I have the same set up (20G long, 24" T8 light). There is actually enough light to grow medium light plants like swords(but they get big), water wisteria, vallisneria, ludwigia, etc. In fact, you should have some of the faster growing plants to balance out the low light plants and take up extra nutrients, or you will start to grow algae. I currently have vallisneria, Windelov fern (LOVE this plant), crypt wendtii, mystery crypt, spiralis crypt, ozelot sword (new), and water wisteria (new, recommend).
I'm currently dosing Flourish Excel once a week after water change, and no CO2
Don't try for anything with high light needs but there are lots of medium light need plants available.