I agree with all that say Shellies...they are fantastic! in 20 gallons you can get 3 different spicies in small colonies (2-5 each) and still have space for a pair of something else. Do not mix them with Julidochromis since they can get in to the shells and get the fry.
I have a 20 gallon with:
(Neo)Lamprologus Similis (4)
(Neo)Lamprologus Brevis (2)
(Neo)Lamprologus Ocellatus "Gold" (3)
(Neo)Lamprologus sp. Kiku (started with a pair, now 1 year later eighty billion trillion gazillion)
I have a 20 gallon with:
(Neo)Lamprologus Similis (4)
(Neo)Lamprologus Brevis (2)
(Neo)Lamprologus Ocellatus "Gold" (3)
(Neo)Lamprologus sp. Kiku (started with a pair, now 1 year later eighty billion trillion gazillion)