Sounds like you purchased
this
I think before you add anything to your tank you should know what your PH Alk and Ca are. Before adding the buffer I would purchase test kits for all 3 and measure. 7.9 PH isn't too bad, When was that measurement taken?
Alk is not something that would be regulated by RO/DI water.
There are a ton of Articals in books and on the web related to
Reef Chemistry here is one