Three-lined Pencilfish

Quick Summary

The Three-lined Pencilfish is a stunningly beautiful but somewhat demanding small characin best suited to experienced hobbyists who can provide soft, acidic water in a calm, dimly lit aquarium. If you can replicate its blackwater origins and house it with equally gentle companions, this peaceful species rewards you with subtle iridescence and fascinating behaviour.

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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

Introduction

The Three-lined Pencilfish (Nannostomus trifasciatus) is one of the most attractive members of the pencilfish family, Lebiasinidae. Native to the slow-moving blackwater tributaries and flooded forests of the Amazon and Orinoco basins in South America, this diminutive characin has captivated aquarists since it first entered the hobby in the early twentieth century. Its three bold horizontal stripes, interspersed with flashes of red and gold iridescence, make it a genuinely eye-catching fish despite its modest size.

While the Three-lined Pencilfish is entirely peaceful and well-mannered, it is not necessarily the easiest species for a beginner to keep long-term. It demands soft, acidic water with minimal hardness, subdued lighting, and a tranquil tank environment free from boisterous or fast-moving companions. Water quality must be consistently high, and diet requires more thought than simply scattering flake food across the surface. For aquarists willing to invest in these conditions, however, a shoal of Nannostomus trifasciatus hovering among fine-leaved plants and tannin-stained water is one of the most rewarding sights in the freshwater hobby.

This guide covers everything you need to know to keep Three-lined Pencilfish successfully, from tank setup and water chemistry through to breeding, feeding, disease prevention, and choosing compatible tank mates.

Appearance

The Three-lined Pencilfish earns its common name from the three prominent dark horizontal stripes that run the length of its torpedo-shaped body from snout to caudal peduncle. The uppermost stripe traces along the dorsal ridge, the central stripe passes through the eye and continues to the tail, and the lowest stripe runs along the ventral edge. Between these dark bands sit lighter zones that, depending on the fish’s condition and the ambient lighting, can appear cream, gold, or a warm coppery hue. Scattered along the flanks between the stripes are small but vivid iridescent spots in shades of red, orange, and gold that catch the light beautifully when the fish moves through dappled illumination.

The fins are largely transparent to lightly tinted, though mature males often display subtle red or orange colouration in the anal and pelvic fins. The caudal fin may carry faint red markings at its base. Like all pencilfish, Nannostomus trifasciatus has a small, slightly upturned mouth adapted for picking tiny food items from the water column and the surfaces of plants and driftwood.

Sexual dimorphism is subtle but discernible. Males tend to be slimmer and slightly more intensely coloured than females, particularly in the ventral region. Females are typically a touch deeper-bodied when mature and in breeding condition. Both sexes share the characteristic pencilfish posture: the body is held at a slight oblique angle, with the head pointing gently upward, especially when resting among vegetation. Like other Nannostomus species, the Three-lined Pencilfish displays a distinctive nocturnal colour pattern. At night or when stressed, the horizontal stripes fade and are replaced by a series of broad, pale vertical bars across the body – a transformation that can alarm keepers who are seeing it for the first time but is entirely normal.

Natural Habitat

Nannostomus trifasciatus is found across a broad swathe of tropical South America, from the Rio Negro and its tributaries in Brazil through parts of the Peruvian and Colombian Amazon to the river systems of Guyana, Suriname, and French Guiana. Throughout this range, the species occupies a remarkably consistent type of environment: slow-moving or near-still blackwater streams, forest pools, and the margins of flooded forests (igapo) where the water is deeply stained with tannins released by decomposing leaf litter and submerged wood.

These blackwater habitats are characterised by extremely soft, acidic water with negligible mineral content. The pH can drop below 4.0 in some locations, general hardness is often barely measurable, and conductivity is very low. The water itself appears tea-brown to almost black, which dramatically reduces light penetration. The substrate is typically composed of sand, mud, and thick layers of fallen leaves. Submerged roots, branches, and aquatic vegetation provide shelter and foraging surfaces.

