If your kH and calcium balance is OK, I wouldn't worry as much about pH, as it really comes down to CO2 at that point. CO2 levels go up at night and thus the pH falls. I also noticed the deeper I place the pH probe, the lower the pH is, so keep that in mind.
As for using that buffer, it's not the best idea to use it until you know your alk and calcium levels. To a lesser extent also magnesium.
Since you're putting the calcium reactor on hold, instead of that pH buffer, I would actually recommend to you a two-part. Kent makes a decent one IMO. There's part A and part B, both in liquid forms, added to different parts of the tank, or at different times, always in equal amounts.
http://www.jlaquatics.com/product/k-...+-+64+oz..html
If low pH is still a concern, you could also drip limewater (kalk) at night and/or have a reverse lighting refuge with algae.