IME, years ago i had the same problem minus the lowish pH. I was chasing the Ca and alk and could not balance them. I then stopped with the additives other than kalk for top off. I started doing 15% water changes every 5 days to get things back in order. it is difficult to maintain high Ca...
I believe Its the lighting, duration & color temp I run 2 175watt 10k for 11 hrs and a 400watt 14k for 8hrs over my 36wx36dx25h (140g) sps. I also feed my corals cyclopeeze and brightwells zooplanktos L daily... overall I feel your duration and color temp needs to be adressed