Purple Pencilfish (Nannostomus rubrocaudatus)

The Purple Pencilfish (Nannostomus rubrocaudatus) is a tiny Peruvian pencilfish with a dark three-stripe pattern and red to purple-red colour between the stripes. This is the accepted identity of the fish on this page. It is not the Golden Pencilfish Nannostomus beckfordi, and it should not be confused with the separate Coral Red Pencilfish Nannostomus mortenthaleri. Older trade lists may call this species Purple Pencilfish, Peru Red II, Nannostomus sp. "Red" or N. marginatus "Purple". The scientific name Nannostomus rubrocaudatus, formally described in 2009, is the clearest way to identify it.

This is a specialist small-fish choice rather than a generic community tetra. Adults are only around 3-3.5 cm in overall aquarium length, yet males can be assertive with one another. A successful group needs a mature, softly filtered aquarium with dense cover, open feeding space, calm companions and reliably soft, slightly acidic water. In return, the fish develops remarkable colour and intricate social behaviour that are easy to miss in a bright dealer tank. The guide below separates verified species information from practical aquarium planning, so you can decide whether your water, tank and existing fish are a genuine match.

Purple Pencilfish Identity: What Are You Buying?

The product identity matters because three red pencilfish names are often mixed in online catalogues. Nannostomus rubrocaudatus, N. mortenthaleri and N. beckfordi are not interchangeable colour forms. They are separate species. The fish sold here is the Purple Pencilfish, N. rubrocaudatus, matching the current source label and species image for this item. Its historical trade labels include Purple, Red and Peru Red II. Those labels describe the aquarium trade history; they do not replace the accepted scientific name.

The California Academy of Sciences' Catalog of Fishes lists Nannostomus rubrocaudatus as a valid species in the family Lebiasinidae. It records the species from the Maranon River drainage in Loreto, Peru. FishBase also recognises the taxon and reports a maximum recorded standard length of 2.6 cm. Standard length excludes the tail fin, so a living adult can appear a little longer when viewed nose to tail. That distinction is why this page uses roughly 3-3.5 cm as a practical visual planning size without inflating the scientific measurement.

The species was formally described by Axel Zarske in 2009 after fish had already circulated in the aquarium trade under provisional red or purple names. It resembles N. mortenthaleri, but the original description and subsequent taxonomic records maintain them as separate species. A useful visual clue discussed in specialist accounts is the underside: N. mortenthaleri retains a conspicuously pale or white belly, while mature male N. rubrocaudatus can carry red pigmentation lower on the body. Colour alone is still not a safe identification method because stress, hierarchy, age, sex, lighting and photography can alter what the eye sees.

Colour, Pattern and Sex Differences

Purple Pencilfish have the compact, shortened profile shared by several dwarf Nannostomus species. Three dark horizontal bands organise the pattern. In well-conditioned mature males, red or purple-red pigment can fill much of the space between those bands and extend towards the tail. The dorsal and lower fins may also show red. Females are generally less intensely coloured and become fuller through the abdomen, but sex should not be promised from young retail fish.

Colour is responsive rather than fixed. A newly moved fish can look grey, brown or washed out. A subordinate male may suppress its red and resemble a female, while a dominant male in settled conditions can become far brighter. Night-time pattern changes occur across the genus, so a fish seen immediately after the lights switch on may not resemble the same individual later in the day. Dark substrate, floating plants and a shaded background improve visual contrast, but welfare comes first: colour should develop from security, suitable water and complete nutrition rather than intense lighting or colour-enhancing food alone.

Do not use redness as the only health test. A healthy fish should also hold itself level, breathe steadily, use the group, respond to food and carry intact fins. A strongly coloured fish can still be underweight; a pale new arrival may simply be settling. Look at body condition behind the head and along the belly, not only at the flank stripe.

Natural Distribution and Habitat

Nannostomus rubrocaudatus is known from Peru, with the accepted catalogue distribution centred on the Maranon drainage in Loreto. Available species accounts associate it with slow tributaries, backwaters, flooded margins and swamp-like water where plants, roots, branches and decomposing leaves create dense cover. These are not bright, bare, fast-flowing habitats. Water is often soft, acidic and stained by dissolved organic material.

The natural setting explains nearly every useful aquarium recommendation. Overhanging or floating vegetation reduces light. Submerged wood and leaf litter break sightlines. Slow water allows a tiny fish to feed on small drifting invertebrates without fighting a strong current. Tannin staining does not need to be copied exactly, but a calm, mature, structurally complex aquarium is more important than ornamental perfection.

