The reason you can't easily adjust the pH by mixing tank water, high pH, with distilled water, low pH, is that pH is not a linear measurement. If we were talking about KH or GH, which are measured in parts per million, the calculation for the final concentration when mixing water having two different concentrations is easy. But, pH is the logarithm of the concentration of OH- ions in the water. (I may have that slightly wrong). So, water with a pH of 6 has a tenth of the OH- ions as water with a pH of 7. To further complicate it, we can't add ions of anything to the water, we can only add a compound that dissociates into an equal number of negative and positive ions when in water. And, some compounds have those ions changed when in solution when they are there with certain other ions, and those changes can be quick or slow. It is best to just look at a pH reading as something interesting, and then forget it!