One-lined Pencilfish

Quick Summary

The One-lined Pencilfish is a delicate, slender beauty ideal for mature, softwater community tanks with calm companions – but its shy nature and need for dim, acidic conditions mean it is not the best pick for a standard beginner setup or a busy, brightly lit aquarium.

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In this guide:

• Introduction
• Quick Stats
• Appearance
• Natural Habitat
• Tank Size and Setup
• Water Parameters
• Diet and Feeding
• Behaviour and Temperament
• Tank Mates
• Breeding

Introduction

The One-lined Pencilfish, Nannostomus unifasciatus, is one of the most elegant members of the pencilfish family and a long-standing favourite among aquarists who specialise in softwater biotope setups. Its slender, torpedo-shaped body, subtle golden tones, and characteristic oblique resting posture set it apart from almost every other small community fish. Native to slow-moving blackwater creeks and flooded forest margins across a broad swathe of the Amazon, Orinoco, and Guiana Shield river systems, this species has been available in the hobby for decades, yet it remains underappreciated compared to flashier tetras and rasboras.

What makes N. unifasciatus so rewarding is precisely what makes it a poor fit for the wrong tank. It is slow, gentle, and easily intimidated. It feeds with a tiny mouth and prefers to drift among plant stems and leaf litter in near-darkness, picking at microscopic invertebrates. Place it in a bright, sparsely decorated tank with boisterous barbs or hungry tetras and it will fade, refuse food, and succumb to stress. Give it the dim, tannin-stained environment it has evolved for, alongside equally tranquil companions, and it will reward you with years of fascinating behaviour – including a striking colour shift between day and night patterns.

This guide covers everything you need to know to keep One-lined Pencilfish successfully, from tank dimensions and water chemistry to breeding behaviour and disease prevention. Whether you are building your first blackwater biotope or looking to add a gentle centrepiece shoal to an established softwater planted tank, read on to decide whether this species is the right match for your setup.

Appearance

Nannostomus unifasciatus is named for the single, bold, dark lateral stripe that runs from the tip of its snout, through the eye, and all the way to the base of the caudal fin. This line is jet black in healthy, well-conditioned fish and sits against a body that ranges from pale silver-gold to warm amber depending on water conditions and diet. The dorsal surface is a muted olive-brown, while the belly is lighter, often with a faint silvery sheen. In mature males, a subtle flush of red or orange may appear along the lower flanks and at the base of the anal fin, though this is never as vivid as in some other Nannostomus species.

The body shape is unmistakably pencilfish – extremely slender, laterally compressed, and somewhat cylindrical, giving the fish its common name. The mouth is small and slightly upturned, adapted for picking tiny invertebrates from the water surface and plant surfaces. The fins are largely transparent to faintly tinted, with the caudal fin displaying a slightly rounded or truncate shape. Males tend to be slimmer and may show slightly more intense colouration, while females are marginally deeper-bodied, especially when carrying eggs, though sexing can be tricky outside of breeding condition.

One of the most captivating aspects of this species is its oblique resting posture. Unlike most fish, pencilfish often hover at an angle of roughly 30 to 45 degrees, head tilted slightly upward, drifting among plant stems. This posture is entirely normal and is one of the reasons the group has such a distinctive visual appeal in a planted aquarium.

Like other Nannostomus, the One-lined Pencilfish displays a marked difference between its daytime and night-time colour patterns. During the day, the single lateral stripe dominates. At night, or when the fish is stressed, the stripe fades and is replaced by a series of darker blotches or bars across the body. This nocturnal pattern can alarm new keepers who turn on the lights suddenly, but it is perfectly normal and the daytime colouration returns within minutes of the lights coming on.

Natural Habitat

The One-lined Pencilfish has an extensive natural range spanning several major river systems in northern South America. It is found throughout the Amazon basin in Brazil, Peru, and Colombia, across parts of the Orinoco drainage in Venezuela and Colombia, and in the coastal river systems of Guyana, Suriname, and French Guiana. This broad distribution encompasses a variety of habitats, but the species shows a clear preference for slow-moving or still blackwater environments.

