It is unusual to have such a low GH and such a high pH. Are you sure that the GH is 13 ppm and not 13 dGH? And, contrary to what so many people post here, a high KH does not keep the pH from dropping as you add CO2. If you manipulate the equation that relates CO2 ppm to KH and pH, you get:
initial pH - final pH = log(final ppm of CO2 divided by the initial ppm of CO2)

So, the change in pH for a given change in ppm of CO2 is not related to the KH at all. KH is important only because some plants need low KH to do well, and some need moderately high KH to do well. But, most plants do well over a big range of KH.

If that were my tank I wouldn't worry about having pH 8.0 tap water.