Leaf litter can be used carefully if it suits the aquarium. Choose aquarium-safe leaves, add a modest amount, and monitor the effect on pH and organic load. Botanicals do not magically create safe blackwater, and they do not replace filtration or water changes. In very low-alkalinity water, a large addition can change chemistry faster than expected. The best biotope cue is stability: soft water, low dissolved waste, gentle flow and plenty of secure cover.

Tank Size and Layout

Body length alone makes this fish look suitable for a very small nano aquarium. Social behaviour says otherwise. Males can defend small areas and spar repeatedly. In a short or open tank, the lower-ranking fish cannot leave the dominant male's view. A practical home starts with a tank around 60 cm long and roughly 60-80 litres for a group. More length and planting are valuable when keeping several males or adding another shoal.

A 50-litre aquarium can work in experienced hands when its footprint is long enough, stocking is restrained and the layout is dense. It should not be treated as an automatic minimum for every group. Volume printed on the box is less useful than usable length, mature filtration and the number of visual barriers. A tall narrow aquarium with the same litres may provide less social space than a longer shallow one.

How to Arrange the Aquarium

• Rear and side planting: use fine-leaved or bushy plants to make several sheltered pockets.
• Floating cover: leave broken patches rather than sealing the entire surface, so food and gas exchange remain accessible.
• Open feeding lane: keep a calm central or front area where every fish can see tiny food.
• Wood and roots: arrange smooth pieces to interrupt long views without trapping fish or collecting excessive waste.
• Dark visual base: darker substrate and background reduce glare and make the natural pattern easier to see.
• Secure lid: cover cable gaps and feeding openings because small surface-oriented fish can jump when startled.

Keep the filter outlet gentle. A sponge filter, diffused spray bar or outlet aimed against hardscape can provide biological filtration and surface movement without turning the entire upper level into a current. Watch the fish rather than relying only on a flow-rate label. If the whole group is pinned behind wood or continually swimming against the outlet, the usable current is too strong. If the surface is stagnant or fish breathe rapidly, improve oxygen exchange and check temperature and water quality.

For planting ideas, the site's beginner live-plant guide explains straightforward species and maintenance. The goal is not a dense display at the expense of swimming space. Build a sequence of shelter, open water and shelter so a chased fish has more than one route away.

Water Parameters and Stability

Purple Pencilfish are best planned as soft-water fish. A useful routine target is 24-26°C, pH 5.5-7.0 and roughly 2-8 dGH. Published aquarium ranges vary because locality, acclimation history and measurement methods differ. Do not chase the lowest possible pH. A stable pH of 6.6 in suitable soft water is safer than repeatedly forcing the aquarium between 5.5 and 6.5 with chemicals.

Measure	Practical target	What matters
Temperature	24-26°C	Keep stable within a broader 23-27°C care range
pH	5.5-7.0	Prefer stable mildly acidic water; avoid abrupt correction
GH	About 2-8 dGH	Soft water supports the species; test source and tank water
Ammonia	0 mg/L	Any detectable result needs immediate investigation
Nitrite	0 mg/L	Also toxic; a mature biofilter is essential
Nitrate	Low and stable	Control through measured feeding, plants and regular partial changes

Test the water that actually enters the aquarium. UK tap water varies widely. Some areas provide naturally soft water; others require blending with remineralised reverse-osmosis water to reach a suitable, repeatable profile. Pure RO water is not complete aquarium water. It needs controlled remineralisation and enough buffering to remain stable. If you are unfamiliar with GH, KH and pH interactions, read the water-chemistry guide before changing the supply.

Weekly maintenance should be boring and repeatable. Replace a moderate proportion with dechlorinated, temperature-matched water of similar chemistry. Vacuum accessible waste without stripping every surface at once. Rinse filter media in removed aquarium water when flow falls, and never replace all biological media together. In a lightly stocked planted tank, the correct change volume depends on test results, feeding and source water, not a universal percentage.

A Practical Observation Routine

Small fish benefit from short, consistent checks. Before the first feed, look across the surface and count the group. Confirm that no individual is isolated behind the filter, trapped among plants or breathing faster than the others. At feeding time, watch the smallest and palest fish rather than only the dominant male. After feeding, remove obvious uneaten food and check that the filter is still moving the surface. These observations take only a few minutes but reveal social and nutritional problems earlier than a monthly deep clean.

Once a week, record temperature, pH, GH, ammonia, nitrite and nitrate until the aquarium has a dependable pattern. Continue ammonia and nitrite checks whenever fish behaviour changes, filtration is disturbed or new livestock is added. Compare replacement water with the aquarium before a change, especially when using an RO blend. Also inspect the lid, heater guard, intake sponge and narrow gaps around wood. A pencilfish can enter spaces that look harmless for a larger tetra.

