O to bardzo dziwne z tym PH.
CO2 w chacie zmierz 
Tu fajnie opisane :
Suppressed or low kh levels can cause ph drops, but generally the solution is simple. Here are a few thing to try.
I know you say you have a good water surface movement and circulation which is good but...
1 - Are the light close to the surface - High temp generated near water surface will suppress the ion exchange making it inefficient - solution raise lights, small fan to blow air across the water surface
2 - If you are in a modern house they are pretty air tight these days as buildings don't breath properly (commonly mistaken for a draughty house), air tight houses generally have a increased CO2 content as air exchange is more limited as the exchange with fresh air is more limited- solution open a window and let some fresh air in the room where the tank is in
3 - Skimmer - On the air intake put some air line and run it to outside to inject fresh air in the skimmer reaction chamber over stuffy air in the sump cabinet/room.
4 - Are you are getting large swings day reading (take one a hour before lights out) and night reading (hour before lights on), tanks get a swing going as the algea within the system work like a tree or any plant does...during the day algeas will absorbs CO2 and exhales 02 pushing up pH, but during night period the cycle reverses and the removal of 02 and production of CO2 from the algea suppress pH...you can even these swings out generally via a sump using the reverse lighting methods with algae.
5 - What do you does as some calcium/carbonate additive will suppress the pH due to the high acidity, perhaps putting the out put near a skimmer intake can quickly add oxygen to reduce the acidity of the reactor/dosing effluent.
Also overstocking especially with large bodied and very active fish can deplete oxygen and if you are not keeping up with demand because you may have poor ion exchange or suppressed oxygen availability within the room the fish can keep the pH suppressed also.