Well I have found is if you have to much Co2 the fish will be gasping for air at the top. It is high light system that needs Co2.
Thanks for correcting me.
I think it's simply from adding too much, it has little to do with the light intensity however. More light means you have to add CO2......but you still get 10X more growth using CO2 on a low light tank(say 40-80 micromols), which is 1000% more more growth, pretty significant difference.
Many think they get less/little benefit out of CO2 on low light tanks so they do not use it, moistly out of ignorance or fear, not realizing it can offer really good benefits as far as growth and health.
So they think of CO2 as optional, yet add nutrients liberally.........
But C is about 40-50% of the plant bioamss, N is maybe 1%. CO2 comes in a little from the air, but the rates are 10,000x slower in water vs air, so the system is always CO2 limited. Adding about 10x the normal amount takes care of this limitation however and is still fine for fish etc.
So you can see, you get a lot more from the addition even in low light.
Here's a 1.5 W/gal low light tank with CO2:
Basically you get a lot more efficient use from the light by adding CO2, so this applies to all light levels that can grow plants........