pH always dropping

It will. The reason is because when you have a refugium on the opposite lighting schedule, you have the macroalgaes present in the refugium producing oxygen during the night. It helps to prevent the carbon dioxide levels in the tank from rising significantly during the night.

when there's just a DT on a regular schedule, all photosynthetic organisms (microalgae, coralline, and corals among others) stop performing photosynthesis, instead respirating just like any other organism (like fish and people). When this happens, their net output turns from oxygen creation to carbon dioxide creation. Carbon dioxide in the water lowers pH.

During the day, the respiration will change back to photosynthesis, and the pH will rise again because of decreased carbon dioxide.

Running the tanks together will create a situation where the pH will drop at night and rise during the day. The cycles will pretty much create the situation that happens with "rogue waves" out in the ocean: the two cycles will be on top of one another, so you'll get a larger swing between the high point and low point of the pH.

When you alternate the cycles, you create a situation where the high point of the DT cycle corresponds with the low point of the refugium cycle, and thus the total variance from the average is minimized, creating a more stable pH.

The biggest issue with this is in more sensitive tank setups - ones loaded with SPS, clams, or other organisms that can't handle pH swings.


^^ If anyone wants to do a TL;DR version, go ahead.