I agree with Ned - I'd go with a shell dwelling tangenikan - neolamprologus multifaciatus etc - they are extremely easy to breed, and don't need large tanks.
In fact - I've got a "colony" now in a 55g. I started with maybe 8-12 fish - now I've got at least 50 of all sizes (adults are 1-1.5 inches). I don't "do" anything -- no special care, no special food. I give the tank pretty much the same food as all my other tanks ( although I tend to feed smaller pellets and tiny frozen foods like daphnia), and don't give the tank any more water changes than any of my other tanks.
I use sand on the bottom, and crushed coral in a corner "bubble-up" filter, along with lots and lots of snail shells (the fish live in them).
Unlike you, I've got extremely soft water (liquid cotton) - so I've been using seachem's Lake Tang buffer to set my pH and alk.
But crushed coral substrate should do the trick.
Because I'm using a 55g, I've also got piles of rockwork in the ends of the tank, and a second cichlid species in there (
Neolamprologus marunguensis ) -- in a 20g I'd just put a handful of "shellies" in and watch the multiplication !