Black Arrow Tetra

Quick summary

The Black Arrow Tetra is a peaceful, moderately challenging shoaling fish best suited to aquarists with some experience keeping stable, soft, acidic water, as it needs a group of six or more, a well-planted tank, and consistent water quality to thrive. Complete beginners may want to start with a hardier tetra species first before taking this one on.

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Identity note: this fish was formally described as Inpaichthys luizae in 2025 (Dagosta, Neotropical Ichthyology). The paper does not use the trade name Black Arrow, so the link between the two names rests on Aquarist Atlas and retailers, not on the description itself.

In this guide:

• Introduction
• Quick Stats
• Appearance
• Natural Habitat
• Tank Size and Setup
• Water Parameters
• Filtration and Equipment
• Diet and Feeding
• Behaviour and Temperament
• Tank Mates
• Breeding
• Common Diseases
• FAQs

Introduction

The Black Arrow Tetra, traded under the provisional name Inpaichthys sp. “Black Arrow”, is a striking small characin from South America that has been steadily gaining popularity among aquarists who appreciate rare and visually distinctive tetras. Sporting an iridescent blue-purple body bisected by a bold, dark lateral stripe that runs from the snout to the caudal peduncle, this fish cuts a sleek, arrow-like silhouette in the aquarium – hence its evocative common name.

Taxonomically, the Black Arrow Tetra sits in genuinely unresolved territory. The genus Inpaichthys currently contains only a single formally described species, Inpaichthys kerri, commonly known as the Blue Emperor Tetra or Purple Emperor Tetra. While the Black Arrow Tetra clearly belongs within or very close to this genus, it has not yet received a formal scientific description. It is therefore best referred to by its provisional trade designation rather than any assumed species epithet. Aquarists should be aware that future taxonomic work may place this fish in a revised genus or assign it a species name that differs from anything currently circulating in the hobby. For now, it remains an undescribed species, and care information is drawn from a combination of observations by experienced keepers and inferences from its close relationship to Inpaichthys kerri.

In the aquarium, the Black Arrow Tetra is a rewarding fish to keep. It is relatively peaceful, adapts well to a range of soft to moderately hard water conditions, and its shimmering colouration becomes especially vivid under subdued lighting and against a dark substrate. It is best suited to community tanks with other small, non-aggressive species, and it thrives in groups where its natural shoaling behaviour can be observed. This care guide covers everything you need to know to keep and breed Black Arrow Tetras successfully.

Appearance

The Black Arrow Tetra is a compact, laterally compressed fish that typically reaches between 3 and 4 cm in total length. Its body shape is typical of many small characins – streamlined and slightly elongated, tapering to a forked caudal fin that underscores the “arrow” imagery behind its common name.

The most immediately striking feature is the broad, dark lateral stripe that extends from the tip of the snout, passes through the eye, and continues along the entire flank to the base of the tail. This stripe is deep black and sharply defined, giving the fish a bold graphic quality. Above and below this stripe, the body exhibits a luminous iridescence that shifts between blue and purple depending on the angle of light and the fish’s mood. Under optimal conditions and subdued lighting, this iridescence can appear almost electric, with flashes of violet and cobalt blue rippling across the body as the fish moves.

The dorsal, anal, and caudal fins are generally translucent to lightly tinted, sometimes showing a subtle wash of colour that complements the body. Males tend to display more intense and saturated colouration than females, particularly during courtship or when competing with rivals. Females are typically slightly fuller-bodied, especially when gravid, and their iridescence may lean more towards a muted silvery-blue rather than the vivid purple seen in dominant males.

Compared to its close relative Inpaichthys kerri, the Black Arrow Tetra is distinguished by its deeper, more prominent lateral stripe and a somewhat different colour profile, with the “arrow” appearance being more pronounced. However, the two can appear superficially similar, and misidentification in the trade is not unheard of. Careful observation of stripe width, body proportions, and the exact hue of the iridescence can help differentiate them.

Natural Habitat

While the precise collection localities for the Black Arrow Tetra are not always clearly documented in the trade, the genus Inpaichthys is associated with the upper Amazon basin in Brazil. Inpaichthys kerri, the sole described congener, is known from the Rio Aripuana system in the state of Mato Grosso, and it is reasonable to suppose that the Black Arrow Tetra originates from a similar or nearby drainage, though this has not been confirmed by formal ichthyological study.

