.6 ph swing day to night

Hi i have a mixed reef tank mainly Lps. Tank is about 2 years old. Its 75 gallon display with a 15 gallon sump. I have recently added a phons ban reactor using bio pellets. I also have a euro reef skimmer and a razor led. My DKH is 8, calcium 420, nitrates >5, light period of 11 hours a day (seems long) however they gradually get bright. and my ph swings from about 8.15 to 7.50 at night. I have a clip on light with a led bulb that grows one mangrove and about 80 heads of a pest like zoanthid that grows like crazy in display tanks. I do regular water changes 15-20 gallons using kent reef salt and RO/DI every week followed by a bi weekly change. The PH swing is noticeable in my corals throughout the night. Any help would be appreciated.