Taxonomy & naming
Hemigrammus filamentosus was described by the German ichthyologist Axel Zarski in 2011, based on specimens that had entered the European ornamental trade from the Rio Araguaia in Brazil. Before the formal description was published, the fish circulated in the hobby provisionally under the name Hyphessobrycon cf. stegemanni, reflecting uncertainty about both its generic placement and its species identity.
The valid name, as listed in Eschmeyer's Catalog of Fishes, is Hemigrammus filamentosus Zarske, 2011. The species is placed in Hemigrammus, a large and morphologically diverse genus in the family Characidae. Hemigrammus is distinguished from the closely related Hyphessobrycon primarily by the presence of scales on the lower lobe of the caudal fin, a character that must be examined directly on fresh or preserved material. The specific epithet filamentosus refers to the thread-like, elongated fin rays that distinguish breeding males.
Morphology
Hemigrammus filamentosus is a small tetra reaching 1–1.5 in in length, with the slender, laterally compressed body plan typical of the genus. The base colouration is silver to pale gold, with a darker midlateral stripe running from behind the gill cover toward the caudal peduncle. A caudal spot — a dark mark at the base of the tail — is present, as in many Hemigrammus.
Sexual dimorphism is pronounced for the genus. Males develop markedly elongated fins, particularly in the dorsal, anal, and caudal rays, and the caudal fin is flushed deep red; this fin colouration and fin elongation are the species' most distinctive field marks and the origin of both the common name and the scientific epithet. Females lack the elongated fins and show a transparent or only weakly coloured caudal fin; they are more heavily built in the body, a reliable indicator of ripe condition. A golden colour form occurs in the wild alongside normally pigmented individuals, and both have entered the hobby.
Habitat
The species is known from the Rio Araguaia basin in the state of Mato Grosso or Pará, central Brazil — one of the major southern tributaries of the Amazon system. The Araguaia is characterised by white-sand and clear-water reaches, slow-moving backwaters, and gallery forest, with seasonal flooding that temporarily connects river channels to oxbow lakes and flooded forest floor.
Available hobbyist data place the fish in soft, slightly acidic water: pH 5.5–7.5, general hardness up to 15 dH, and temperatures of 72–82 °F. In the wild the species likely inhabits slower, well-vegetated margins rather than main-channel rapids. Its biology as a shoaling tetra is consistent with the typical Araguaia white-water and clearwater microhabitats occupied by many Hemigrammus species, where tannin-stained or clear water flows over pale sandy substrate beneath dense riparian cover.
Feeding
Hemigrammus filamentosus is an omnivore, as is typical for Hemigrammus. In the wild, small tetras of this type feed opportunistically on aquatic and terrestrial invertebrates — microcrustaceans, insect larvae, small insects falling to the surface — as well as fine particulate organic matter.
In the aquarium the species is undemanding and readily accepts quality dried foods: flakes, micropellets, and granules. Colour and condition benefit from regular supplementation with live or frozen invertebrates such as Daphnia, baby brine shrimp, bloodworm, and Cyclops. A varied diet with some live or frozen component is particularly important when conditioning fish for spawning, as richer nutrition stimulates courtship activity and improves egg quality.
Mating
Like all Hemigrammus, the Phoenix Tetra is an egg-scattering open spawner. Males court females through vigorous fin-displaying behaviour, fanning their elongated, red-flushed caudal fins and driving females through the water column. Courtship intensity increases with water temperature; sources note that activity begins to peak around 82 °F.
The species does not form pairs in the strict sense. In a well-planted group tank, multiple males may display simultaneously, and spawning fish drive through fine-leaved vegetation or over substrate, releasing and fertilising small batches of adhesive eggs among the plants. There is no parental care — both sexes will consume eggs if they encounter them — so separation of adults from the spawning medium is standard practice for breeders.
Breeding
Hemigrammus filamentosus breeds in the typical tetra pattern and has been produced in captivity; captive-bred stock as well as wild-collected fish of both normal and golden colour forms are available in the trade. To prompt spawning, condition a group of adults on live and frozen foods and raise the temperature to the upper end of the range, around 79–82 °F. A dedicated breeding tank fitted with fine-leaved plants such as Java moss or synthetic spawning mops, with soft, acidic water — pH below 6.5 is often recommended — mimics the spawning conditions of the Araguaia tributaries.
Eggs are adhesive and hatch in approximately 24 hours at breeding temperatures. The tiny fry are free-swimming within a few days and initially require very fine foods — infusoria or commercial liquid fry preparations — before graduating to newly hatched brine shrimp nauplii as they grow. Adults must be removed after spawning to prevent egg and fry predation. The species is reported to live up to four to five years in good aquarium conditions.
In the aquarium
The Phoenix Tetra is rated as a peaceful, sociable species suitable for community tanks housing other small, non-aggressive fish. It occupies the middle water column and, like most tetras, does best in a group of six or more individuals, which reduces individual stress and brings out natural schooling and courtship behaviour. A group of males kept together will display more vigorously and show deeper red fin colouration than isolated fish.
Water conditions should lean soft and slightly acidic — pH 5.5–7.5, hardness up to 15 dH, temperature 72–82 °F — reflecting the fish's Araguaia origins. A planted aquarium with a dark substrate and some surface shade, perhaps from floating plants, recreates the subdued, tannin-touched character of its natural habitat and shows the red caudal fin colouration to best effect. The species is described as undemanding in care but genuinely uncommon in the hobby, meaning prospective keepers may need to seek it out from specialist importers or breeders. It is considered appropriate for experienced beginners through intermediate hobbyists.
Conservation
Hemigrammus filamentosus has not received a dedicated IUCN Red List assessment, and its conservation status is therefore Not Evaluated. The species was described only in 2011 from aquarium-trade imports, and the precise wild distribution within the Araguaia basin and the size of wild populations remain incompletely characterised. One hobby source (Fishipedia) lists the species as Least Concern, but this does not reflect a formal IUCN assessment.
The Rio Araguaia faces documented pressures from deforestation, soy and cattle agriculture in the Cerrado and transitional forest zones, and water abstraction — pressures shared by many fish species in this basin. The ornamental trade is a relatively minor consideration compared with habitat change for a species whose collection appears to have been modest. The existence of captive-bred stock from European breeders means the hobby does not depend solely on wild imports, which is a positive factor for any future assessment.