Typical collection localities include small forest streams, flooded igapo and varzea forests, oxbow lakes, and quiet backwaters where the current is negligible. The water in these habitats is stained dark brown by humic acids and tannins leaching from decomposing leaf litter, fallen branches, and submerged root tangles. pH values in the wild frequently fall between 4.0 and 6.0, with general hardness close to zero. Water temperatures are stable and warm, typically ranging from 24 to 28 C depending on the exact locality and season.

The substrate in these environments is usually fine sand overlaid with a thick bed of decomposing leaves. Vegetation can be sparse in deeply shaded forest streams or more abundant at the margins of flooded areas, where emergent and submerged plants trap pockets of still water. Light levels beneath the forest canopy are low, creating a dim, sheltered world that pencilfish have adapted to exploit. They forage among leaf litter and fine root structures for microscopic crustaceans, insect larvae, worms, and other tiny invertebrates.

Understanding this habitat is the key to successful captive care. The combination of very soft, acidic water, minimal current, low light, abundant cover, and a diet of tiny live prey items defines the conditions under which N. unifasciatus thrives. Attempting to keep this species in hard, alkaline, brightly lit water with strong filtration flow is likely to result in chronic stress and shortened lifespan.

Tank Size and Setup

A group of eight to ten One-lined Pencilfish can be comfortably housed in a tank of 60 litres or more. Because this species is a mid-water to upper-water swimmer that rarely ventures far from its preferred patch of vegetation, horizontal swimming space matters more than depth. A tank measuring roughly 60 cm in length is a practical minimum; longer tanks of 80 to 100 cm provide a more natural setting and allow the group to spread out and display more interesting social behaviour.

The aquascape should aim to replicate the dim, cluttered feel of a blackwater forest stream. Use a fine sand substrate – a dark colour helps reduce reflected light and brings out the fish’s colouration. Add a generous layer of dried leaf litter (Indian almond leaves, oak leaves, or similar) to release tannins and provide foraging surfaces. Driftwood and branching roots create structure and break sightlines, giving individual fish places to retreat.

Planting should be moderately dense, particularly with species that tolerate low-light conditions and soft water. Cryptocoryne species, Java fern, Java moss, Anubias, and floating plants such as Amazon frogbit, Salvinia, or red root floaters are all excellent choices. Floating plants are especially important because they dim the light reaching the water column and provide the shaded canopy that pencilfish feel safest beneath. A tank without floating plants or other overhead cover is likely to leave One-lined Pencilfish perpetually stressed.

Lighting should be subdued. If you are growing plants that need moderate light, use floating plants to create shaded areas. Alternatively, adjust the photoperiod or use dimmable LED fixtures. Pencilfish kept under intense lighting tend to wash out in colour and spend most of their time hiding.

Minimal water flow is essential. In the wild, these fish inhabit almost still water, and strong currents will exhaust them and make feeding difficult. Position filter outlets so that the main swimming area remains calm, or use a spray bar to disperse flow across the surface.

Water Parameters

Getting the water chemistry right is arguably the single most important factor in long-term success with One-lined Pencilfish. This species has evolved in extremely soft, acidic blackwater and, while captive-bred specimens may tolerate slightly harder conditions than wild-caught fish, they still do best when water chemistry leans decidedly towards the soft and acidic end of the spectrum.

For the best colouration, health, and any chance of breeding, aim for a pH of 5.0 to 6.5 and a GH of 1 to 4 dGH. Many keepers achieve this by using RO (reverse osmosis) water remineralised with a small amount of a GH-only remineraliser, then acidified naturally through tannin-rich botanicals such as Indian almond leaves, alder cones, and driftwood. Peat filtration is another traditional method for lowering pH and softening water.

Avoid large, sudden swings in pH or hardness. In very soft, low-KH water the pH can be unstable, so monitor it regularly and make water changes gradually. A consistent environment matters more than hitting a precise number. Weekly water changes of 10 to 20 percent, using pre-treated water matched closely to tank parameters, will help maintain stable conditions without shocking the fish.

Temperature should remain in the range of 24 to 28 C. A reliable, thermostatically controlled heater is necessary in most climates. Pencilfish are sensitive to temperature swings, so avoid placing the tank where it might be affected by draughts or direct sunlight.

