Explanation on pH cycles

[KyleP]

I've read about the reverse fuge cycling people do. Fuge light on when display is off and vice versa. Can someone explain how this affects the pH?

[landlord]

In its simplest form. When the lights are on the pH rises so having the fuge on a reverse lighting cycle from the main tank will ensure that the lights are always on somewhere.

[landlord]

....resulting in less of a swing.....

[seapug]

pH will only rise if there are photosynthetic algae or corals under the lights.

Photosynthesis uses CO2 and releases oxygen, which raises pH. At night, the photosynthetic algae and animals consume oxygen and release CO2, which causes pH to drop. Having the display and refugium on opposite cycles will help offset the pH drop that occurs when one or the other is on its dark cycle.