Dwarf Pencilfish (Nannostomus marginatus)

Dwarf Pencilfish (Nannostomus marginatus) are tiny South American shoaling fish with a calm manner, a neat horizontal pattern and flashes of red in the fins. They are best appreciated in a mature, planted aquarium where floating cover, gentle flow and a proper group allow their unhurried hovering behaviour to develop. Adults reach about 3.5 cm total length, yet their need for stable water and social company means they should not be treated as bowl fish or squeezed into an immature desktop tank.

This listing is for one unsexed fish in the live size selected above. Plan the whole group before ordering: authoritative references advise keeping at least five together, and a larger group usually gives these naturally social fish more security and a better display. A tank with a footprint of at least 60 cm and roughly 45 litres or more is a sensible starting point for a small species group; choose a larger aquarium when adding other fish. Check the live variant selector for the sizes currently available.

What is Nannostomus marginatus?

Nannostomus marginatus is a valid species in the pencilfish family Lebiasinidae. The Catalog of Fishes and FishBase record it from Amazon and Orinoco basin drainage areas across Brazil, Colombia, Ecuador, Guyana, Peru, Suriname and Venezuela. These records describe a freshwater fish associated with quiet tropical habitats rather than a British native or food fish. That distinction matters because older copy on this page had attracted irrelevant UK river-fishing phrases that offered no help to aquarium keepers.

The common name comes from the species' slim, elongated body. Its dark horizontal markings and pale-to-bronze ground colour are relieved by red accents, particularly in settled adults. Colour varies with age, condition, lighting, social confidence and the background of the aquarium. A newly delivered juvenile may therefore look paler than a settled adult photographed under display lighting.

Sexual differences can be subtle. Mature females often appear fuller through the abdomen, while mature males may be slimmer and show stronger red markings. Juveniles and newly imported fish should be sold as unsexed unless a specific variant explicitly states otherwise. One fish is not a complete shoal, so calculate the planned number and total adult stocking load before checkout.

Natural range and habitat clues

Across its South American range, this small species is associated with sheltered freshwater habitats. For the aquarist, the useful lesson is not to reproduce one river scene exactly, but to provide the features that make a small, cautious fish feel secure: visual cover above and behind it, subdued areas, gentle movement and clean, stable water. Roots, fine branches, leaf-like tones and patches of floating vegetation can create that structure without turning the aquarium into a dark or cluttered box.

A planted community can work just as well as a strict biotope. Leave a clear route through the upper and middle water so the group can move together. Dense planting on the sides and rear gives retreat points, while a darker substrate and broken overhead light can help the fish show more confident colour. Any natural leaves or wood added for tannins should be aquarium-safe and introduced with an understanding of how they may alter pH.

Aquarium size and group planning

Small adult length does not remove the need for swimming room, water stability or a social group. FishBase gives a minimum aquarium length of 60 cm for this species, while OATA's general guidance for small pencilfish starts at around 45 litres. Taken together, a mature aquarium of roughly 45 litres with a 60 cm footprint is a responsible lower starting point for a modest species group. A longer or larger tank is the better choice for a mixed community, a bigger shoal or less experienced keepers.

Keep at least five together. Where the aquarium has enough room and biological capacity, a larger group can improve security and lets the fish distribute normal social attention across more individuals. Do not solve nervous behaviour by simply adding fish to an already overstocked tank. Use the aquarium volume calculator, consider the adult size of every inhabitant and allow capacity for the filter to process the resulting waste.

Situation	Planning guidance	Reason
Small species display	60 cm footprint, about 45 litres or more, with at least five fish	Provides social company, cover and usable horizontal room
Planted community	Choose more volume and length than the minimum	Allows space for compatible companions and steadier water
Immature or uncycled tank	Wait until testing proves the filter is cycled	A calendar alone cannot confirm safe ammonia and nitrite processing
Bowl or unfiltered desktop container	Not suitable	Cannot provide the required group, stability, filtration and swimming room

Water parameters and filtration

For species-specific care, FishBase records 24-26°C, pH 5.8-7.5 and hardness up to 15 dGH. Aim for the stable middle of a suitable range rather than moving pH or hardness rapidly to chase an exact number. Ask the retailer about the water in which the fish have been held, compare it with your aquarium and acclimate carefully when there is a meaningful difference.

