Neolamprolgus furcifer.
As people who know me might expect, it's an unusual Tanganyikan cichlid. It has basically the same body form as Neolamprologus brichardi which is one of the commonest aquarium cichlids, sometimes called the Princess of Burundi, but it is rather larger with longer fins and it is completely purple except for yellow rings around the eyes. Moreover it is said to be specialised for living in underwater caves, often swimming upside down with its belly near the cave's roof. I have never seen this species for sale, but I did see one when I visited the fish room of a Rift Valley cichlid specialist, which I think was in August 1991.