Buy a test kit and test your kH...kH is crucial to prevent pH drops. Did you take note of the time you measure your pH? pH levels are lowest during early morning before lights are on...pH levels tend to be highest just before lights off...
You can consider dosing kalkwasser to prevent pH drops during the night.
Check the stocking of your tank..overstocking can lead to low pH levels too..
LPS tend to prefer meaty foods like mysis...DT phyto is mainly for filter-feeding organisms like tubeworms, scallops, clams etc.