If you can bring the pH down to about 7.2-7.3 using CO2, I think that'll be enough for the tank. A higher KH just needs more CO2 to bring it down, if your using DIY CO2, just add another bottle or change the recipe, if pressurized, just turn it up. 7.5-7.8pH according to the charts, is giving you less than 10ppm of CO2. You could use RO water to bring the KH down, but it's easier to do it with CO2. HTH