That pH is acceptable. The most important aspect of reef pH is stability. As long as your pH is stable it doesn't matter much if 7.8 or 8.6, relative to stability. What you don't want are large changes in pH such as those that come when trying to alter the pH with buffers, kalk and other pH altering products.
10% water changes will do little to change the pH. You are best to leave it alone and let your tank "naturally" find its pH sweet spot based on your stocking level, maintenance, CO2 saturation etc.