It was a looong night...
Thanks for the replies IslandReefer and Nemo!
Re: the pH monitor; the first thing that I did was check it with pH 7 and pH 10 solutions. It was exactly on - didn't have to make any adjustments. The probe is 6 months old and I always use a 9v battery - had heard that the adapter could give strange readings with interference.
The tank's daytime high is ~8.2 and I've had lows down to 7.7 in the early am. I generally try to keep dKH at 10.5, so it dropped a bit lower than usual yesterday. I find that I need to add SC Reef builder every 2 days to maintain that level. I do have plans to run a lighted refugium opposite to the tank's photoperiod, but (like everything else) it's still not completed...
The lights had been off for approx 3 hrs when I got home, and most everything was in sleep mode, so it was difficult to tell if anything was stressed. I did end up dripping in a dose of reef buffer (pH had dropped to 7.60 by that time). It slowly came up to 7.70 But this am it had dropped back down to 7.59
When the actinics come back on, I'll have a good look at the inhabitants
Thanks again for the help!
~Tamara