20 us gal = 75 liters... Quite small, but...
- Apistogramma agazizi
- Apistogramma bitaeniata
- Mikrogeophagus altispinosa
- Mikrogeophagus ramirezi
Some of above are quite angry, so find information about them before buying any. Most Apistogrammas lives in harem, 1 male 2-3 females. However, your tank is quite small, so be sure what you're doing.. Probably Mikrogeophagus ramirezi will be best choice for you. It demands soft/turf-filtered water.