In these habitats, the Three-lined Pencilfish lives among dense marginal vegetation and tangled root systems, hovering in loose groups in the mid-water and upper water column. Current is minimal, and the fish rarely ventures into open water. It shares its environment with a vast array of small characins, dwarf cichlids, corydoras catfish, and other species adapted to the same demanding water chemistry. Understanding this natural habitat is the key to successful captive care: the closer you can approximate these quiet, tannin-rich, dimly lit conditions, the healthier and more colourful your fish will be.

Tank Size and Setup

A minimum tank size of 60 litres is recommended for a small group of Three-lined Pencilfish. Since this species must be kept in a shoal of at least six individuals to feel secure and display natural behaviour, the tank needs to provide enough horizontal swimming space for the group to spread out and interact without constant territorial friction. A tank measuring approximately 60 cm in length is a sensible starting point. For a larger community or a group of ten or more, aim for 80 to 120 litres.

The aquascape should prioritise dense planting, particularly towards the sides and rear of the tank, with fine-leaved species such as Java moss, Cabomba, or various Cryptocoryne species working well. Floating plants are highly recommended and arguably essential: they diffuse overhead light, create the dappled, shaded conditions this species prefers, and provide a sense of security that encourages the fish to spend more time in open view. Amazon frogbit, Salvinia, and red root floaters are all suitable choices.

Driftwood is another important element. Pieces of bogwood, Mopani wood, or spider wood release tannins into the water, naturally lowering pH and lending the amber tint that mimics the species’ blackwater origins. Indian almond leaves (Terminalia catappa) and dried oak leaves scattered across the substrate serve the same purpose while also providing surfaces for biofilm growth that the fish will graze upon. A dark substrate – fine dark sand or a dark aquasoil – complements the overall aesthetic and helps the fish feel comfortable. Pale or reflective substrates can cause stress and wash out the fish’s colours.

Avoid strong water flow. Three-lined Pencilfish are slow, delicate swimmers that become exhausted in strong currents. Position the filter outlet so that flow is gentle and diffused, or use a spray bar to spread the output across the surface. If necessary, baffle the filter to reduce flow further.

Lighting should be subdued. If you are growing plants that require more light, the floating plant layer will help mitigate the brightness reaching the mid-water zone where the pencilfish spend most of their time. A dimmer or a timer that provides a gradual dawn and dusk transition is beneficial.

Water Parameters

Getting the water chemistry right is the single most important factor in keeping Three-lined Pencilfish successfully. This is a true blackwater species, and while it can tolerate conditions slightly outside its ideal range once well acclimatised, it will never truly thrive in hard, alkaline water. If your tap water is moderately hard or above, you will almost certainly need to use reverse osmosis (RO) water or a mixture of RO and tap water to achieve the necessary softness.

Stability is just as important as hitting the right numbers. Sudden swings in pH or temperature are dangerous for any fish but particularly so for a species adapted to the remarkably stable chemistry of blackwater environments. When performing water changes, ensure the replacement water is matched for temperature and chemistry before adding it to the tank. Smaller, more frequent water changes (10-15% twice a week) are preferable to large weekly changes for this reason.

Tannins from driftwood and dried leaves will naturally help buffer the pH on the acidic side and contribute to the soft-water chemistry that Nannostomus trifasciatus requires. Peat filtration is another traditional method for achieving these conditions, although it can be less predictable than using RO water. Commercially available blackwater extracts can be used to supplement tannin levels but should not be relied upon as the sole means of adjusting water chemistry.

Filtration and Equipment

Filtration for a Three-lined Pencilfish tank should be efficient but gentle. A small hang-on-back filter or a sponge filter is ideal for tanks at the lower end of the size range. For larger setups, a small external canister filter with a spray bar or lily pipe to diffuse the output works well. The key requirement is to keep water movement minimal while maintaining excellent biological filtration. In the soft, acidic conditions this species requires, the beneficial bacterial colonies that drive the nitrogen cycle can be somewhat less robust than in harder, more alkaline water, so ensuring adequate biological media is important.

