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I am a big fan of crypts, amazon swords (with root tabs), Money wort, Wistera, Java moss. you'll need to dose ferts, depending on what you have and what their needs are.
You could do Vals, corkscrew ones are a better size IMO for a 20g. Ambulia, Cabomba, Anacharis (if you not using liquid carbon), Rotala Indica (Rotala roundifolia), Pennywort, Myrio's, Narrowleaf Temple, just to name a few. Crypt Wendtii would add alittle color.
All you really need for a well planted, low to medium light tank, in addition to your Java fern are: Anacharis (Brazilian water weed) and Pennywort both are floated, Singapore moss, and a couple of different species of Anubias, like nana and nangi.
Attached is a pic of a 55 G with those plants in it. The lighting is a couple of florescent bulbs from the hardware store. No special fertilizers, just what the fish produce and a lot of pure, treated tap water flushed through the tank every week.