buffering ph by adding minerals

my tap water is ph 7 with a kh of 11 degrees
my tank water is also ph 7 with a maximum kh of 8 or 9 (it keeps
dropping see my other post called ph kh snails and clown loach)
i get falling kh and ultimately ph, so i want to add buffering
capacity to the water to maintain ph 7.0 if possible, but i am worried
that coral will buffer it up to 8 ish.....i just dont want to kill any
fish by experimenting.

chris