Obviously extra light helps and so does fertilizer.  However without the right balance of these with CO2 the plant has a hard time digesting and using these resources.  It is almost a triangle that is required for each to be proportionate to the other.  Without light the CO2 and fertilizer is useless.  Without fertilizer the CO2 and lights are useless as well.  If you've added extra light (around 2-3 watts total per gallon normally is ok) and you have fertilizer (iron suplements as well) then CO2 will likely be your solution.  Be sure to check your water hardness for both KH and GH as compared to your plants needs as well first.