In its native environment, this species is expected to inhabit slow-moving streams and tributaries shaded by dense tropical forest canopy. These waterways are typically characterised by soft, acidic water stained a tea-brown colour by tannins leaching from decaying leaves, wood, and other organic matter on the forest floor. The substrate is usually composed of fine sand or mud, often blanketed with a layer of leaf litter. Submerged roots, fallen branches, and overhanging vegetation provide abundant cover and create a complex, dimly lit habitat.

Water in these environments is generally very soft (often below 5 dGH) and acidic (pH 5.0 – 6.5), with temperatures in the range of 24 to 28 degrees Celsius. Dissolved mineral content is low, and the dark water limits light penetration, which in turn influences the types of aquatic plants and algae present. Small invertebrates, insect larvae, and microorganisms form the basis of the food web in these habitats, and small characins like the Black Arrow Tetra are typically mid-water swimmers that forage in loose shoals among the leaf litter and woody debris.

Understanding this natural environment is key to providing appropriate aquarium conditions. Replicating the soft, acidic water chemistry, subdued lighting, and abundant cover of their native habitat will help bring out the best colouration and most natural behaviour in captive specimens.

Tank Size and Setup

A minimum tank size of 60 litres is recommended for a small group of Black Arrow Tetras. However, as these are active shoaling fish that benefit from swimming space and the company of their own kind, a tank of 80 to 120 litres or more will allow a larger group to be kept comfortably and will result in more natural behaviour and better colouration. A rectangular tank with a standard footprint is preferable to tall or unusually shaped aquaria, as horizontal swimming space matters more than depth for this species.

The aquascape should aim to replicate the shaded, structure-rich environment of a South American forest stream. A dark, fine-grained substrate such as black sand or a dark aquasoil is ideal, as it will enhance the fish’s iridescent colours through contrast. Scattered dried leaves – Indian almond leaves (Terminalia catappa) or oak leaves are popular choices – can be added to the substrate to release tannins, lower the pH slightly, and provide a naturalistic appearance. These leaves also serve as surfaces for biofilm growth, which the fish may graze on.

Driftwood and bogwood should be incorporated to provide cover, visual barriers, and additional tannin release. Twisted roots, branching wood pieces, and small caves or overhangs all contribute to a sense of security for the fish. Live plants are not strictly necessary but are highly recommended. Species that tolerate soft, acidic water and lower light levels work best. Java fern (Microsorum pteropus), various Cryptocoryne species, Anubias varieties, and floating plants such as Amazon frogbit (Limnobium laevigatum) or red root floater (Phyllanthus fluitans) are all excellent choices. Floating plants are particularly useful for diffusing overhead light and creating the subdued conditions under which these tetras look their best.

Lighting should be moderate to low. Very bright lighting will cause the fish to appear washed out and may make them feel exposed and stressed. If stronger lighting is required for plant growth, generous floating plant coverage can help temper the intensity at the water’s surface. A tightly fitting lid or cover is advisable, as small tetras can and do jump, particularly when startled or during the initial settling-in period.

Water Parameters

The Black Arrow Tetra is a soft-water species that shows its best colour and most robust health in acidic to neutral conditions. Ammonia and nitrite must always be maintained at zero, as with all freshwater fish. Nitrate should be kept as low as practicable through regular partial water changes, ideally below 20 ppm and certainly never allowed to climb above 40 ppm.

While the species can adapt to slightly harder water than it would encounter in the wild, very hard or alkaline conditions are not recommended and may lead to stress, diminished colouration, and reduced lifespan. If your tap water is moderately hard, the use of reverse osmosis (RO) water remineralised with an appropriate GH/KH product can help achieve suitable chemistry. The addition of botanicals such as dried leaves, alder cones, or peat filtration can further soften and acidify the water while imparting a gentle tannin stain that replicates the blackwater conditions these fish have evolved in.

Temperature should be stable within the stated range. Sudden fluctuations in temperature or water chemistry should be avoided, as these can trigger stress-related illness. When performing water changes, aim to match the temperature and chemistry of the replacement water to that of the tank as closely as possible. Weekly water changes of 20 to 30 per cent are a good baseline for maintaining water quality.

Filtration and Equipment

Effective biological filtration is essential, but the Black Arrow Tetra does not require a particularly powerful current. In fact, these fish naturally inhabit slow-moving or still waters, so an excessively strong flow can cause stress and force the fish to expend energy swimming against the current rather than behaving naturally. A sponge filter is an excellent choice for smaller tanks, providing gentle but effective biological and mechanical filtration without creating problematic water movement. For larger setups, a hang-on-back filter or a small canister filter with an adjustable flow rate or a spray bar to diffuse the output works well.

If using a canister or power filter, consider directing the outflow against the back glass or through a spray bar positioned just below the water surface to reduce direct current in the main swimming area. Pre-filter sponges on intakes are recommended to prevent small fish or fry from being drawn in.

