You're really limited with African tanks in a 20 long..
There is a tetra species that you can find on aquabid if you're in the USA: African red eyed tetra.
But they get to 4", like congos so can't fit in a 20 comfortably.
I've down a lot of work looking for African species that aren't cichlids, I'm setting up a 75 gallon West African community. Really, these biotopes are best in big tanks, the fish are large. And African fish are really scarce in the trade in modern times, it's important to remember that Africa isn't a peaceful continent around the jungle areas, and wading through the jungles looking for aquarium fish isn't really worth the risk, plus importing animals from Africa is crazy expensive (Owner of an LFS explained this to me).
If you got a breeding pair of kribensis you could have an African butterfly in there, maybe 2, but that's about it for a 20 longs.
If you have soft acidic water and are willing to search a bit, these are a beautiful African barb that you should be able to find online at least. A group of 15ish in that 20 gallon would be a gorgeous sight to see: ‘Barbus’ hulstaerti (African Butterfly Barb) — Seriously Fish
I've been looking at plants too, lately. Also limited. Anubias are African, easy to find. There's also African water fern, and crinum natans, which I'm going to ask about at my fish store. African fern is a gorgeous plant and it's pretty sought after since it's unique looking.