Dwarf Pencilfish Care Guide

The dwarf pencilfish is a small (~2.5-3.5 cm) soft-water characin whose defining trait is not aggression but timidity: it is a shy, slow, deliberate feeder, and its core welfare risk is being out-competed and intimidated rather than eaten. Kept with fast or boisterous tankmates it retreats to the shadows, stops feeding and slowly starves — so it is 'calm or starve', a peaceful planted-nano fish for similarly small, calm company only, never a robust general-community starter.

Dwarf Pencilfish at a glance

The sourced figures the welfare engine uses to judge Dwarf Pencilfish — the parseable key facts.

Where it comes from

It comes from northern South America — Guyana, Suriname and the lower-to-middle Amazon basin, with a type locality at Maduni Creek, Guyana, and named drainages from the Rupununi and Essequibo to the Rio Negro and Branco. Its habitat is sluggish tributaries, small rivers and swampy areas with dense marginal vegetation: shallow, near-still water with very little detectable hardness, frequently blackwater (a sampled stream measured pH 5.51-6.01 and ~25 °C), though it occurs in clearwater too. That biotope explains the care. The soft, acidic, mineral-poor origin calls for soft, acidic, tannin-stained, dimly-lit, densely-planted water with leaf litter and driftwood, which lowers stress and brings out the red. The near-still swamp water means gentle flow — a strong current is unnatural and stressful for a small deliberate fish. And the shaded, vegetation-dense habitat is why this shy fish needs cover: paradoxically, plenty of planting makes it more visible, because it only comes out when it feels safe.

Did you know?

• It changes colour at night: the dark lateral stripes disappear and two oblique bars appear behind the dorsal fin, so it is effectively a day-fish and a night-fish in one body.
• It is one of the smallest pencilfish in the trade — body length ~2-2.5 cm (standard length), ~3-3.5 cm including the tail.
• It has no 'boy versus girl' colours — uniquely among many aquarium fish it lacks sexual dichromatism, so you sex it by the female's fuller belly, not by colour.
• It is a peaceful fish with an aggressive twin: its trade name is shared with the genuinely feisty coral-red and purple pencilfish, same nickname but very different temperament.
• It is 'greedy' only when it feels safe — branded a shy slow feeder, in the right calm company it becomes a reliable, even greedy eater, proof that its feeding problems are social, not dietary.

Tank size — and why

About 10 US gallons is a sound practical minimum for a group of eight to ten — it matches Aquarium Co-Op's floor and sits a little above the Seriously Fish 45 x 30 cm base. The limiting factor is group size plus the calm, planted, low-stress environment this timid fish needs, not bioload, which is low. A longer ~60 cm / 20-gallon footprint is better for a bigger, bolder, more visible shoal kept with calm tankmates. Prioritise footprint over height for the mid-water swimming lane and to let the shoal spread out.

As a guide, a 20-gallon tank comfortably suits about 8–12 Dwarf Pencilfish as a single-species display, leaving room for tankmates.

See it to scale

Adult Dwarf Pencilfish reach only about 2.5 cm (1 in) long — close to the size they are sold at, so what you see is roughly what you get. The catch is the group: a proper shoal still needs about a 10-gallon tank, around 51 cm long.

Adult size is sourced; tank length is approximate for a standard 10-gallon aquarium.

Water parameters in practice

Soft and acidic is strongly preferred. Aim for a stable mid-20s °C comfort target — roughly 24-26 °C — rather than the warm end of the tolerated band (sources span 21-28 °C, but most cap around 26-27 °C). Steer the pH towards the soft acidic end, around 5.5-6.5, in very soft water (~1-8 dGH), which is where colour, comfort and any spawning come good; store-acclimated stock tolerates near-neutral, moderately soft water. Most stock is wild-caught and adapted to extremely soft acidic blackwater, so it dislikes hard, alkaline or unstable water — it can survive harder water but is then more prone to stress-related conditions. Acclimate slowly and keep parameters stable; the danger window is the first weeks after import. (One care source lists a hard-water hardness figure that contradicts every other source and is best ignored.)

Will it thrive in your water?

The comfortable range for Dwarf Pencilfish is about 22–28 °C (72–82 °F) and pH 4–7. Test your own tap water against it below.

These are the sourced comfortable ranges. Stable water matters more than chasing an exact number — a steady reading inside the band beats a "perfect" one that drifts. Some fish also need a particular water hardness (GH); where that applies, the prose above covers it.

