Taxonomy & naming
Phenacogrammus deheyni was described by the Belgian ichthyologist Max Poll in 1945, based on material collected from the Congo River basin in what is now the Democratic Republic of Congo. It belongs to the family Alestidae, the African tetras, which includes several popular aquarium species such as the Congo tetra (Phenacogrammus interruptus) and the red-eye tetra relatives of West and Central Africa. Eschmeyer's Catalog of Fishes lists the valid name as Phenacogrammus deheyni Poll, 1945, with the authority unparenthesised because the species has remained in Phenacogrammus since its original description.
The genus Phenacogrammus is diagnosed by features of the jaw dentition and caudal fin scalation and unites a number of riverine alestids from the Congo basin and surrounding systems. P. deheyni has sometimes been confused in the trade with its more widespread relative P. interruptus, the Congo tetra, from which it differs in several meristic and coloration characters. The two species overlap broadly in range.
Morphology
Phenacogrammus deheyni is a moderately elongate, laterally compressed characin reaching a maximum recorded total length of 4 in, with most individuals in the wild and aquarium measuring somewhat less. The body shape is typical of the genus: a relatively narrow, streamlined profile suited to open-water schooling, with a forked caudal fin and the characteristic small adipose fin common to characins.
Colouration is subdued compared with the showier Congo tetra — males carry an iridescent sheen along the flanks, with a diffuse lateral stripe and bluish reflections depending on angle of light and mood. Females are plainer and rounder-bodied when ripe. The species is unlikely to be confused with the Congo tetra by anyone examining live fish side by side, but meristic counts are necessary to separate preserved specimens.
Habitat
FishBase records Phenacogrammus deheyni from the middle Congo River and tributary drainages, specifically the Ruki, Mongala, and Aruwimi systems of northern and central DRC. It is classified as a benthopelagic species, indicating it ranges through the lower to middle water column in association with riverine habitats, though as a schooling characin it is also found in open midwater areas of its river reaches.
Water conditions in these lowland Congo tributaries are typically warm and softly acidic, reflecting the vast catchment's laterite soils and dense equatorial forest cover. FishBase records a temperature range of 75–81 °F for the species; the Aquaryus database gives a slightly broader span of 73–82 °F with a pH of approximately 6.9–7.2. The species apparently tolerates moderate reductions in dissolved oxygen, a useful trait in the seasonally variable waters of the Congo basin.
Feeding
Phenacogrammus deheyni is an omnivore with a slight carnivore bias. FishBase assigns it a trophic level of 3.1 and records the natural diet as small insects and crustaceans, consistent with the feeding ecology of riverine alestids generally, which take surface-fallen invertebrates, aquatic insect larvae, and small crustaceans from the water column.
In the aquarium the species is undemanding. It accepts good-quality flake, micro-pellets, and a range of frozen and live foods including brine shrimp, Daphnia, bloodworm, and finely chopped invertebrates. A diet varied with live or frozen invertebrate items keeps the fish in good condition and supports colour development in males.
Mating
Like other Phenacogrammus species, P. deheyni is an open-water egg-scatterer. Males display to females by intensifying their lateral iridescence and spreading their fins; there is no territory defence and no pair bond. Spawning takes place in open water or among fine-leaved aquatic vegetation, with the pair driving together and releasing eggs and milt simultaneously before separating.
The species apparently has a relatively extended spawning season: the Aquaryus database notes a six-month reproduction period, suggesting that under stable conditions individuals spawn repeatedly over many months rather than in a single compressed event. Males may spar and display frequently within a group, and keeping several males together encourages the displays that show the species at its best.
Breeding
Phenacogrammus deheyni is an egg-scattering species that deposits small, adhesive eggs among fine-leaved aquatic plants or over open substrate. Like most alestids, it provides no parental care and will consume its own eggs if left in the spawning tank. Breeders typically condition adults on varied live and frozen foods, then move a ripe pair or small group to a separate, well-planted breeding tank.
Females can lay hundreds of eggs per spawning event according to available data. Eggs hatch in approximately one week at typical spawning temperatures in the mid-20s Celsius. Newly hatched fry are small and require infusoria or commercial liquid fry food initially, graduating to newly hatched brine shrimp and finely powdered dry foods as they grow. The species is not commonly bred in captivity and detailed husbandry accounts are sparse, but it is expected to follow the general pattern of the genus.
In the aquarium
Phenacogrammus deheyni is an active, gregarious species that should be kept in groups of at least six. A school of well-conditioned adults is most rewarding — males display their lateral iridescence most intensely in the presence of rivals and females, and a mixed-sex group in a spacious, well-lit tank brings out the best in the species. A tank of 35.5 in or more in length is appropriate for a group; ample open swimming space combined with areas of fine-leaved planting suits the species' riverine habits.
Water conditions should reflect the Congo basin origin: soft to moderately hard, slightly acidic to neutral water at 75–81 °F is ideal, though the species tolerates a modest range. Filtration should maintain good water quality without excessive current. The species is peaceful with similarly sized tankmates and can be kept alongside other Congo basin fishes, medium-sized rasboras, or other soft-water tetras. Its relative scarcity in the trade means wild-caught or specialist-bred stock must be sourced with care.
Conservation
Phenacogrammus deheyni is assessed as Least Concern on the IUCN Red List (assessed February 2009), reflecting its distribution across multiple drainages of the Congo basin — the middle Congo, Ruki, Mongala, and Aruwimi systems — a broad enough range that local declines in any one tributary are unlikely to threaten the species as a whole. No major conservation concerns have been documented.
The Congo River basin as a whole faces increasing pressures from deforestation, artisanal and industrial mining, and the collection of ornamental fish for export, all of which affect water quality and fish populations over time. P. deheyni is not a prominent trade species and is unlikely to be subject to heavy collection pressure, but continued monitoring of the Congo basin's ecological health is relevant to the long-term security of all its endemic fishes.