Neolamprologus brevis
Neolamprologus brevis is a species uniquely suited to small
tanks, far more so than many of the active schooling fish (white clouds,
neons, or, worst of all, danios) that are more frequently recommended
for them. Brevis pairs maintain such tiny territories that a pair, alone
in a 5 gallon tank, may not even venture into the far corner - for the
entire period of time they live in the tank! The pair is much more
preoccupied with guarding a territory, flirting, and raising fry to be
swimming all over.
The variety of brevis (which may turn out to be a separate species)
known as "Minuta" is an even better choice since it stays at most half
the size of normal brevis; there are size differences between the more
common varieties as well but those are comparatively minor.
It should be noted that groups of brevis, bachelors, and a male and a
female who aren't breeding do not make half as good choices for a five
gallon – more willing to explore and less tied to a territory, they lead
much more peregrinating lives. A couple of bachelor brevis in my 6' tank
managed to use about a quarter of it, the front lefthand corner, which
isn't much but is certainly more than a five gallon would have afforded
them. On the other hand, a single brevis makes a "pet" fish as
interesting as an oscar, if more retiring, for the smaller tank.
(sa shelldwellers.com)