A reliable heater is essential, as tropical temperatures must be maintained year-round. A heater rated for the tank volume with an accurate thermostat is sufficient. Place it near the filter outlet to ensure even heat distribution. A thermometer – ideally a digital one for accuracy – should be positioned where you can easily read it during routine checks.

Lighting, as discussed above, should be kept subdued. LED units with adjustable brightness or colour temperature are useful because they allow you to dial down the intensity. A timer that provides 8-10 hours of light per day is recommended. If the tank is positioned near a window, be mindful of direct sunlight, which can raise temperatures unpredictably and promote algae growth.

An air pump is not strictly necessary if the filter provides adequate surface agitation for gas exchange. However, in heavily planted or tannin-rich setups where oxygen levels can drop at night, a gentle airstone on a timer (running during the dark hours) can be a useful safeguard.

Diet and Feeding

In the wild, Three-lined Pencilfish are micro-predators, feeding on tiny invertebrates, insect larvae, zooplankton, and organic particles gleaned from plant surfaces and the water column. Their small mouths limit them to very small food items, and their slow, deliberate feeding style means they can easily be outcompeted by faster or more aggressive species at feeding time.

In captivity, a varied diet is essential for health and colour. High-quality micro-pellets or finely crushed flake food can form the staple, but live and frozen foods should be offered regularly – ideally several times a week. Baby brine shrimp (Artemia nauplii), micro-worms, grindal worms, daphnia, and cyclops are all excellent choices and will bring out the best colouration and condition in your fish. Frozen foods should be thawed and rinsed before feeding to avoid introducing excess phosphates into the tank.

Feed small amounts two to three times daily rather than one large feeding. This approach better mimics the continuous foraging behaviour of wild pencilfish and reduces the risk of food sinking to the substrate and fouling the water before the fish can eat it. Three-lined Pencilfish tend to feed in the mid-water and upper water column, so foods that remain suspended or sink slowly are preferable to rapidly sinking pellets.

Watch carefully during feeding to ensure all individuals are getting their share. In a community tank, more assertive species may consume the food before the pencilfish have had a chance to locate it. If this becomes a problem, consider target-feeding the pencilfish using a pipette or feeding at opposite ends of the tank to distract faster feeders.

Behaviour and Temperament

The Three-lined Pencilfish is a peaceful, somewhat shy species that spends much of its time hovering in the mid-water among plant stems, driftwood branches, and floating plant roots. It swims with the characteristic pencilfish posture – body held at a slight upward angle, moving in slow, deliberate glides punctuated by brief pauses. In a well-planted tank with subdued lighting, groups will often spread out across the mid-water zone, each fish occupying a small area of personal space while remaining within visual contact of the shoal.

Although generally peaceable, males can be mildly territorial with one another, particularly in smaller groups. This behaviour rarely escalates beyond brief chases and fin-flaring displays, but keeping a group of at least six, and ideally eight to ten, helps distribute any aggression and prevents any single individual from being persistently harassed. A larger group also produces more confident, natural behaviour, with the fish spending more time in open water rather than hiding among the plants.

One of the most fascinating aspects of pencilfish behaviour is their nocturnal colour change. As the lights dim or go off, the bold horizontal stripes fade and are replaced by broad, pale vertical bands. This cryptic pattern presumably helps the fish avoid nocturnal predators in the wild. The transformation is rapid, often occurring within seconds as the lights change, and reverses equally quickly when daylight returns. New keepers sometimes mistake this for a sign of illness, but it is entirely normal and healthy behaviour.

Three-lined Pencilfish are not particularly active swimmers and do not require large open swimming areas. They prefer complexity – dense plant growth, root tangles, and shaded retreats where they can hover and forage. Providing this complexity is key to reducing stress and encouraging the fish to display their full behavioural repertoire.