OATA advises 0 mg/L ammonia, 0 mg/L nitrite and nitrate no more than 20 mg/L above the nitrate level in the source tap water. Test regularly with a reliable kit, especially after adding livestock, changing filter media or noticing altered behaviour. Our water chemistry guide explains the role of these measurements and why stable biological filtration matters.

Use a filter sized for the aquarium, but soften any forceful outlet. An air-driven sponge filter, a gentle internal filter or a baffled return can all work when they provide biological filtration and gas exchange without pushing the group continuously around the tank. Position the flow so there are calm areas and watch the fish: persistent struggling, hiding behind equipment or being pinned into one zone means the current needs adjustment.

Never rinse all biological media under untreated tap water or replace every mature filter component at once. Maintain it in removed aquarium water when required and preserve enough established media to support the nitrogen cycle. Regular partial water changes should closely match the aquarium temperature and avoid sudden swings in chemistry.

Plants, lighting and substrate

A useful layout combines cover with open water. Plant the rear and sides, add some fine-leaved growth and provide floating cover over part of the surface. Leave gaps for feeding and observation. Floating plants can diffuse intense LEDs, but they should not seal the entire surface or prevent adequate gas exchange.

Dark sand or fine, smooth gravel often gives the fish visual security and makes the lateral pattern easier to see. Substrate colour is an aesthetic and behavioural choice, not a substitute for water quality. Keep sharp decor away from narrow routes, secure wood and stone, and make sure every opening is either too small to enter or large enough for a fish to leave safely.

Moderate, consistent lighting suits a planted display. There is no universal number of lighting hours that works for every aquarium because plant mass, lamp strength, nutrients and ambient daylight differ. Begin with a controlled photoperiod, watch fish comfort and algae growth, then adjust gradually. Plenty of shaded structure is more important than making the whole tank dim.

Social behaviour and what to watch

Dwarf Pencilfish are peaceful rather than robust. In a settled group they often hover at a slight angle, make short movements between positions and feed from small items in the upper half of the aquarium. They may spread out instead of moving as a tight school every minute; grouping more closely can be a response to uncertainty as well as normal social behaviour.

Brief display and chasing can occur within a group, particularly among mature fish, but it should not leave one individual constantly isolated, damaged or unable to feed. A larger social group, broken sight lines and adequate room help prevent attention from being concentrated on one fish. Check that all individuals obtain food because quiet species can be outcompeted without obvious fighting.

Hiding after delivery is expected, but persistent clamped fins, rapid breathing, loss of balance, refusal of food, spots, sores or progressive weight loss require investigation. Begin with water tests, temperature and observation of tank-mate behaviour. A behavioural change is a warning sign, not a diagnosis.

What should Dwarf Pencilfish eat?

These fish are omnivorous and need appropriately tiny food. A good routine alternates high-quality fine flake or micro-granules with small frozen or live foods such as daphnia, cyclops or suitably sized brine shrimp. Crumble any prepared food that is too large, and make sure pieces remain accessible in the part of the water column the fish use.

OATA's general pencilfish guidance recommends offering only what is eaten within a few minutes, typically once a day. In practice, very small fish may also be managed with smaller divided portions where the aquarium's routine and water quality support that approach. The key is total quantity: food should reach every fish without leaving a layer to decay. Remove obvious leftovers and do not use extra feeding as a substitute for separating aggressive competitors.

