Yes, to explain in more detail, photosynthetic critters (algae, corals, trees, grass, etc.) take in carbon dioxide (CO2) and release oxygen. When carbon dioxide mixes with water, it creates carbonic acid, which drops pH. Photosynthesis primarily happens in the light (it's a process fueled by light). So when photosynthesis is happening (when lights are on), CO2 is constantly being taken up by these organisms, so it's not getting a chance to sit in the water and create carbonic acid.
At night time, most of the photosynthesis shuts down, so CO2 accumulates in the water, forming carbonic acid, and dropping the pH (making the water more acidic).
This is why the pH is lowest first thing in the morning, after the lights have been off for a long time. And this is why pH tends to be highest at the end of the day, after the lights have been on and organisms have been photosynthesizing all day long and removing that CO2 from the system.
If you run your fuge 24/7 or opposite the tank lights, it allows whatever is in your fuge to continue photosynthesizing during that time. So there will always be something in your system taking up carbon dioxide, and preventing the pH from dropping. Running a refugium on an opposite or 24/7 light schedule is one of the easiest and most effective ways to prevent pH swings.
