Measured at the start and end of the photo period 8.28 -8.34 although the drift is a fraction wider in summer months with warmer air temperatures ( i feed my skimmer air lines in from outside)
I've had mine drop to those levels before. I usually add a little buffer to bring the high end back up and then it doesnt swing past 8.0. I tend to add a bit every 2-3 weeks.