. A ph of 7.6 aint bad to start with and it can be lowered with co2 and water softening pillows. Again if theres something in the tank that keeps yo-yoing your ph back up you'll be forever fighting this ph battle. By the way I posted the "Anonymous" post, I gues I wasn't logged on.