Baking soda will raise it slowly. I forgot what the formula was, but it's on this site somewhere.
Also, some LFS
sell a buffer that will raise it to 8.3, even if you over dose.
I have a slow PH
dropping issue as well in one of my tanks. It's because of my tap water, I'm not using RO
water. Are you using RO
? What kind of substrate do you have?