South American cichlids give you lots of choices:
One of the dozen or so commonly available Apistogramma species.
A pair of blue rams.
A pair of Laetacara curviceps.
A pair of Golden Eye dwarfs (Nannacara anomala)
And if you want to play with the water chemistry, that opens up a whole lot more dwarves such as Bioteocus, Taeniacara, and Dicrossus species as well as even more apistos.
For west african, the only three krib species I'd try in a twenty long are Pelvicachromis rollofi, P. taeniatus, or P. subocellatus. But there are other options like Buffaloheads or dwarf jewels.