During the day, the algae within coral tissue (zooxanthellae) along with algae within the tank are photosynthesizing, so they are assimilating co2 and releasing o2. This drop in co2 causes ph to rise and stabilize with a higher hydrogen reading, thus higher ph. At night, when that photosynthesis stops, co2 begins to build up and ph drops. CO2 has an acidic effect on water, thus lowering ph. There are two options to buffer that and minimize these fluctuations. One, a large refugium (about 25% the water volume of your tank) filled with macroalgae that is reverse lit (meaning the light kicks on at night, when your tank lights go out, and kicks off when your tank lights come on). Then, the photosynthesizing algae can absorb some of the excess co2. Two, adding a co2 reactor to your protein skimmer. These use medical grade co2 absorber to pull co2 from the air entering the skimmer, thus helping to dilute co2 in the tank water. This may be an issue with carbonate/calcium absorption by the corals within your tank. Are you dosing and if so, what are you using. Likely a more effective way to minimize these swings and buffer ph would be the addition of a kalkwasser reactor.