Tank Mates

Choosing tank mates for the Three-lined Pencilfish requires careful thought. This is a slow, delicate species that will be stressed and outcompeted by boisterous, fast-moving, or aggressive fish. The ideal companions are other small, peaceful species that share similar water chemistry requirements – soft, acidic conditions – and will not dominate at feeding time or harass the pencilfish. Blackwater community tanks built around South American species are the natural choice, though some Southeast Asian species adapted to soft water can also work well.

Good tank mates

• Cardinal Tetra – A classic blackwater companion that thrives in identical water conditions and is equally peaceful.
• Green Neon Tetra – Another soft-water tetra that is small, gentle, and occupies a similar mid-water zone without causing conflict.
• Ember Tetra – Tiny and undemanding, ember tetras add a warm splash of colour and are far too small and mild to bother pencilfish.
• Chocolate Gourami – A soft-water labyrinth fish that is equally shy and slow-moving, making it an excellent upper-water companion.
• Pygmy Corydoras – A bottom-dwelling species that stays small and peaceful, occupying a different zone entirely.
• Otocinclus – A gentle algae grazer that largely ignores other fish and helps keep the tank clean.
• Apistogramma agassizii – A dwarf cichlid that coexists well with pencilfish, as it primarily occupies the lower water column and is not overly aggressive outside of breeding.
• Beckford’s Pencilfish – A closely related pencilfish species with similar care requirements that can cohabit peacefully in a spacious tank.

Fish to avoid

• Tiger Barb – Notorious fin nippers that are far too boisterous and aggressive for delicate pencilfish.
• Convict Cichlid – A territorial and aggressive cichlid that would quickly dominate and stress pencilfish.
• Angelfish – Although sometimes kept in soft water, adult angelfish are large enough to view pencilfish as prey.
• Buenos Aires Tetra – A large, fast, and somewhat nippy tetra that will outcompete pencilfish at feeding time.
• Chinese Algae Eater – Becomes aggressive and territorial with age and may harass slow-moving tank mates.
• Oscar – A large predatory cichlid entirely incompatible with any small, delicate species.
• African Cichlids – Require hard, alkaline water and are generally far too aggressive for pencilfish communities.

Breeding

Breeding the Three-lined Pencilfish in captivity is possible but requires patience, careful preparation, and precise water conditions. It is not a species that breeds readily in a community tank, and most successful spawnings occur in dedicated breeding setups.

To condition a breeding pair or group, feed heavily with live and frozen foods for two to three weeks. Males will intensify in colour and begin displaying more actively to females, flaring their fins and chasing potential mates with gentle persistence. Females in breeding condition will appear noticeably rounder when viewed from above.

The breeding tank should be small – 20 to 30 litres is sufficient – and set up with very soft, acidic water (pH 5.0-5.5, GH below 2 dGH). The temperature should be at the upper end of the species’ range, around 27-28 C. Use a sponge filter for gentle filtration and include clumps of fine-leaved plants such as Java moss or spawning mops, which serve as egg-deposition sites. Dim lighting or even near-darkness encourages spawning behaviour.

Spawning typically occurs in the early morning. The pair will come together among the fine-leaved plants, and the female will scatter small, adhesive eggs among the vegetation. The eggs are tiny and translucent, making them difficult to spot. Parents will readily eat their own eggs, so remove the adults promptly after spawning is observed.

The eggs hatch in approximately 24 to 36 hours at 27 C, and the fry become free-swimming after a further two to three days. The fry are extremely small and require infusoria or liquid fry food for the first week or so before they are large enough to accept freshly hatched brine shrimp nauplii. Growth is slow, and the fry are delicate, requiring consistently clean water and frequent small feedings throughout the day. Maintaining stable water parameters in the rearing tank is critical, as even minor fluctuations can cause losses in very young fish.

Common Diseases

Three-lined Pencilfish are not unusually susceptible to any species-specific diseases, but their requirement for soft, acidic water and their sensitivity to poor water quality mean that stress-related illnesses can arise quickly if conditions deteriorate. Prevention through consistent water quality management is always preferable to treatment.

