During the day when the tank lights are on, anything that photosynthesizes light gives off o2 and use up Co2. The more o2 and the less Co2 in the tank the higher the pH. At night when the lights are out, all the animals are using the o2 in the tank and giving off Co2, plus no photosynthesis taking place. The only new o2 going into the tank is through air exchange. A big refugium with macro algaes and other algaes in it, runniing at night is a place for photosynthesis to take place, using up some of the Co2 and adding some o2. Thus off setting some of the pH drop you would find in a tank without the refugium. And hopefully minimizing the pH swings between the high after a day with all the lights on and how low it would go after a night without the lights.