Food type	How to use it	Watch for
Fine flake or micro-granule	Use as a measured staple and crush to mouth size	Large or fast-sinking pieces being missed
Daphnia or cyclops	Offer as small live or frozen variety	Overfeeding frozen liquid and fragments
Suitably sized brine shrimp	Useful variety and conditioning food	Prey larger than small juveniles can swallow
Large bloodworm	Usually not a practical staple for this tiny mouth	Oversized pieces, waste and uneven feeding

A slightly rounded abdomen after feeding can be normal; continual swelling, wasting or stringy waste is not something to diagnose from a product page. Isolate and seek specialist aquatic veterinary advice when illness is suspected. Avoid adding medication, salt or copper to a community tank without identifying the problem and checking the sensitivity of every animal present.

Compatible tank mates

Choose small, peaceful species that accept similar temperature and water chemistry. Suitable companions must also feed calmly enough for a slender pencilfish to get its share. Adult Amano shrimp and non-predatory snails are often considered practical invertebrate companions, although any fish may investigate very small shrimplets. Every proposed combination still needs to be checked against the actual aquarium's volume, water and stocking level.

Potential fish companions include gentle small tetras, rasboras and smaller Corydoras species, but a shared common label does not guarantee compatibility. Compare adult size, temperature, hardness, activity and feeding style before purchase. Currently available examples worth researching include Ember Tetra and Green Neon Tetra. For another member of the same genus, see Beckford's Pencilfish; it is a related option, not an instruction to mix species without a stocking plan.

Amano Shrimp can add activity lower in the aquarium, provided their own needs are met and they are not expected to replace routine cleaning. Avoid large cichlids and catfish capable of swallowing a 3.5 cm fish, persistent fin nippers, forceful barbs and very fast feeders. A fish that is peaceful toward humans or members of its own species is not automatically safe with tiny tank mates.

Companion type	Assessment	Checks before mixing
Small calm tetras or rasboras	Often suitable	Adult size, temperature, hardness, group sizes and feeding pace
Small peaceful Corydoras	Potentially suitable	Floor space, substrate, temperature and total stocking
Adult Amano shrimp and snails	Often suitable	Invertebrate-safe treatment plan and adequate food
Large cichlids, predatory fish or fin nippers	Avoid	Predation, injury and chronic stress risk
Very fast, boisterous feeders	Usually avoid	Whether every pencilfish can feed without being displaced

Delivery, acclimation and the first days

Prepare the aquarium before the parcel arrives. Test water, check the heater and filter, reduce strong lighting and make sure the lid is secure. Read our live-fish delivery information for the current dispatch process and Live Arrival Guarantee terms; those live policy pages are the authority if a promotional sentence elsewhere becomes outdated.

On arrival, inspect the sealed bag before opening it and follow the acclimation instructions supplied with the order. Do not leave fish in a shipping bag while performing an unnecessarily long procedure. Temperature, transit time and the difference between bag and aquarium water all affect the appropriate method. Never pour shipping water into the display tank if the supplied instructions advise transfer by net or container.

Keep lighting subdued while the group settles. Offer a very small meal only when the fish are upright, responsive and ready to feed. It is normal for colour to be reduced after transport. Check breathing, balance and group behaviour repeatedly during the first day, then test water after the addition rather than assuming the established filter has absorbed the increased load.

A separate observation or quarantine aquarium can reduce the chance of introducing disease to an established community. It should be cycled, heated and appropriate for a social fish; a fixed number of days cannot guarantee health. Use observation, water quality and qualified advice to determine the process, and do not apply preventive medication without a reason.

A practical weekly care routine

1. Observe before feeding: count the fish, note breathing, posture, colour and whether any individual is excluded.
2. Test the water: confirm ammonia and nitrite remain at 0 mg/L and track nitrate against the source water.
3. Change water as needed: use conditioned water matched closely for temperature and avoid abrupt chemistry changes.
4. Inspect equipment: confirm heater operation, gentle filter flow and clear surface movement.
5. Maintain plants and substrate: remove trapped waste and dead growth without stripping all cover at once.
6. Review feeding: reduce portions if food remains, and make sure shy fish are not becoming thin.

