I agree with
@fishguy1978: you can keep shell dwellers and
Julidochromis together, but that's unlikely to work in a small tank. This is a 48" x 12" x17" tank that contains three breeding groups (left, center, and right) of
J. ornatus plus several pairs of
N. multifasciatus. The adult
ornatus don't bother much with the Multi fry- they're too busy breeding and defending their territories- but once the
J. ornatus young put on some size, then they will prey on the Multi fry. That's not necessarily a bad thing unless you have a way to sell the inevitable abundance of Multi fry.
In terms of which are fishes are more interesting, I agree with
@knut_ that this is largely a matter of personal taste. Multi's are cute with their small size, constant digging, and ritualized aggression, but my favourite shell dweller is
N. brevis.
Brevis form very stable pair bonds, with the much larger male defending a single shell (usually) that contains his tiny mate. N. brevis is also very pretty-
The one shell dweller I would avoid at this point is
Lamprologus ocellatus, because you're just asking for trouble with such aggressive fishes in a 20-gallon tank.
That said, there is nothing quite like a breeding group of
Julidochromis, with fry of various sizes cohabiting a territory with their parents. And
Julidochromis are largely unconcerned with gravity, so you will find them swimming at odd angles, even upside down, amongst the rockwork. For me, a tank of
Julidochromis, especially my favourite,
J. ornatus (below) is hard to beat for interest value.
