First of all, test pH at the same time of day. pH naturally varies throughout the day especially if you have plants.
If your KH is reasonably high, that will stop the pH falling even with tannins in the water. With a GH of 8, KH is probably similar and this would be enough to keep the pH stable.
What substrate do you have on the bottom of this tank? Cuts from sharp particles letting an infection get in is the usual cause of damaged barbels.