The hardness of tank water being much higher than tap water means that you are not changing much water very often. Simply adding water is not enough, it lets the mineral content of the water build up over time since only pure water evaporates off. You will also get a build from the food that goes into the tank since it has nowhere to go unless you do a water change. If your tap water and tank water are quite far apart on hardness, make several small water changes over several days so that the tank water gradually becomes more like the tap water. Once you get the water closer to tap water, you can do larger water changes to keep the nitrates down without stressing the fish because they won't have become acclimated to the high mineral content in the tank. If you are worried about the pH that much, draw the water a few hours before the water change and let it gas off. It should come to the same value as the tank.