Filtration and Equipment

The filtration requirements for One-lined Pencilfish centre on two priorities: effective biological filtration to keep ammonia and nitrite at zero, and minimal water flow. A small sponge filter powered by a gentle air pump is often the ideal choice. Sponge filters provide excellent biological filtration, create negligible current, and offer a surface on which microorganisms can colonise – these become a supplementary food source for tiny fish. For larger tanks or community setups, a small hang-on-back filter or a canister filter fitted with a spray bar and turned to its lowest flow setting can also work, provided the output is dispersed enough that no area of the tank has a strong current.

Because pencilfish inhabit very still water, any turbulence at the surface or mid-water level can be detrimental. If using a power filter, point the outlet towards the back glass or use a lily pipe that returns water gently below the surface. Some keepers add a pre-filter sponge to the intake to prevent any risk of tiny pencilfish being drawn against it.

A heater rated appropriately for the tank volume is essential. For a 60-litre setup, a 50-watt heater is usually sufficient. Position it horizontally near the filter outlet for even heat distribution.

Lighting, as discussed in the setup section, should be dimmable or naturally attenuated by floating plants. Standard aquarium LED units are fine as long as the intensity reaching the water column is reduced. Many keepers of blackwater biotopes choose warm-toned LEDs (around 3000 – 4000 K) to complement the amber hue of tannin-stained water.

An air pump running a sponge filter will provide gentle surface agitation, which is adequate for gas exchange in a modestly stocked pencilfish tank. Additional aeration is rarely needed unless stocking levels are high or temperatures are at the upper end of the range.

Diet and Feeding

Feeding is an area where many keepers underestimate the needs of One-lined Pencilfish. Their mouths are extremely small – even by nano fish standards – and they are not aggressive feeders. In a community tank with faster, more assertive species, pencilfish can easily be outcompeted for food and slowly starve.

Offer food in very small particle sizes. Micro pellets (0.5 mm or smaller), finely crushed high-quality flake food, and powdered or granular foods designed for fry or micro fish are all suitable dry food options. However, a diet based solely on dry foods is unlikely to produce the best health and colouration. Live and frozen foods should form a significant part of the diet.

The best live foods for pencilfish include baby brine shrimp (Artemia nauplii), micro worms, Walter worms, Grindal worms, vinegar eels, and Daphnia (particularly small strains). Frozen equivalents – cyclops, baby brine shrimp, daphnia, and finely chopped bloodworm – are also readily accepted. When offering frozen foods, thaw them first and rinse away any packing liquid to avoid fouling the water.

Feed small amounts two to three times daily rather than one large meal. This mimics the fish’s natural foraging pattern and ensures that food remains in the water column long enough for every individual in the group to eat. Scatter food across the surface or mid-water where pencilfish naturally hover. If you notice certain fish consistently missing out at feeding time, consider using a feeding ring or target-feeding with a pipette to direct food into their preferred area.

In a well-established tank with leaf litter and mature biofilm on hardscape surfaces, pencilfish will also supplement their diet by grazing on microorganisms and infusoria throughout the day. This natural foraging behaviour is one more reason to include botanical materials in the setup.

Behaviour and Temperament

Nannostomus unifasciatus is one of the most peaceful and mild-mannered fish available in the hobby. It poses absolutely no threat to any tankmate – indeed, the concern is almost always the reverse, that other species may bully or outcompete the pencilfish. It spends its time hovering among plant stems and drifting gently through the mid-water and upper-water zones, adopting its characteristic head-up diagonal posture.

This is a shoaling species and must be kept in a group. A minimum group of six is necessary, but eight to twelve or more is strongly recommended. In small numbers, pencilfish become nervous and reclusive, spending most of their time hidden. In a larger group, they gain confidence, display more natural behaviour, and the subtle social interactions between individuals become visible. Males may engage in gentle sparring – flaring their fins and displaying laterally to one another – but these encounters are entirely harmless and rarely involve any physical contact.

One-lined Pencilfish are not strongly territorial, though dominant males may claim a small area near a favoured patch of plants and display to rivals. Aggression never escalates beyond posturing. The group will typically occupy a loose shoal, with individuals drifting apart to explore and then regrouping periodically.