Diet & feeding

In the wild it is a micropredator feeding on very small animals and zooplankton. In the tank it accepts dried foods of a suitable size plus small live and frozen fare — Artemia nauplii, moina, grindal worm, baby brine shrimp, cyclops, daphnia and crushed flake. The genus has small, upturned mouths, so favour tiny foods that float or sink slowly. The core welfare point is feeding behaviour: this is a shy, unhurried, even 'nervous' feeder that loses every race against fast or aggressive eaters like some barbs and larger tetras, sticking to the shadows and picking off whatever drifts over, which puts it at high risk of not getting enough food. The fix is not exotic food but calm tankmates plus small, slow-sinking fare, fed as two small meals a day rather than one large one. The reassuring nuance: once settled in the right company it becomes a reliable, even 'greedy' feeder that will eat anything that fits its tiny mouth — its feeding problems are social, not dietary.

Gear & setup

A mature, densely-planted, dimly-lit soft-water tank with a dark substrate is the natural fit. Dense fine-leaved planting and floating plants for shade, plus driftwood and leaf litter or botanicals for tannins, are welfare-critical for this shy species — cover is what coaxes it out of hiding. A heater holds the stable mid-20s. Keep flow gentle, matching the near-still swamp habitat; avoid a strong current. As an upper-water characin, a lid is a sensible jump precaution.

Temperament & behaviour

Peaceful and gregarious but timid — Seriously Fish calls it 'very peaceful but does not make an ideal community fish'. It occupies mid-to-upper water in a loose shoal and shows essentially no aggression toward other species; males may hold small territories with no real aggression toward tankmates, and within-group squabbling is minor display, not damage. It is not a fin-nipper. Crucially, this peacefulness is specific to Nannostomus marginatus — the look-alike coral-red and purple pencilfish are genuinely feistier (see below). The behaviour the buyer wants appears only with numbers, cover and calm company: get those right and it is an active, coloured, confident shoal that feeds in the open; get them wrong and it is a pale, skittish, hidden, under-fed fish.

Group & social needs

Keep a group — Seriously Fish says to buy as many as possible, ideally 10 or more, AquaInfo recommends at least 10 because of their skittish nature, and FishBase and Aquarium Co-Op set 5-6 as a floor. Treat eight as a reasonable welfare minimum and aim for 10+. In small groups they are shy, washed-out and hidden; numbers plus cover give them the confidence to come out, colour up and feed in the open. They are described as 'brave once settled' but the first species to disappear when frightened, so a big secure group is what keeps them visible.

Compatible tank mates (preview)

A short, engine-cleared shortlist — the species TankStocking's welfare engine clears with Dwarf Pencilfish and that suit its size and temperament best. Tap a name for its care guide, or use + to load the pairing in the planner.

• Amano Shrimp+Peaceful temperament, similar adult size
• Assassin Snail+Uses the bottom zone, peaceful temperament, similar adult size
• Black Neon Tetra+Peaceful temperament, similar adult size
• Black Phantom Tetra+Peaceful temperament, similar adult size
• Cardinal Tetra+Peaceful temperament, similar adult size
• Celestial Pearl Danio+Peaceful temperament, similar adult size
• Checker Barb+Peaceful temperament, similar adult size
• Cherry Barb+Peaceful temperament, similar adult size

A note on the shrimp and snails here: Dwarf Pencilfish is peaceful and generally invertebrate-safe — but almost any fish will take very small shrimplets given the chance, so give shrimp dense cover (moss, leaf litter) if you want a colony to grow, rather than expecting every baby to survive.

This engine-cleared shortlist is Dwarf Pencilfish's tankmate surface for now — a dedicated tank-mates guide can follow for high-demand species.

Breeding & sexing

There is no colour difference between the sexes — the species lacks sexual dichromatism, so sex by body shape: males are noticeably less stocky, females rounder and fuller-bodied (described as 'chunkier, more blimp-shaped'). Breeding is moderate-to-advanced and rarely achieved in aquaria. Use a mature, densely-planted, dimly-lit, soft acidic tank and let them spawn over fine wool mops, Java moss or other fine-leaved plants. They are egg-scatterers with no parental care that eat their own eggs, so use thick cover or mesh and remove the adults after spawning. Relatively few eggs are scattered among the plants; they hatch in about one to two days, with fry visible a day or two after the adults are out. The tiny fry need infusoria or rotifers first, progressing to microworms and Artemia nauplii, so a mature, microfauna-rich tank helps.

Lifespan

Roughly three to five years with good care, toward the upper end in a stable, well-fed soft-water tank. What shortens it is slow starvation and chronic stress from being out-competed by faster, bolder tankmates (the species-specific number-one killer), hard, alkaline or unstable water for soft-water wild stock, bare bright low-cover tanks that keep a shy fish permanently stressed, and predation by anything large enough to swallow a 3 cm fish.

