I agree that light is probably one issue here. And lack of nutrients another. But there is also a third issue, and that is the GH of your source water, presumably tap water.
Prepared fertilizers for aquarium plants like the Seachem Flourish Comprehensive Supplement
[and make sure you get this exact product, as Seachem make several different things under the "Flourish" name, you want the Comprehensive Supplement] contain all necessary nutrients, back those we call the "hard" minerals are not in sufficient supply because many of us have these in our tap water. So knowing your tap water GH (general hardness) is also important in case these may be lacking as they are in my tap water for instance. You can ascertain the GH from the water supply folks, they probably have a website.
Over a 20g, a single T8 tube will work but it has to be a good one. In this case, I would follow Boredomb's suggestion and go to the fish store and get something like a Life-Glo (made by Hagen) or the UltraSun (made by ZooMed). These are more intense light and will be fine. Make sure they are T8, not T5 which will not fit, and take your existing tube to make sure of the length as B said.
Lastly, welcome to Tropical Fish Keeping forum.