I would agree that water parameters are better left alone unless you are trying to breed a specific fish that requires soft water or if your water supply is at a radical end of the pH and hardness scale.
If you feel compelled to change it you should either cater to a local fish store and buy their RO water and figure out a ratio of your tap water to mix with RO water to achieve the desired parameter or go to walmart and buy distilled water to mix with your tap.
If you are going to try and change your water parameters, and again I think you shouldn't,,,but if you are,,,then you should create water at the desired parameter BEFORE it goes into your tank. And then when you do your water changes use this manipulated water and then the change will be very gradual. You do not want to suddenly change the parameters of the main tank overnight.
Bill