It would really help to know what lighting you're using. If it's low lighting, you don't have a huge selection. Most plants like 1.5+ watts per gallon. Java ferns will do well in low lighting, they don't grow too tall and if they do you can just trim them, they reproduce so you can start with a couple and then spread them around. Moneywort requires a good amount of lighting. Also, make sure you have a good bulb with the proper light spectrum. What all plants are you interested in? Obviously since it's a 20g long, tall plants are almost out of the question.