You still need the correct levels of all the nutrients (inc CO2) in the
water its just that you will be adding less to keep it at that level.
So you still would be better with 20-30ppm of CO2 in the water whatever the
amnount of light you have. It would be the same with any nutrient like
potassium which yo would want to be approx 20-30ppm, the plants would just
be using less of the nutrients and you would not have to add as much to keep
it at that level.