Common mistakes

• Housing them with fast or boisterous feeders. The single biggest error — barbs, large or active tetras and other greedy fish out-compete and starve a slow, deliberate pencilfish even without any real aggression.
• Putting them in a busy general community tank. They are timid; constant bustle keeps them hidden, pale and under-fed.
• Keeping too few. A group of three to five stays skittish, washed-out and hidden — buy eight to ten or more.
• A bare, bright tank with no cover. No plants or shade means permanent stress for a shy fish — plant densely and dim it.
• Hard, alkaline or unstable water. Wild stock is adapted to soft acidic blackwater and gets stress-related conditions in hard water.
• Buying the wrong 'dwarf pencilfish'. The genuinely feisty coral-red (N. mortenthaleri) and purple (N. rubrocaudatus) species are sold under overlapping names — know which one you're getting; care advice calling dwarf pencilfish 'feisty' is usually describing those, not this peaceful species.

Signs of trouble

• A thin, sunken belly — the starvation tell, almost always from being out-competed by faster, bolder tankmates.
• Colour fading and a fish hanging away from the shoal — water quality, an immature or bare tank, or too small a group.
• Persistent skittishness and a fish that vanishes at the slightest disturbance — too little cover, too small a group, or boisterous company.
• Clamped fins, hiding and erratic swimming, especially in newly-bought wild fish — settling stress, often worsened by hard or unstable water.
• Flicking, white spots or fungal patches — standard characin ailments; treat cautiously, as small soft-water fish can be sensitive to full-dose medications.

Is this fish right for you?

Don't buy dwarf pencilfish for a community of fast, boisterous or large fish; for a bare, bright, high-flow tank; for hard alkaline water you can't soften; or if you can't commit to a group of eight to ten or more with calm, similarly small tankmates and small, slow-sinking food twice a day. Above all, don't buy them expecting a robust general-community fish — they are peaceful toward others but easily bullied and starved themselves, so the wrong tankmates are a slow death sentence. Watch out for the look-alike trap, too: the aggressive coral-red and purple pencilfish are sold under the same nickname, so confirm you are getting true Nannostomus marginatus. Stock is largely wild-caught (the species itself is IUCN Least Concern), so quarantine and slow-acclimate new arrivals; there are no dyed or balloon morphs to avoid.

Bringing one home

Most stock is wild-caught and adapted to extremely soft acidic water, so acclimate slowly — float to match temperature, then add tank water a little at a time over a good twenty minutes before netting the fish across and leaving the shop water behind. Add it only to a mature, soft, acidic, stable, densely-planted, dimly-lit tank, and quarantine new arrivals, since transport-stressed wild fish are at their most fragile in the first weeks.

Common questions

Is the dwarf pencilfish aggressive?

No — true Nannostomus marginatus is very peaceful, with no real aggression toward tankmates and only minor within-group display. The confusion comes from look-alikes: the coral-red (N. mortenthaleri) and purple (N. rubrocaudatus) pencilfish sold under overlapping names are genuinely feisty, and care advice calling 'dwarf pencilfish' feisty is usually describing those.

What tankmates suit a dwarf pencilfish?

Calm, similarly small fish only — small peaceful tetras, other peaceful Nannostomus, harlequin and other gentle rasboras, Boraras, pygmy or dwarf Corydoras, and quiet upper-dwellers like hatchetfish. Avoid fast, greedy or boisterous feeders such as tiger barbs and larger active tetras, and anything big enough to swallow a 3 cm fish. Adult dwarf shrimp are usually safe, but it will prey on shrimplets.

Why is my dwarf pencilfish hiding and not eating?

Almost always because it is being out-competed or intimidated. It is a shy, slow, deliberate feeder that loses food races to faster fish and retreats to the shadows, risking slow starvation. The fix is calm small tankmates, dense cover and a big group, plus small slow-sinking food fed as two small meals a day — once settled in the right company it feeds readily.

What water does a dwarf pencilfish need?

Soft and acidic — a stable mid-20s °C (around 24-26 °C) and pH ~5.5-6.5 in very soft water, dimly lit and densely planted with tannins from leaf litter or driftwood. Wild stock dislikes hard, alkaline or unstable water and becomes stress-prone in it.

How many dwarf pencilfish should I keep?

Aim for eight to ten or more; five to six is the absolute floor. Small groups stay skittish, washed-out and hidden, while a big secure group with plenty of cover gives them the confidence to come out, colour up and feed in the open.

Sources & confidence

Sources & confidence (9 species)

These back the Dwarf Pencilfish figures and the previewed tank mates above. Each figure is read from the TankStocking species database (v2026.06); below is the care reference behind it and how confident we are in that data. Confidence reflects the source quality, not whether any pairing is safe. Full source list and the welfare model are on the methodology page.

