I think kribensis would be a good choice for that size tank, but only a pair, and probably nothing else in the tank.
Otherwise you'd need to stick with apistogramma types or rams, as mentioned, perhaps a couple of keyholes, or shelldwellers if you have hard water. You might be able to keep a pair of N. brichardi, but that is a big if, and again, only the pair. They would spawn, however, and they permit several generations to remain in the tank with them - kindof cool.
It also depends on whether it is a 20 long or a 20 high.