The day-night colour change mentioned earlier is accompanied by a behavioural shift. As lights dim, the fish settle lower in the water column, their lateral stripe breaks up into blotches, and activity decreases. When the lights come on, normal colouration and activity resume within a few minutes. Sudden exposure to bright light after darkness can cause a brief startle response, so using a gradual sunrise-sunset lighting cycle is beneficial.

Because of their calm nature and small size, pencilfish can be difficult to observe in overly large or densely planted tanks. Choosing a setup where you can see into the mid-water zone clearly will enhance your enjoyment of the species.

Tank Mates

Choosing tank mates for the One-lined Pencilfish requires careful thought. The overriding principle is simple: nothing in the tank should be fast enough, large enough, or aggressive enough to stress or outcompete the pencilfish. Ideal companions are other small, slow, peaceful species from similar softwater habitats. Avoid anything with a reputation for fin nipping, boisterous behaviour, or food-hogging. Even some commonly recommended “community” fish, such as larger tetras or active barbs, can be too much for these gentle pencilfish.

Good tank mates

• Cardinal Tetra – A classic softwater companion that shares similar water chemistry preferences and is too peaceful to bother pencilfish.
• Green Neon Tetra – Tiny, calm, and perfectly suited to the same dim, acidic conditions pencilfish prefer.
• Ember Tetra – A small, gentle tetra that occupies similar mid-water space without competing aggressively for food.
• Pygmy Corydoras – A diminutive bottom-dweller that stays out of the pencilfish’s way and thrives in soft, acidic water.
• Otocinclus – Peaceful algae grazers that occupy a different niche entirely and pose no competitive threat.
• Chocolate Gourami – Another softwater specialist with a similarly shy, gentle disposition.
• Dwarf Pencilfish – Nannostomus marginatus is another peaceful pencilfish that coexists well in the same biotope setup.
• Harlequin Rasbora – A calm, schooling species that does well in soft, slightly acidic water and is not an aggressive feeder.

Fish to avoid

• Tiger Barb – An active, often nippy species that will harass and outcompete pencilfish at feeding time.
• Buenos Aires Tetra – Too large, too boisterous, and too fast a feeder for a pencilfish tank.
• Convict Cichlid – Aggressive and territorial, especially when breeding; completely incompatible.
• Angelfish – Large enough to eat adult pencilfish and will dominate the mid-water zone.
• Chinese Algae Eater – Becomes aggressive with age and may harass small, slow-moving tankmates.
• Common Plecostomus – Grows far too large for a pencilfish setup and produces excessive waste.
• Serpae Tetra – Known fin nipper that will target the slow-moving pencilfish relentlessly.

Breeding

Breeding One-lined Pencilfish in captivity is achievable but requires dedication to providing the right conditions. It is considerably more challenging than breeding many common egg-scattering tetras, primarily because the eggs and fry are tiny and the parents (as well as tankmates) will eat eggs and newly hatched fry if given the opportunity.

To condition a breeding group, feed heavily with live and frozen foods for two to three weeks. Select the plumpest females and the most colourful males for a dedicated breeding tank. The breeding setup should be small – 10 to 20 litres is sufficient – with very soft, acidic water (pH 5.0 to 6.0, GH below 2 dGH). The temperature can be raised slightly to 26 to 28 C. Use a bare bottom or a thin layer of fine-leaved moss such as Java moss, which gives the eggs somewhere to fall out of reach of the adults. A clump of spawning mops or fine-leaved floating plants also works. Lighting should be extremely dim.

Spawning typically occurs in the morning. The male courts the female with lateral displays, and the pair will scatter a small number of adhesive eggs among fine-leaved plants or moss. The total number of eggs per spawning event is relatively low, often fewer than 30. Remove the adults immediately after spawning to prevent egg predation.

The eggs are tiny and translucent, making them difficult to spot. They hatch in approximately 24 to 36 hours at 27 C, and the fry remain attached to the substrate or plant surfaces, absorbing their yolk sacs for a further two to three days before becoming free-swimming. At this stage, the fry are extremely small and need the finest of foods – infusoria, Paramecium cultures, or commercially available liquid fry foods. After a week or so, they can graduate to freshly hatched baby brine shrimp and micro worms.

Growth is slow. Expect several months before the fry begin to show adult colouration and proportions. Maintaining pristine water quality in the rearing tank is critical; small, frequent water changes with matched parameters are essential to prevent losses. Keep the rearing tank dimly lit, as the fry are photosensitive.

