What's your Salinity and what's your Calcium level? If your Calcium is very high (over 500ppm) then that could be the problem. If your Slinity is low (below 35ppt) then there's another potential problem.
IMO your low PH is either due to inadequate circulation (excess CO2) or more likely you have an accumulation of detritus somewhere and it is rotting (A dirty filter, dead snail, etc.).
I am interested in your evening reading.