Planted Tank Guru
Join Date: Jun 2004
Location: Baton Rouge, LA
CO2 is used in photosynthesis. No light, no photosynthesis. CO2 levels are ignored in the dark, so no need for it to be there at all.
When the lights turn back on, the CO2 is again needed. And if it isn't already there, or isn't yet close enough to the normal amount, then that's potentially harmful. The plants may not have enough CO2 for the amount of light they're given. And even if there's technically enough CO2, if it's less than the amount they're accustomed to, it will take them a while to stop being "lazy" and collect it more efficiently; during which they also won't get enough CO2. There's a better and longer scientific explanation of course, but this sums it up well enough.
That's why folks who turn it off at night, turn it back on before lights-on; giving it an hour or however long needed to come back up to near normal daytime levels.
When a DIY system starts to burn out, the decline in CO2 levels occurs progressively, regardless of day or night. It's the lack of CO2 during the day that gets you.