Taxonomy & naming
Inpaichthys luizae was formally described by Fernando Carvalho Dagosta in 2025 in the journal Neotropical Ichthyology (DOI: 10.1590/1982-0224-2025-0052). The type locality is a tributary of the Rio dos Peixes in the municipality of Juara, Mato Grosso State, central Brazil; the holotype measured 1.5 in SL and paratypes ranged from 1–1.5 in SL. The Catalog of Fishes records the valid name as Inpaichthys luizae Dagosta, 2025.
The genus Inpaichthys was established for I. kerri, the Royal Tetra, which has for decades occupied a somewhat uncertain position within the broad family Characidae. FishBase currently places Inpaichthys within Acestrorhamphidae, though the genus has also been treated within Characidae sensu lato by other authorities; the top-level family assignment here follows Characidae as the broader encompassing grouping. Inpaichthys luizae is one of three species now recognised in the genus, the others being I. kerri and I. parauapiranga, and the three are considered one another's closest relatives.
The specific epithet luizae is a Latin genitive honouring the author's daughter, Luíza Krauss Dagosta — a dedication recorded in the original description. The fish first appeared in the ornamental trade in late 2023, roughly a year before the formal description, and circulated under the common name 'Black Arrow Tetra' before Dagosta's paper assigned it a valid scientific name.
Morphology
Inpaichthys luizae reaches approximately 1.5 in standard length, with some individuals growing slightly larger in aquarium conditions. It is a slender, laterally compressed characid with the torpedo-shaped body typical of the genus and a small adipose fin. The overall body plan closely resembles Inpaichthys kerri, and the two species were long conflated in the ornamental trade.
The key diagnostic character separating I. luizae from its congeners is a prominent oblique dark ventrolateral stripe that runs from the base of the pectoral fin to the middle rays of the caudal fin — a bold, angled marking that gives the fish its common name. This stripe is more oblique in orientation than markings seen on I. kerri and I. parauapiranga and is unique within the genus. Males are expected to display brighter overall colouration, as in the related I. kerri, though detailed published descriptions of male-specific colour pattern for I. luizae remain limited given the species' recent formal description.
Habitat
Inpaichthys luizae is known from small tributaries of the Rio dos Peixes in the upper Tapajós basin of Mato Grosso State, central Brazil. The type locality is a private tributary in the Juara municipality, accessible in the field primarily through private landholdings. The Aquarium Glaser documentation notes the fish inhabits small, easily accessible tributaries in this part of the Tapajós drainage.
Water chemistry data for the natural habitat, referenced from hobbyist documentation of related Glaser material, suggests cool, very soft, and acidic conditions typical of Mato Grosso tributary headwaters: temperatures in the range of approximately 72–79 °F, extremely low conductivity (TDS around 40, corresponding to roughly 2° dGH), and pH near 5.5. These conditions reflect the oligotrophic nature of many Tapajós headwater streams, which drain ancient, nutrient-poor soils far from the carbonate-rich geology of the Amazon várzea system.
Feeding
No detailed study of the natural diet of Inpaichthys luizae has been published. Like other small characids of similar body form from Amazonian tributary systems, it is presumed to be an opportunistic micro-predator and omnivore, targeting small invertebrates — including insect larvae, micro-crustaceans, and other zooplankton — along with incidental plant or algal material.
In the aquarium, Aquarium Glaser reports that it is easily maintained and accepts standard prepared foods. A varied diet including good-quality micro-pellets or small flake alongside frozen or live foods such as Daphnia, Artemia nauplii, and micro-worms will support good colour and condition. As with most small characids, feeding small amounts frequently is preferable to a single large daily ration.
Mating
Inpaichthys luizae has not been the subject of a dedicated reproductive study, but Aquarium Glaser reports that it spawns in a manner similar to Inpaichthys kerri — an egg-scattering, open spawner with no parental care. Males are expected to court females with colour display, and spawning is likely to occur over fine-leaved vegetation or spawning mops.
The closely related I. kerri is a well-documented aquarium spawner: males drive receptive females into dense plant material, and the pair scatter small adhesive eggs. Spawning activity in I. kerri follows a broadly continuous pattern under good conditions rather than a discrete seasonal cycle. Given the close phylogenetic relationship described by Dagosta (2025), I. luizae is expected to share these fundamentals, though aquarium reports are still accumulating.
Breeding
Aquarium Glaser characterises Inpaichthys luizae as productive and easy to breed, comparable in this respect to I. kerri. Conditioning adults in a species tank or with a small group of conspecifics and providing fine-leaved plants or spawning mops should bring them into spawning condition without difficulty.
Following the model of I. kerri, eggs are small and adhesive and should be removed to a separate rearing container to avoid predation by the adults. At typical aquarium temperatures in the low-to-mid 20s Celsius, incubation is expected to last several days, with fry requiring infusoria or commercial liquid fry food initially before graduating to freshly hatched Artemia nauplii. Soft, acidic water matching the natural habitat is likely to improve both egg fertility and fry survival, though hobbyist reports suggest the species is adaptable.
In the aquarium
Inpaichthys luizae is a peaceful, social fish that should be kept in a group of six or more. Aquarium Glaser notes that it shows better colouration when maintained in a planted tank with a dark substrate, which intensifies the contrast of its ventrolateral stripe and general body colours — a recommendation consistent with most small Amazonian characids that inhabit shaded tributary environments.
Water chemistry should lean toward the soft, acidic conditions of its native Mato Grosso headwaters: temperatures of roughly 72–79 °F, pH in the range of 5.5–7.0, and low hardness will keep the fish in best condition and encourage breeding. It is compatible with other small, peaceful tetras, dwarf cichlids, corydoras, and similarly sized schooling fish. A densely planted aquarium with open swimming lanes suits the species well. Given its small adult size, it is well suited to nano and mid-sized planted aquaria.
Conservation
Inpaichthys luizae was assessed as Least Concern (LC) under IUCN criteria at the time of its formal description in 2025, evaluated in accordance with IUCN Standards and Petitions Subcommittee criteria (2024). The original description notes that the type locality lies within private land with no immediate environmental threats identified at the time of assessment.
As a recently described species with a localised known range in Mato Grosso, ongoing monitoring of its distribution and population status would be prudent, particularly given the pace of land-use change in parts of the Brazilian Cerrado-Amazon transition zone. For the moment, the species appears stable, and the emergence of captive-bred stock in the ornamental trade — Aquarium Glaser documented it from trade sources — means that hobbyist demand can be met without pressure on wild populations.