My tap water pH is also about 9 before mixing, after mixing will drop to ~8.2. The other thing is maybe wanna check your ph monitor, might not be calibrated correctly.
Dunno why I can't seem to increase the pH of my tank above 8. It seems to stabilize at 8 no matter how much seachem reef buffer I use. There was once I added so much tt my KH shot up to 19 over 5 days!!! Prob one of the reasons Gouldian stated. As long as it doesn't drop below 8, I guess it should be fine right?