• Dwarf Pencilfish Nannostomus marginatus — Seriously Fish (seriouslyfish.com/species/nannostomus-marginatus) high confidence
• Amano Shrimp Caridina multidentata — Aquarium Co-Op amano shrimp care; Aquadiction high confidence
• Assassin Snail Anentome helena (Clea helena) — The Shrimp Farm (theshrimpfarm.com/posts/assassin-snail-care) high confidence
• Black Neon Tetra Hyphessobrycon herbertaxelrodi — Seriously Fish / Aqua-Fish (Hyphessobrycon herbertaxelrodi) high confidence
• Black Phantom Tetra Hyphessobrycon megalopterus — Seriously Fish (Hyphessobrycon megalopterus) high confidence
• Cardinal Tetra Paracheirodon axelrodi — Seriously Fish (seriouslyfish.com/species/paracheirodon-axelrodi) high confidence
• Celestial Pearl Danio Celestichthys margaritatus — Seriously Fish (seriouslyfish.com/species/celestichthys-margaritatus) high confidence
• Checker Barb Oliotius oligolepis — Seriously Fish — Oliotius oligolepis (https://www.seriouslyfish.com/species/oliotius-oligolepis/) high confidence
• Cherry Barb Puntius titteya — Seriously Fish (Puntius titteya) seriouslyfish.com/species/puntius-titteya high confidence

Care-guide sources (6)

This guide synthesises the references below; where they disagree, the range and the disagreement are noted in the text above. The figures in the key-facts box are read from the TankStocking species database (v2026.06). Full welfare model on the methodology page.

• FishBase — Nannostomus marginatus — authority (Eigenmann 1909), family Lebiasinidae, max 3.5 cm TL, temp 24-26 °C, pH 5.8-7.5, dH up to ~15, trophic level 3.2, range (lower-middle Amazon, Colombia/Guyana/Peru/Suriname/Venezuela), 'in groups of 5 or more', min aquarium 60 cm, IUCN Least Concern (assessed 21 Sep 2020)
• Seriously Fish — Nannostomus marginatus — authority/family, synonym N. m. picturatus, Maduni Creek type locality + range, blackwater biotope + measured stream chemistry (pH 5.51-6.01, 25 °C), aquarium envelope 22-28 °C / pH 4.0-7.0 / 18-179 ppm, 20-25 mm SL, 45x30 cm tank, micropredator diet, 'very peaceful but does not make an ideal community fish', 'gregarious', 'buy as many as possible, ideally 10 or more', sexing (males less stocky), lacks sexual dichromatism, short anal fin, night colour-change, egg-scatterer breeding rarely achieved
• Aquarium Co-Op — Care Guide for Pencilfish — genus-level: 10-gal min (20-high/29 preferred), 75-80 °F (24-27 °C), pH 6.5-7.5, school of at least 6, upturned mouths need tiny floating/slow-sinking food, tankmate list, prey on cherry/crystal shrimp, coral-red and purple species feisty (look-alike-temperament flag), breeding (eggs hatch 1-2 days)
• AquaInfo — Nannostomus marginatus — max ~4.5 cm (outlier), temp 21-26 °C, pH 5.5-7.0, GH 0-8, range and blackwater/clearwater biotope, peaceful (males small territories, 'no real aggression'), group at least 10 (skittish), micropredator diet, 'nervous' feeder that 'may not receive adequate food with bold companions', two small daily meals, 'females much fuller than males', egg-scatterer that eats eggs, fry on infusoria then artemia
• TinyMenagerie — Dwarf Pencilfish Care Guide — size up to 1.3 in, lifespan 3-5 yr (longer with optimal care), temp 22-26 °C, pH 6-7, soft-water preference ('more prone to stress related conditions' in hard water), group of at least 6, 60 cm tank, 'brave once settled' but 'first to disappear' when frightened, slow-but-greedy-once-settled feeder out-competed by 'barbs and larger tetras', good mates (small tetras/cories/hatchetfish), females 'chunkier/blimp shaped'
• aqua-fish.net — Dwarf Pencilfish — lifespan 2-5 yr, usual size 3-4 cm, temp 24-27 °C, pH 5.8-7.3, group of at least 10 (skittish), peaceful, 'they cannot compete with other tank mates so only house ... with other peaceful species', egg-scatterer in soft acidic water, eggs hatch ~2 days (one hard-water hardness figure flagged as an unreliable outlier and not used)

More on Dwarf Pencilfish

Related guides on TankStocking — each scored by the same welfare engine as the planner.

This care guide is a sourced planning reference, not veterinary advice — individual fish, filtration and maintenance all matter. Cycle the tank, test your water, and observe your fish. How TankStocking works →