Nynbaul

My tap water has a really high ph around 9.. I keep my tank around 7. How should I do water changes to keep the pH from changing to much in the tank?

I have a 60 gallon with one bristle nose pleco but he is lazy and doesn't eat any algae. I am curious if I put 4-5 ottos and 4-5 Corys in there if they can all get enough to eat.