The necessary water-change frequency depends on stocking, feeding, source-water nitrate, plant growth and filter performance. Testing provides a better guide than a universal percentage. Keep a simple record so gradual changes are visible before they become an emergency.

Health and welfare checks

Most health protection comes from husbandry: a cycled tank, stable temperature, suitable water, a proper social group, compatible companions and varied food. Small fish can deteriorate quickly, so respond promptly to loss of balance, gasping, rapid gill movement, severe colour loss, sores, white spots, swollen eyes, persistent isolation or failure to eat.

Test the aquarium first because poor water can imitate or worsen disease. If only one fish is affected, compare it with the rest of the group and review recent additions. Photographs, water results and a timeline are useful when consulting an aquatic veterinarian or experienced specialist retailer. Do not combine medicines casually, and check whether any treatment is safe for plants, shrimp, snails and biological filtration.

Breeding overview

Dwarf Pencilfish are egg scatterers. A mature, well-conditioned group may show courtship, but consistently raising fry is a specialist project because eggs and newly hatched young are tiny and may be eaten in a community aquarium. OATA recommends a dedicated breeding setup for pencilfish and suitably small first foods for fry.

A breeding aquarium should be fully cycled, gently filtered and matched to the species' water needs. Fine-leaved plants or spawning material can provide egg cover. Any attempt should be researched in detail before adults are moved; rapid changes in chemistry made only to trigger spawning can compromise welfare. Adult removal, egg protection, microscopic first foods and stable water are all more important than simply seeing courtship.

How does it compare with similar small fish?

Fish	Typical appeal	Planning difference
Dwarf Pencilfish	Very small, subtle horizontal pattern, calm hovering behaviour	Needs gentle feeding competition, cover and a social group
Beckford's Pencilfish	Related pencilfish with a larger, more robust appearance	Research its own adult size and social behaviour rather than copying this guide
Ember Tetra	Warm orange colour and an active small-tetra display	Compare group size, adult stocking and water before mixing
Green Neon Tetra	Blue-green reflective stripe in a small South American tetra	Confirm soft-water suitability and feeding access for both groups

The right choice depends on the water you can maintain, the aquarium's footprint, existing inhabitants and the kind of movement you enjoy. Choose this species for quiet detail rather than as a substitute for a bright, fast schooling display. Browse the freshwater fish collection only after writing down your aquarium's tested parameters and remaining adult capacity.

Is Dwarf Pencilfish the right choice for your aquarium?

This species is a strong match when the aquarium is already cycled, at least 60 cm long and arranged for a group of small, peaceful fish. It is especially rewarding for a keeper who enjoys watching subtle social behaviour rather than expecting one large centrepiece fish. Plants, partial floating cover and a measured feeding routine all support that kind of display.

It is a poor match when the tank is new, the water changes rapidly, or existing fish dominate every feeding. Dwarf Pencilfish should not be used to fill a visual gap in a community that is already at capacity. Their small body does not make their waste or social needs disappear, and adding only one or two to save space leaves the fish without an appropriate group.

Your aquarium	Likely fit	Action before ordering
Mature, planted, gently filtered and at least 60 cm long	Potentially very good	Check temperature, pH, hardness, current stock and group capacity
Quiet South American-style community	Potentially good	Compare every species' adult size, social number and feeding speed
New aquarium with no established biological filter	Not ready	Complete cycling and prove 0 mg/L ammonia and nitrite with tests
Tank with large, territorial or predatory fish	Unsuitable	Choose livestock appropriate to the existing inhabitants instead
Very hard, alkaline water outside the species range	Unsuitable without a safe long-term plan	Do not create unstable chemistry through repeated emergency adjustment
Community of rapid surface feeders	High risk of food competition	Choose calmer companions or a separate suitable aquarium

Measure the real aquarium rather than relying on the model name printed on its box. Gravel, rocks, wood and an air gap reduce actual water volume, while a cube can hold many litres without providing the 60 cm horizontal footprint this species needs. The volume calculator is useful, but it should be combined with dimensions, filtration and the adult behaviour of the planned stock.

