CO2 helps plants grow better and faster whatever the light intensity is. When you add CO2 to a low light tank, the plants just do better, and some plants that wouldn't grow at all, will then grow, but slowly. A FishNeedIt light with 4 bulbs should give you about twice as much light as with 2 bulbs, which would be medium light. Then you would need CO2 to keep the plants growing and healthy, or algae would certainly take over the tank. I think you would enjoy the tank a lot more, once the excitement of seeing fast plant growth goes away, if you used low light.