I've been dealing with this for months. There's a few threads on it recently. On a typical day pH in my reef peaks around 8.10 and then over night bombs out below 7.80. Last night it bottomed out at 7.75, as it has all week, and it looks like today's high is going to be on 8.08 ish. dKH is typically around 8 and Calcium 440 ppm.
In my case, it's 100% based on CO2 levels in the water which is 100% based on CO2 levels in the air in my apartment. I've employed a CO2 scrubber and it helps dramatically - but at a steep price so it's not an option long term. The air input to the skimmer runs through a canister of CO2 absorbent media (soda lime) from BRS. With a fresh canister of media tank pH will likely rise to 8.30. The price/problem is that the CO2 media exhausts in a day, maybe two. Possibly even hours. The media changes from white to purple when spent. In my case, the media turns purple usually in 12 hours. If I was to change the media right now (midnight here) I'll probably see the pH bottom out much higher than last night (7.95) and probably peak over 8.20 tomorrow...but by tomorrow at this time the media will be 100% purple, aka spent. So yay. Changing the media daily isn't going to work both in terms of my sanity or my wallet. The stuff isn't cheap.
Opening a window helps DRAMATICALLY - problem - it's winter. Wife will murder death kill me if I leave the living room window open - she's a girl, she gets cold in July. Also the window is 25 feet from the tank so...yeah.
I'm willing to guarantee if I had an EPA approved CO2 meter in my apartment it would say "your home is unhealthy".
There are 2 return vents from my building in my kitchen (near the tank) and the bathroom (not near the tank) Neither suck they way they should - or at all. My super doesn't know why not. It's frustrating.
I now use both saturated (pH 12.4) kalkwasser via a stirrer dosed every hour and BRS 2-part (soda ash, the kind of stuff that raises your pH). I have noticed pH spike after 2 part doses, but it creeps back down after each dose. Kalkwasser just seems to help me not have to dose so much 2-part, especially calcium.
The only thing I've got left to change is to bring back a refugium, light it 18 hours a day, and fill it with big green leafy macro algae (ulva, mangroves, not sure what else)
pH < 7.80 has been known to cause coral calcification to halt. In my case I keep murdering SPS. It's not anything else in my reef...even my lights are a known winner (ATI T5) so...yeah.
As you can tell, this is a sore subject with me. At this point if the refugium doesn't work I'm out of options and will just live with what I can do and probably not add much else in terms of SPS, ever, until I move into a house and can either put the tank near-er a window or duct fresh air to the tank. (which will cause my wife to think I'm nuts)