Check your source water before deciding to alter it. If the tap-water pH or hardness is outside the suitable range, a stable specialist method may be possible for an experienced keeper, but continual unmeasured additions of “pH down” or mineral products can create dangerous swings. It is often kinder to choose a species suited to the water you can reproduce reliably each week.

Ordering checklist

• Group: the basket contains enough fish to maintain at least five, accounting for any healthy Dwarf Pencilfish already present.
• Space: the tank is at least 60 cm long and has room for the full group at adult size.
• Cycle: recent tests show 0 mg/L ammonia and 0 mg/L nitrite.
• Water: the aquarium can remain at 24-26°C, pH 5.8-7.5 and no more than 15 dGH.
• Companions: no resident fish can swallow, harass or consistently outcompete them.
• Food: fine prepared food and appropriately small frozen or live variety are ready.
• Delivery: someone can receive the parcel and follow the current live-fish instructions promptly.
• Contingency: a suitable observation plan and access to qualified help are available if a fish becomes unwell.

When those checks are satisfied, select the available live size rather than assuming a larger or smaller juvenile changes the adult care. Supplied length affects acclimation and feeding particle size, but the species still requires the same group, adult footprint and stable water. Prices and stock can change, so the selector and checkout remain the current commercial source.

Dwarf Pencilfish questions

How many should I keep?

Keep at least five together, as recorded by FishBase and reflected in OATA's pencilfish guidance. A larger group can be beneficial where the aquarium has enough room, filtration and adult stocking capacity. Do not keep a single individual as the permanent plan.

What tank size do they need?

Use at least a 60 cm aquarium footprint. Around 45 litres is a practical lower starting point for a small species group, while a community should be larger. Dimensions, usable swimming room and water stability matter as much as the nominal litre figure.

Are they suitable for beginners?

They can suit a careful keeper who already has a fully cycled, stable aquarium and can provide tiny food and gentle companions. They are less forgiving of an immature tank, strong current or aggressive feeding competition than many more robust community fish, so “moderate” is the fairest care description.

Can they live with shrimp?

Adult Amano shrimp and peaceful snails are often reasonable companions in a compatible setup. Very small shrimplets may be investigated or eaten, and any medication used for fish must also be checked for invertebrate safety.

How large do they grow?

FishBase records a maximum total length of about 3.5 cm. The live size option refers to the approximate size supplied, not the final adult length. Growth varies with age, diet and care.

What temperature should I use?

Use 24-26°C for this species. Stability is important, so use a reliable heater and thermometer and avoid rapid changes during maintenance or acclimation.

How long do they live?

OATA describes pencilfish as living for several years when cared for correctly. A precise five-year promise is not justified because lifespan varies with age on arrival, genetics, water quality, diet, social conditions and health history.

Are they aggressive?

They are generally peaceful. Minor social display can occur, but continual pursuit, injury or exclusion is not normal acceptable care. Review group size, space, cover and sex balance if one individual is repeatedly targeted.

Will I receive the exact fish shown?

No. The gallery uses representative illustrations, and livestock naturally vary. You will receive one unsexed Dwarf Pencilfish in the live size selected, subject to the current variant availability shown on the page.

Evidence and sources

Accepted identity and distribution were checked against the Catalog of Fishes record for Nannostomus marginatus. Family, adult size and species-specific water values were checked against the FishBase species record. General welfare, aquarium, group, feeding, water-quality and breeding guidance was checked against the Ornamental Aquatic Trade Association care sheet for tetras and pencilfish. Product sizes, stock, pricing, delivery dates and guarantee terms remain live commercial data and should be read from the selector, checkout and policy pages rather than inferred from this care guide.

Last care-content review: 27 July 2026. Taxonomy and husbandry knowledge can change, so source records and current welfare guidance take precedence over older saved copies of this page.