Taxonomy & naming
Cyathopharynx sp. 'blue flush' is an undescribed form — a recognizable geographic colour morph, or small cluster of morphs, documented in the hobby and biotope literature (notably Konings' Cichlid Press titles and the photographic survey at tanganyika.si) but never formally described. Cyathopharynx are "featherfin" cichlids: lek-spawning sand-and-rock dwellers whose males build crater bowers and trail long pelvic fins tipped with egg-dummy lappets. Forms in this group are followed by trade name until a formal revision sorts them out. The Catalog of Fishes is the authority for valid names.
Morphology
A streamlined cichlid of the intermediate zone where rock meets sand, reaching around 8 in TL. The sexes look very different: a nuptial male switches on intense metallic blue-green and gold and grows long pelvic streamers tipped with swollen yellow lappets, while females and non-displaying males stay plain silvery and short-finned. Populations differ mainly in the colour a male turns when he displays, so the form is best told apart by that local nuptial colour — something photographs capture far better than words.
Habitat
Like other featherfins, this form lives in the well-lit shallows where rock gives way to open sand, the zone where displaying males clear their crater bowers. It is documented from Lake Tanganyika by collection point rather than as a described range, and the water is what it is everywhere in the lake: hard and strongly alkaline.
Feeding
A micro-feeder. It grazes fine algal biocover and the tiny invertebrates living within it off rock and sand, and picks plankton from the water column when it is available. The long, coiled gut belongs to a low-protein grazer — "a lot of food but only a little meat," as Konings put it of the genus — so captive fish do best on a vegetable-based diet.
Mating
Spawning happens at a lek, where males gather and each builds and guards a sand-crater bower, showing off in full nuptial colour to any female that passes. There is no pair bond. The male gives only his genes and his crater; a female visits, spawns, and leaves to brood on her own.
Breeding
A maternal, ovophilous mouthbrooder. The female lays a small clutch — genus-typically a few dozen large eggs, often fewer — into the male's crater, takes them straight back into her mouth, and is fertilized at the egg-dummies on the male's anal fin. She then broods the eggs and larvae in her mouth for several weeks, and the male plays no part in raising them.
In the aquarium
Kept for the male's spectacular display, a featherfin needs space, open sand for its craters, and hard alkaline water. A single male colours up best, and the long pelvic filaments are easily nipped, so tankmates have to be chosen with care. As with all Tanganyikan locality forms, these fish are kept as single-provenance groups so the local nuptial colour is not lost to cross-breeding.
Conservation
Undescribed forms are not individually assessed by the IUCN. Featherfins as a whole are widespread, but any single locality population is small and easily over-collected, so captive-bred, provenance-labelled stock is the responsible way to keep this fish.