A reliable heater with a thermostat is necessary to maintain stable tropical temperatures, particularly in cooler climates or homes without consistent ambient warmth. Choose a heater rated appropriately for the tank volume and consider using a heater guard if keeping species that may rest against the glass. A thermometer – preferably a digital one for accuracy – should be used to monitor the temperature regularly.

Lighting, as discussed in the tank setup section, should be moderate to low. LED fixtures with adjustable brightness or a dimmer are ideal, as they allow you to tailor the light level to the needs of both the fish and any live plants. A timer set to provide 8 to 10 hours of light per day helps establish a consistent photoperiod and reduce algae issues.

An air pump is not strictly necessary if the filter provides adequate surface agitation for gas exchange, but it can be useful in warmer months when dissolved oxygen levels may drop. Avoid excessive surface turbulence, as these fish prefer calm conditions.

Diet and Feeding

The Black Arrow Tetra is an omnivore with a diet that, in the wild, likely consists primarily of small invertebrates, insect larvae, zooplankton, and organic detritus. In the aquarium, it is not a fussy eater and will readily accept a wide range of foods, but a varied diet is important for maintaining vibrant colouration, robust health, and breeding condition.

A high-quality micro-pellet or crushed flake food formulated for small tropical fish should form the staple diet. Look for products with a good balance of protein, vitamins, and natural colour-enhancing ingredients such as astaxanthin or spirulina. Supplement this staple with regular offerings of live or frozen foods such as baby brine shrimp (Artemia nauplii), daphnia, cyclops, micro worms, and bloodworm. These protein-rich foods are particularly valuable for conditioning fish for breeding and for maintaining the intense iridescent sheen that makes this species so attractive.

Feed small amounts two to three times per day rather than one large feeding. The fish should be able to consume all offered food within two to three minutes. Overfeeding leads to water quality issues, which small tetras are particularly sensitive to. Remove any uneaten food promptly to prevent it from decomposing and fouling the water.

Variety is the key to good nutrition. Alternating between different food types throughout the week ensures that the fish receive a broad spectrum of nutrients and keeps them interested and active at feeding time. Freeze-dried foods can also be offered occasionally but should be pre-soaked to prevent digestive issues.

Behaviour and Temperament

The Black Arrow Tetra is a peaceful, shoaling species that is best kept in groups of at least six individuals, though groups of ten or more are strongly recommended where tank size permits. In larger groups, the fish feel more secure, display more confident behaviour, and the males in particular will show their most vivid colouration as they compete for the attention of females through subtle displays rather than outright aggression.

Within the shoal, a loose hierarchy typically develops. Males may engage in brief fin-flaring displays and parallel swimming contests, but these interactions are generally harmless and rarely result in any physical damage. Such sparring is, in fact, one of the most entertaining aspects of keeping this species, as the males’ colours intensify dramatically during these exchanges.

In the aquarium, Black Arrow Tetras tend to occupy the mid-water column, though they will explore all levels of the tank throughout the day. They are moderately active swimmers but are not as frenetic as some smaller tetra species. Their movement through the water is graceful and purposeful, and a well-settled group will move through planted areas and around driftwood with a relaxed, flowing motion.

When kept in too-small groups or in sparsely decorated tanks, these fish may become shy, hide frequently, and lose much of their colouration. They may also become nippy towards each other or towards other small fish if they feel insecure. Providing adequate group size, cover, and subdued lighting are the most effective ways to ensure calm, confident behaviour.

Black Arrow Tetras are not aggressive towards other species and can be considered excellent community fish provided their tank mates are chosen with care. They may be intimidated by large, boisterous, or fast-moving fish, so the overall community should be composed of similarly sized, peaceful species.

Tank Mates

The Black Arrow Tetra is well suited to a peaceful community aquarium alongside other small South American species and similarly tempered fish from other regions. The ideal tank mates are those that share a preference for soft, acidic water and will not outcompete or intimidate these relatively gentle tetras. Avoid any fish large enough to consider the Black Arrow Tetra as food, and steer clear of known fin-nippers or highly territorial species that may cause stress.

