the carbonate concentration (what LPS needs to form their calcium carbonate bones) is approx 50% less in pH of 7.7 vs pH of 8.0. In those conditions, LPS will likely need to use bicarbonate to form calcium carbonate, which it can do, but takes more work. When it uses bicarbonate to make calcium carbonate, extra +H ions remain which causes the pH inside the coral tissue to be actually lower that 7.7 and the corals need to use extra energy to pump them out. the "lower than 7.7 pH" environment inside coral tissues, in turn, make calcium carbonate formation even more difficult. This leads to corals just generally more brittle and more susceptible to other environmental stressors and lessens the long term success rate. When the tanks other params are optimal, 7.7pH is passible but you will have little margin of error. When there are other environmental stressors, as may be the case in OP's tank, a 7.7pH will cause the LPS to be the first to go.It is worth checking, but I've never seen tissue recession and polyp bailout from low pH. A pH of ~7.7 is the generally accepted minimum "safe" pH. Test it in early morning before the lights come on when pH is generally at it's lowest. Some tanks swing as much as 0.4pH throughout the course of the day.