Dang, 7.4PH with all those tannins and wood in there? That's crazy. However I'm in the same boat, my water is almost 8 out of the tap. Takes a lot to get my water down to a lower PH. I've had cory's flash like this before, usually a water change fixes the problem, sometimes they just do it because they like the sand. I'm not sure. If it becomes excessive flashing, then you could do some salt in the tank and see if that helps ease things.