Ahhh I use RO filtered water. You can go to a grocery store and get RO/DI water from the U-fill it water container. OR you can buy it from teh LFS (around $0.35 to $0.45 around here).
I would not use tap water. Too many uncontrollable elements in tap water (as well as toxic elements that can't be totally filtered out with your tank's filtration - tin, copper, iron, chlorine, chloramine, etc.)
If nothing else there is a tap water purifier that will help reomve some of the harmful elements.