Good tank mates

• Blue Emperor Tetra (Inpaichthys kerri) – A close relative with very similar care requirements, making it an ideal companion that will shoal in proximity without conflict.
• Cardinal Tetra (Paracheirodon axelris) – Another soft-water South American tetra that thrives in similar conditions and adds complementary colour to the tank.
• Ember Tetra (Hyphessobrycon amandae) – A tiny, peaceful tetra whose warm orange tones contrast beautifully with the Blue Arrow’s cool iridescence.
• Pygmy Corydoras (Corydoras pygmaeus) – A diminutive bottom-dwelling catfish that occupies different tank zones and poses no threat.
• Otocinclus Catfish (Otocinclus vittatus) – A small, peaceful algae eater that stays out of the tetras’ way and helps keep the tank clean.
• Beckford’s Pencilfish (Nannostomus beckfordi) – A calm, slender characin that thrives in soft, acidic water and coexists peacefully.
• Chocolate Gourami (Sphaerichthys osphromenoides) – A quiet, soft-water labyrinth fish that complements the tetras’ temperament, though it requires experienced care.
• Kuhli Loach (Pangio kuhlii) – A peaceful, nocturnal bottom dweller that adds interest to the lower levels of the tank without causing any disturbance.

Fish to avoid

• Oscar (Astronotus ocellatus) – A large, predatory cichlid that would readily eat small tetras.
• Tiger Barb (Puntigrus tetrazona) – A notorious fin-nipper that can harass and stress smaller, more peaceful species.
• Convict Cichlid (Amatitlania nigrofasciata) – An aggressive, territorial cichlid wholly unsuitable for a peaceful tetra community.
• Red-Tail Black Shark (Epalzeorhynchos bicolor) – Highly territorial and aggressive towards smaller fish, especially in confined spaces.
• Green Terror (Andinoacara rivulatus) – A large, aggressive South American cichlid that would dominate and endanger small tetras.
• African Rift Lake Cichlids – Require hard, alkaline water that is fundamentally incompatible with the Black Arrow Tetra’s needs, and many are aggressive.
• Common Pleco (Hypostomus plecostomus) – While not aggressive, it grows far too large for tanks suitable for small tetras and produces excessive waste.
• Bucktooth Tetra (Exodon paradoxus) – A highly predatory tetra that will attack and kill smaller fish despite its relatively modest size.

Breeding

Breeding the Black Arrow Tetra in captivity is achievable with some preparation, though it requires more attention to water chemistry and setup than keeping the fish in a community tank. The species is an egg scatterer, and its breeding behaviour is broadly similar to that of Inpaichthys kerri and other small characins.

To condition a breeding group, select the healthiest and most colourful males and the plumpest females and feed them a protein-rich diet of live and frozen foods for one to two weeks prior to spawning attempts. Baby brine shrimp, daphnia, and micro worms are particularly effective conditioning foods.

A separate breeding tank of 20 to 30 litres is recommended. This should be set up with very soft, acidic water (pH 5.5 to 6.5, GH below 5 dGH) at a temperature of 26 to 28 degrees Celsius. Dim lighting is important, as the eggs and fry are sensitive to light. The bottom of the tank can be left bare or covered with a thin layer of dark sand, and clumps of fine-leaved plants such as Java moss (Taxiphyllum barbieri) or spawning mops should be provided as egg-receiving surfaces. A mesh or grid placed above the substrate can also help protect eggs from being eaten by the adults.

Introduce a pair or a small group (two males to three or four females works well) into the breeding tank in the evening. Spawning typically occurs in the early morning hours. The male will display intensely to the female, swimming alongside her with fins spread and colours at their peak. The female will scatter adhesive or semi-adhesive eggs among the fine-leaved plants or onto the spawning medium, and the male will fertilise them as they are released. A typical spawn may produce 50 to 150 eggs, though this varies.

Remove the adults immediately after spawning, as they will readily consume their own eggs if given the opportunity. The eggs are small and somewhat light-sensitive, so keep the breeding tank in subdued or dim conditions. Hatching typically occurs within 24 to 36 hours, and the fry will become free-swimming approximately three to four days after hatching, once they have absorbed their yolk sacs.

First foods for the fry should be infusoria or commercially available liquid fry food, transitioning to freshly hatched brine shrimp nauplii and micro worms as the fry grow large enough to accept them. Growth is steady but not rapid, and the fry will begin to show adult colouration at around six to eight weeks of age. Maintaining excellent water quality in the fry tank is critical – small, frequent water changes using water matched to the tank’s parameters are essential to prevent losses.

Common Diseases

The Black Arrow Tetra is not particularly prone to any species-specific diseases, but like all small freshwater fish, it can be susceptible to a range of common aquarium ailments, particularly when water quality is poor or the fish are stressed.

