If you have a heavily planted tank with sufficient light, the plants should keep O2 levels up.
Well, the problem is that at night, plants stop photosynthesizing and start respiring, so they actually draw down oxygen levels. I think you'll be fine, I don't know your tank situation but you'd need a lot of fish and plants to drop down O2
levels to the point where the fish are suffering.
, however, is denser than normal air, which could be a problem if you have zero surface agitation on your tank. A layer of CO2
forming above the water in your tank may limit the ability of atmospheric oxygen to diffuse into the water, which would be a huge problem.
What I would do is have a inlet that mixes the CO2
with atmospheric air. You are going to be sucking way more air than your CO2
reactor can produce anyhow, just mix your CO2
inlet with regular air. Then you are still aerating while you add CO2
to the tank.