When I first used DIY CO2 with yeast. I would stick an air pump on over night. I just thought it made sense to make sure there was enough oxygen. I now know I can just use surface agitation.
I could never get to sleep with my air pump but I put up with it, I did so much research on do I need an air pump with DIY CO2 etc and I never found an exact answer, as everyone is different.
However when I went to pressurized I lowered my surface agitation to help CO2 levels, and my regulator came with a solenoid! So I decided no more air pump.
I already new from various research the best time to tell if fish are struggling is around 5-6am in the morning, I'm normally a sleep then so was a bit hard to check that.
When I somehow woke up around them times, I noticed all my fish in the top right hand corner next to my filter, all together. They wasn't gasping or nothing. They was either there because there is hardly any flow there or its where the most oxygen was, being close to the surface and out of the direct flow from the filter.
Another time I've checked when I've increased surface agitation, they have all been at the substrate, some at the middle, just all dotted around. So I presume if your fish are acting fine at 5-6am in the morning then there's your answer. I would not feel comfortable leaving my CO2 on over night, the plants and fish are competing for oxygen, but I read adding CO2 don't drive oxygen away and both remain in the water so I guess its more to do with your tank oxygen levels rather than leaving CO2 on 24-7. Either way I would want my CO2 levels to be falling at night rather than rising. My fish now remain at the bottom-mid level of my tank all night long. I'm happier with them there than being just below the surface.