Ich (white spot disease) is one of the most frequently encountered illnesses in freshwater aquaria. It presents as small white spots on the body, fins, and gills, and affected fish may flash against surfaces or clamp their fins. Ich is caused by the protozoan parasite Ichthyophthirius multifiliis and is highly contagious. Treatment involves gradually raising the water temperature to 28 to 30 degrees Celsius (if the species tolerates it, which the Black Arrow Tetra does within reason) and administering an appropriate ich medication. Ensure the treatment is safe for scaleless tank mates if present.

Fin rot, characterised by fraying, discolouration, or progressive erosion of the fins, is typically a bacterial infection secondary to poor water quality or physical damage. Improving water conditions through increased water changes and addressing any underlying causes (such as aggression from tank mates) is the first step. Antibacterial treatments may be required in persistent or severe cases.

Fungal infections can occur on damaged tissue or in fish with compromised immune systems. They appear as white or grey cottony growths on the body or fins. Antifungal treatments are effective, but prevention through good husbandry is always preferable.

Neon tetra disease (NTD), caused by the microsporidian parasite Pleistophora hyphessobryconis, can affect various tetra species. Symptoms include loss of colouration, cysts or lumps beneath the skin, erratic swimming, and wasting. Unfortunately, there is no reliable cure for NTD, and affected fish should be humanely euthanised to prevent the spread of infection. Quarantining all new fish before introducing them to an established tank is one of the best preventive measures against this and many other diseases.

Internal parasites can occasionally be an issue, particularly in wild-caught or recently imported specimens. Symptoms may include weight loss despite a good appetite, white or stringy faeces, and lethargy. Antiparasitic medications formulated for internal parasites can be administered via medicated food or directly to the water column as appropriate.

The best defence against disease is prevention. Maintain stable, clean water conditions; avoid overstocking; quarantine new arrivals for at least two to four weeks; and provide a varied, nutritious diet. Stress is a major predisposing factor for disease in aquarium fish, so ensuring that the tank environment is suitable – with appropriate water chemistry, adequate cover, correct group size, and compatible tank mates – is fundamental to keeping Black Arrow Tetras healthy.

FAQs

Is the Black Arrow Tetra the same as the Blue Emperor Tetra?

No. While both are placed in or near the genus Inpaichthys, the Black Arrow Tetra is considered a distinct, currently undescribed species. It differs from Inpaichthys kerri (the Blue or Purple Emperor Tetra) in the prominence and width of its lateral stripe and in certain aspects of its overall colour profile. However, because the taxonomy remains unresolved, the exact relationship between the two is not yet formally established. They may occasionally be confused in the trade, so sourcing from reputable suppliers is advisable.

How many Black Arrow Tetras should I keep together?

A minimum of six is recommended, but groups of ten or more are ideal. Larger groups reduce individual stress, encourage more natural shoaling behaviour, and allow males to display their full colouration as they engage in harmless competitive displays. Keeping too few can result in shy, pale fish that spend most of their time hiding.

Can Black Arrow Tetras be kept in a planted tank?

Absolutely. A well-planted tank is, in fact, one of the best environments for this species. Plants provide cover, help maintain water quality by absorbing nitrates, and create the subdued, naturalistic conditions under which these tetras thrive. Choose plant species that tolerate soft, acidic water and lower light levels for the best results.

Do Black Arrow Tetras need blackwater conditions?

While they do not strictly require blackwater conditions, they will generally look their best and behave most naturally in soft, acidic water with some tannin staining. Adding dried leaves, alder cones, or driftwood to the tank can create a mild blackwater effect that benefits the fish without dramatically altering the appearance of the aquarium if a full blackwater setup is not desired.

Are Black Arrow Tetras suitable for beginners?

They are rated as moderate in difficulty. An aquarist with some experience maintaining stable water parameters and managing a community tank should find them straightforward to keep. Complete beginners may find the soft-water requirements and the species’ sensitivity to water quality fluctuations somewhat challenging, and might benefit from gaining experience with hardier tetra species first before progressing to the Black Arrow Tetra.

Related Guides

• Blue Emperor Tetra (Inpaichthys kerri) Care Guide – The only formally described species in the genus and the Black Arrow Tetra’s closest known relative, with very similar care requirements.
• Cardinal Tetra Care Guide – Another iconic soft-water South American tetra that makes an excellent companion species and shares similar habitat preferences.
• Ember Tetra Care Guide – A popular nano tetra suited to the same peaceful, planted community setups as the Black Arrow Tetra.
• Pygmy Corydoras Care Guide – A small, compatible bottom-dwelling species ideal for soft-water community tanks alongside small tetras.
• Blackwater Aquarium Setup Guide – A comprehensive guide to creating the tannin-stained, soft-water environments in which many South American tetras thrive.