Common Diseases

One-lined Pencilfish are not especially disease-prone when kept in appropriate conditions, but their sensitivity to water quality and stress makes them vulnerable to a number of common ailments if husbandry is neglected.

Ich (white spot disease) is the most frequently encountered problem, typically triggered by temperature fluctuations or the introduction of new, unquarantined fish. The classic white cyst-like spots on the body and fins are easy to identify. Treatment involves gradually raising the temperature to 28 to 30 C and adding aquarium salt at a low dose (one teaspoon per 20 litres), though be cautious with salt in very soft water as pencilfish are sensitive. Proprietary ich treatments containing malachite green or formalin can also be used at half the recommended dose for sensitive species.

Velvet disease (Oodinium) presents as a fine, gold or rust-coloured dusting on the skin. It can be more insidious than ich because it is harder to spot until the infestation is advanced. Dim the lights (the parasite is partially photosynthetic) and treat with a copper-based medication, again at a reduced dose for soft, acidic water.

Fungal infections may appear on fish that have been physically damaged or are immunocompromised due to stress. Cotton-like growths on the body or fins should be treated promptly with an antifungal medication.

Bacterial infections, including fin rot and columnaris, can occur in tanks with poor water quality or where fish are chronically stressed. Improving water conditions is the first and most important step; antibiotic treatment may be necessary in severe cases.

Prevention is always better than cure with this species. Quarantine all new additions for at least two weeks before introducing them to an established pencilfish tank. Maintain stable, clean water conditions, feed a varied diet, and avoid stocking with aggressive or overly active species that could cause chronic stress.

FAQs

Why do my pencilfish look different at night?

This is completely normal. Like all Nannostomus species, the One-lined Pencilfish displays a different colour pattern at night. The bold lateral stripe fades and is replaced by darker blotches or bars. This night-time pattern appears within minutes of the lights going off and reverses just as quickly when the lights come back on. It is not a sign of illness or stress, provided the daytime colouration returns promptly each morning.

How many One-lined Pencilfish should I keep together?

A minimum of six is needed, but groups of eight to twelve or more are strongly recommended. In larger groups, the fish feel more secure, display more natural behaviour, and show better colouration. Keeping fewer than six often results in shy, reclusive fish that hide constantly and may refuse food.

Can I keep One-lined Pencilfish in hard, alkaline water?

This is not recommended. While captive-bred fish may survive briefly in moderately hard water, they will not thrive long-term. The species has evolved in extremely soft, acidic conditions, and departing significantly from these parameters is likely to cause chronic stress, poor colouration, increased susceptibility to disease, and a shortened lifespan. If your tap water is hard and alkaline, consider using RO water to create suitable conditions.

Why are my pencilfish not eating?

The most common reasons are food that is too large for their tiny mouths, competition from faster-feeding tankmates, or stress due to inappropriate water conditions, lighting, or lack of cover. Switch to micro-sized foods, offer live baby brine shrimp or micro worms to stimulate feeding, ensure the tank is dimly lit with plenty of plant cover, and check that water parameters are within the recommended range. Target-feeding with a pipette can help ensure food reaches the pencilfish directly.

Do One-lined Pencilfish jump?

Pencilfish are not habitual jumpers, but they can and occasionally do leap from the water, especially when startled. A tight-fitting lid or a dense layer of floating plants (or both) is advisable. Floating plants have the added benefit of providing the overhead cover that helps pencilfish feel secure and reduces the likelihood of jumping in the first place.

Related Guides

• Dwarf Pencilfish (Nannostomus marginatus) – A close relative with similar care requirements, ideal for comparison if you are deciding between pencilfish species.
• Beckford’s Pencilfish (Nannostomus beckfordi) – A slightly hardier pencilfish species that makes a good stepping stone for keepers new to the genus.
• Cardinal Tetra – One of the best companion species for a blackwater pencilfish biotope.
• Chocolate Gourami – Another softwater specialist with overlapping habitat preferences and a similarly gentle temperament.
• Pygmy Corydoras – A peaceful bottom-dwelling companion that shares the pencilfish’s preference for calm, soft, acidic water.