I have a 55 gallon NPT. Soil sub with a gravel cap. Two big amazon swords , one of which currently has four stalks with a whole bunch of plantlets on them. Never used any ferts of any kind and they've been doing great for over a year now.
If you use amazon swords in a 20 I'd think that the plants would out grow your tank in probably less than a year. I have several "melon swords"(no idea what the latin name would be) growing in my 20 long with the same type setup and they are doing great. Also, no ferts.