For me, the primary reason to turn off the CO2 at night (or light-off time) is to save CO2 as during this time, CO2 is of no use as photosynthesis does not take place. The secondary reason is to prevent OD as what you mentioned. This is less likely as my set injection rate is getting me only about 10 to 20 ppm of CO2.
From past experience, DIY CO2 with 2 big COKE bottles to my 3 footer have not give my tank more than 15 ppm of CO2. Hope this can do you some help.
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