Tap water, fish waste, and a build-up of natural acids can make the pH levels fluctuate dangerously.
Experts say an 8.0 to 8.4 range is good. Some have even gone as far as 7.8 to 8.5 but that might alittle to extreme. Natural sea water is about 8.2 for reference.
What is your alkalinity levels at? Higher alkalinity can raise your PH also.
As posted by pogodzib, higher CO2 will lower PH. Check your levels.
Read this article, it may help.
Good Luck
Low pH: Causes and Cures by Randy Holmes-Farley - Reefkeeping.com