It is very desirable to keep the change water the same for every water change, and to have that water be about the same as what was in the tank - not putting high GH water in a low GH tank, or high KH water in a low KH tank, etc. There are not many fish that will be bothered by pH, whether high or low, if you don't dump in water that has a big hardness difference from that in the tank.
When you try to adjust the hardness or pH of the change water you start a never ending problem. Every time you add water you need to adjust that water the same as you did the first time. That can become a really big nuisance.