All depends on what the light was. Was it florescent or incandescent? If incandescent, then the light is almost useless. You need a screwin CF bulb, with a 6500k rating. On a 10G, I have one that has 2 20W bulbs, and another that has 2 25W bulbs, and the one with the 25W bulbs need CO2. The tank with just the 20W bulbs is algae free, and no CO2. So if you have a lamp over the tank with a single bulb, I would recommend getting at least a 25W bulb, as it's the only bulb over the tank. You won't need CO2.
Now if you are wanting a grassy type of plant, you will need higher light than that, as well as adding CO2. Most ground covers require higher lighting.
Even though it is the same planet, underwater is a whole different world.