There is widespread belief that only high-light tanks require injected CO2, or conversely that injected CO2 requires a pairing with high light. But, what about CO2 for low light planted tank? Plants benefit tremendously from CO2 in both strong & low aquarium lighting. This was proved conclusively in controlled experiments that plotted plant growth with varying CO2/light combinations. Read more here.
In fact, low lighting + CO2 is one of the most stable combinations available to aquarists. For aquarists looking for slower but healthy growth, using slightly lower light & CO2 levels is a perfect combination. It is also far easier to control algae issues in a tank with lower lighting.
The chart below shows the findings of a growth experiment run by Tropica on Riccia under different light/CO2 combinations. There is net gain in growth when CO2 levels were increased even under lower light levels.