The primary thing needed to "grow anything" is CO2, not light. As you increase the light the thing that begins to grow better and better is algae. 55 gallon tanks are only 12 inches front to back depth, so a single fluorescent tube can light the whole tank, but it does cause the high growers to shade the low growers. So, it is better to have two tubes. But, T5HO lights are really too bright for a 20 inch high tank, so a 2 tube T5NO light would work best, giving you medium light intensity, and light from two directions to reduce the shading from the higher plants.
If you think you will want to try high light, just to see what it is like, you can use a two tube T5HO light by hanging it about 4 inches above the tank, for high light, or about 8-10 inches above the tank for medium light. A further advantage of doing that is that you can run the tank topless, so, with the light up out of the way, you can easily do routine minor maintenance in the tank anytime you walk by it - pick out dead leaves, prune extra fast growing plants, "fluff" the plants to keep them clean of debris, etc. And, T5HO lights and bulbs are more available than T5NO.