I know people that keep their fish in water of 8.5ph, even discus! No problems! In actual fact, contrary to popular belief, most tropical fish tolerate a wide ph range. My discus are raised in 7.6ph as with most discus breeders as well (tap water ph). However, getting closer to 9.0 is worrisome! Again, steady water parameters are the best. If changing ph, you must ensure that it is steady all the time. To do this, you need to use a holding tank or barrel where you can safely adjust ph. NEVER IN THE TANK. I have tried changing ph when I started into discus from 7.6 to at least 6.8! I never did get a chance to use that water because my ph came down to 7.2 then crashed (under 5.0). So, why bother with the pain.
If you do have high ph (above 9.0), I would recommend using a RO system instead, mixing with a little tap water.