Simple solution:

Whenever you want to change your pH. Use water that is pH adjusted to the pH level you want the soil to be and water with lots of it. continue to flush the plant with pH adjusted water.
In your case, since you want your pH to be 6.5, just water with lots of 6.5 adjusted water.
This saves you major troubles with pH swing in the grow medium making nutrient absorbtion more efficient.

Avoid trying to use high pH water to correct low pH problems and visa versa

hope this helps