As far as I know, both plants and fish respire so co2 builds up regardless, but at night there is no photosynthesis taking place for the plants to use the co2. Unless your fish are breathing heavily in the morning, I wouldn't worry about it.

 

If your water surface is slightly rippled from water flow you should get enough oxygen without any other effort.

 

In the absence of light photosynthetic plants absorb O2 and produce CO2- therefore IMO it is a very good idea to shut off CO2 equipment and run an airstone at night to avoid livestock suffocation.

 

Respiration of plants is only about 1/10 the rate of fish on a per gram basis. Almost always you don't have to worry about it.

 

I had some threadfin rainbowfish and blue-eyed rainbowfish that were gasping at the surface of the water when I over did it with a DIY CO2 set-up I had. I used too much yeast, and I kept the bottle heated in a 5 gallon aquarium and I still over dosed a 65 gallon aquarium with two, 2 litre bottles. I lost some ottos during that time also. The other fish, like rummy nose tetra and cardinals, seemed O.K.

I did a water change and fixed the CO2 reciepe I was experimenting with. It's possible with foolishness, carelessness and stupid logic to overdose your CO2. With CO2 injection, make sure you don't ignore the bubble counter.