ph swings naturally at night due to the respiration period of the corals/algae/etc in lieu of the lighting period in the main display.
corals use co2 during the day for photosynthesis, at night everything respires and releases co2 which is why you're seeing the drop.
1. first thing and easiest change you can make is to crack a window in the room of the tank to vent in fresh air from outside. if your home is air tight and has little exchange... your own breath + pets will raise the co2 in the room and cause the ph to fall. this affect is more noticeable when everyone is home sleeping, then everyone wakes up in the morning telling each other what they have planned for the day over breakfast... hence your ph drop happens consistently at 5-7am.
2. if you have a skimmer; you can route your skimmer air intake line outside to bring in fresh air. this is going to aerate your water with fresh air. bonus: you can further this effect by co2 scrubbing it if you're not lazy about switching media. i hear it burns up quick, but maybe someone else can chime in.
3. if you have a fuge; you can run the light cycle opposite hours of the day time lights to fight the drop with macro algae photosynthesis.
4. if you're dosing kalk, you can do so at night to push your ph up in the evening hours.
no matter what, you're going to see a swing. use these options as ways to minimize that swing. i wouldn't chase it.
tldr: open a window, skimmer air intake from outside, opposite light schedule on fuge, dose kalk at night. don't chase it.
