I use Seachem Reef Buffer that is suppose to maintain if dosed alone at no more than 8.3 however with coinciding chemicals my actual pH ranges at 8.3-8.6 ... if you cannot get your pH above 7.8 there is something battling against it or not supporting it at proper levels ... perhaps another chemical out of whack or deficiency such as calcium & alkalinity. Check both and get back to us buddy! Also let us know if you're dosing calcium?!
Here is a direct question asked on Seachem's Website:
Q:The pH in my reef is low, what could be causing it?
A: The pK of the bicarbonate buffer in seawater is influenced by the calcium & magnesium content. With higher calcium levels, the buffer pK is shifted to a lower range; with high magnesium levels, the buffer pK is shifted to a higher range. Another factor to consider is that certain commercially available salt mixes contain too little carbonate and/or borate buffering and to much bicarbonate; this results in a shift to a lower buffering pK and an inability to buffer above 8.0. Additionally, certain organic acids being sold for the reef aquaria, can cause a constant battle with low pH.