N. multifasciatus (Multies). The young-uns from the previous brood will help raise the newest batch of babies.
If there are enough hiding spots, Brevis might work, too, although they don't really tolerate a high-density of their own kind like Multies do.
In my tanks, the N. caudopunctatus adults will eat the previous brood if I don't remove them by the time they're ready to spawn again.
Julies will also self-populate, albeit a bit more slowly.
The downside of shellies in a 20 tall is that they remain mostly on the bottom. You'll have an empty zone on the top of the tank.