That is some soft water!
Well, buy the GH booster from Gregwatson, you'll need it.
Adding KH is not critical, you might add a 1/4 teaspoon per 100 liters of tank of baking soda after a water change if you are so inclined.
I would add about 1/2 teaspoon of the GH booster definitely though.
Tap sounds pure and very good.
Do not worry about the tap water's pH, it does not matter, what does is the GH and KH.
Those are ppm's and not degree units(which are 17.86 ppms per unit) correct?
Just making sure there.