Ich (white spot disease) is one of the most common ailments seen in freshwater fish and can affect pencilfish, particularly after a stressful event such as transport, a sudden temperature drop, or the introduction of new, unquarantined fish. The characteristic white spots on the body and fins are usually easy to identify. Raising the temperature gradually to 30 C and using a half-dose of a malachite green-based treatment (pencilfish can be sensitive to full doses of some medications) is a standard approach. Always remove activated carbon from the filter before medicating.

Fungal infections may appear as white or grey cotton-like growths on the body or fins, often secondary to a physical injury or bacterial infection. These are typically treated with antifungal medications, but addressing the underlying cause – usually poor water quality or aggression from tank mates – is essential to prevent recurrence.

Bacterial infections, including fin rot and columnaris, can occur in stressed or immunocompromised fish. Symptoms include frayed or eroding fins, white or grey patches on the body, and lethargy. Maintaining pristine water quality is the first line of defence, and mild cases often resolve with improved conditions alone. More severe infections may require treatment with antibacterial medications.

Internal parasites can be introduced through live foods or newly acquired fish. Symptoms include weight loss despite a good appetite, white or stringy faeces, and a gradual decline in condition. Anti-parasitic foods or medications may be needed, though prevention through quarantining new arrivals and sourcing live foods from reputable suppliers is the best strategy.

It is worth noting that many common aquarium medications are formulated for use in neutral to slightly alkaline water and can behave unpredictably in the very acidic conditions preferred by pencilfish. Always research medication compatibility with soft, acidic water before dosing, and consider using half-strength doses initially to gauge the fish’s tolerance.

FAQs

How many Three-lined Pencilfish should I keep together?

A minimum group of six is recommended, but eight to ten or more is preferable. Larger groups dilute any mild male-on-male aggression and produce more natural, confident behaviour. In a group that is too small, individual fish may become chronically shy, spending most of their time hidden among plants and rarely displaying their full colours.

Why do my pencilfish change colour at night?

This is completely normal behaviour shared by all Nannostomus species. At night or in very dim conditions, the horizontal stripes fade and are replaced by broad vertical bars. This nocturnal colour pattern is thought to serve as camouflage against predators. The fish will return to their normal striped pattern within seconds of the lights coming back on.

Can Three-lined Pencilfish live in hard tap water?

They can survive in moderately hard water for a time, but they will not thrive or display their best colours, and long-term health is likely to suffer. This species genuinely needs soft, acidic water. If your tap water is hard, using reverse osmosis (RO) water remineralised with a product designed for soft-water species is the most reliable approach.

Will Three-lined Pencilfish eat shrimp?

Adult pencilfish have very small mouths and are unlikely to threaten adult dwarf shrimp such as Neocaridina or Caridina species. However, they may pick off tiny shrimplets if they encounter them, as this falls within their natural feeding behaviour on small invertebrates. If you are breeding shrimp and want maximum survival of young, dense plant cover and mosses will help provide refuge.

Do Three-lined Pencilfish jump?

Yes, like most small characins, pencilfish are capable of jumping, particularly when startled. A tight-fitting lid or a cover of dense floating plants is advisable to prevent losses. Gaps around filter inlets, heater cables, and other equipment should also be blocked where possible.

Related Guides

• Beckford’s Pencilfish – The most commonly kept pencilfish species, with similar but slightly less demanding care requirements.
• Coral Red Pencilfish – A strikingly coloured relative that shares the same blackwater preferences and makes a beautiful companion species.
• Cardinal Tetra – A quintessential blackwater tetra and one of the best tank mates for any pencilfish community.
• Apistogramma agassizii – A popular dwarf cichlid for soft-water community tanks that pairs well with pencilfish.
• Chocolate Gourami – Another demanding but rewarding soft-water species that thrives in similar blackwater conditions.