water should be a neutral Ph of 7. What are you using to test Ph?
You are seeking stability as much as anything. At lower Ph levels, you need even more calcium saturation for corals.
In older tanks, it is hard to stay above 8.2 as there are so many organisms making C02, even with high Kh
One sure fire way to raise calcium and Ph levels is a Kalkwasser drip at night. I have also installed fans that kick on when Ph starts down to encourage good gas exchange in the system.