If you can't establish a constant pH form the tap, use a buffer, pH to be exact.
Two ways (that I know of);
1) (for African tanks that like high pH) use Limestone for your decor, i.e. caves, hiding spots, aquascaping, and such.
2) Use a pH Buffer chemical like, API's "Buffer Max; African Cichlid"- Maintains pH Between 7.8 -8.8.
I keep a 75g also, but my Tap is coming out at 7, so I use a combination of both my reasons. I have enough Limestone in the tank that when I use the buffer for water changes, I only mix in 10% of what the instructions suggest.
just my $.02...