pH dropping steadily tonight

[Shy_Koi]

Arggg - I hope someone is still up, I need some advice.....
Just got home from work and noticed that my tank's pH was 7.63 and dropping
It's usually still around 8.0 or so at this time of night. Morning low is around 7.8.
Checked the calibration of my pinpoint monitor and it's bang on. Also just ran some tests:
Salinity: 1.026
dKH: 9.5
Ca: 400
Mg: 1280
Temp: 79
No refugium
Water surface has good movement and there's a skimmer running.
I use Seachem's reef advantage calcium and reef builder to maintain Ca and alk.
Should I go ahead and drip in some pH buffer?
I've never used it before, but I'm afraid the tank may crash if the pH keeps dropping like it is....
If anyone has any advice, I'm all ears!
~Tamara