well....its a confusing topic. but on the basic level you have it backwards.
In general low light requires no co2. high light requires co2.
A little more specifically, plants will always benefit from the addition of co2 in a planted tank no matter how much light there is (low or high). The plants uptake depends on available light, available co2 and available nutrients. If one runs out, plant growth is limited. CO2 is the limiting factor of plant growth because of the minimal amount that is naturally present in an aquarium. Aquariums reach their equilibrium and settle at about 2-3PPM of co2. An aquarium that has co2 injection is targeted to have 25-35 (30 is ideal) PPM co2.
