Ang,
I'm not sure if having the lights on for 15 hours would give you such a high pH, but it might. I've never actually kept the lights on our tanks for so long.
Yeah, reading all those chemistry articles takes a lot out of a person. I've read some of them a half dozen times and am still learning each time I read them over
It is normal for there to be a pH swing from just before lights on and just after lights out, but if the swing is over, say, .3 points, I always suspect alkalinity is too low. I try to keep mine between 8 dKH and 11-ish dKH.
A good thing to do when testing and adding additives is to keep a log of when you test, what your test results are, and how much you add of each additive. Here are some calculators for determining system volume and how much additive to add after chemistry testing...
Aquarium System Volume Calculator:
http://home.comcast.net/~jdieck1/volcalc.html
The Reef Chemistry Calculator:
http://home.comcast.net/~jdieck1/chem_calc3.html
HTH