Plant trimming should preserve several layers of cover. Avoid removing all floating plants or cutting every dense stem group on the same day. If the aquarium needs a major redesign, divide the work across sessions so the fish retain secure areas and the biological system is not disturbed all at once. Consistency supports both welfare and the stable colour buyers expect from this species.

Feeding a Tiny Micropredator

Wild pencilfish take tiny invertebrates and zooplankton. In an aquarium, Purple Pencilfish can accept good micro-granules, crushed flake and other complete prepared foods, but particle size and delivery are critical. A food can be nutritionally excellent and still be useless if it sinks too fast or is too large to swallow.

Offer very small portions once or twice a day and watch each fish take food. Rotate a complete staple with appropriately sized frozen or live foods such as cyclops, small daphnia, baby brine shrimp and fine mosquito larvae. Larger bloodworm is often too bulky for such a small mouth unless finely selected. Frozen food should be thawed and portioned; do not drop a large cube into a small aquarium and hope the group clears it.

Food	Role	Serving check
Micro-granule or finely crushed flake	Complete everyday staple	Floats or sinks slowly enough for the group to feed
Cyclops and small daphnia	Frozen or live variety	Particles fit the mouth and spread through the feeding lane
Baby brine shrimp	Useful small live or frozen prey	Rinse where appropriate and avoid excessive salinity entering the tank
Fine insect larvae	Occasional variety	Use only clean aquarium foods of suitable size

Fast tetras, danios or livebearers can consume everything before pencilfish begin. Feed across two or three points, dim intense lighting and confirm the shyest fish swallows food. A hollow belly, pinched area behind the head or repeated missed strikes suggests competition or particle-size problems. Increasing the total food without changing delivery often creates waste rather than solving underfeeding.

Group Size and Social Behaviour

Purple Pencilfish are gregarious but not always a tight synchronised school. In a settled aquarium they may spread among plants, maintain individual spacing and regroup when startled. Plan for at least eight, with ten or more preferable in a suitable larger aquarium. A larger group gives the fish social choice and spreads attention, but it does not cancel male rivalry. Space and broken sightlines remain essential.

Males may intensify colour, display side-on, flare fins and chase one another. Short display bouts with no injury can be normal. Persistent pursuit, torn fins, hiding, weight loss or a fish excluded from food means the setup is failing that individual. Add visual barriers only if doing so preserves swimming and maintenance access. In a cramped tank, the real answer may be a larger aquarium or separation rather than more decoration.

Sex ratios are difficult to guarantee from unsexed young fish. Mature females tend to be fuller, while dominant males are usually brighter, but subordinate males can fade. This listing therefore sells one unsexed fish per quantity. Ordering two does not create a guaranteed pair, and ordering a group does not guarantee a chosen male-to-female ratio.

Purple Pencilfish Tank Mates

The species is peaceful towards suitable neighbours but is not ideal for every mixed community. Its small size, soft-water preference, deliberate feeding and internal male competition favour a quiet aquarium. Choose companions by adult size, temperature, hardness, swimming speed and feeding behaviour rather than by the word "peaceful" on a label.

Companion type	Assessment	Reason
Small calm South American tetras	Often suitable	Choose soft-water species that do not dominate food or crowd the same level
Small Corydoras	Often suitable	Use a proper Corydoras group, suitable temperature and enough bottom area
Otocinclus	Conditional	Only in a mature aquarium with established grazing and species-appropriate group care
Peaceful dwarf cichlids	Conditional	Use more floor space, visual territories and a plan for breeding aggression
Other pencilfish	Conditional	Each species still needs its own group, food access and social space
Dwarf shrimp	Not shrimp-safe	Adults may coexist, but tiny shrimplets are potential prey
Large, aggressive or very fast fish	Avoid	Risk of predation, intimidation and chronic feeding loss
Fin nippers	Avoid	Repeated harassment damages fins and prevents normal group behaviour

Examples Worth Researching

A calm shoal of Ember Tetras can share warm soft water in a tank large enough for both groups, but Ember Tetras are quick at food and must not leave the pencilfish underfed. The Ember Tetra care guide helps compare water and group needs. Rummy Nose Tetras share a soft-water preference but are more active and need substantially more swimming length, so they suit a larger aquarium rather than a compact pencilfish setup.

For another Nannostomus option, compare the Three-Lined Pencilfish, the Red Golden Pencilfish, the Shining Pencilfish and the more surface-angled Tube-Mouthed Pencilfish. These are alternatives, not automatic tank mates. Mixing two pencilfish species can double the required social stocking while forcing both groups into similar feeding and swimming zones.

Small Corydoras may use a different level, but they are also social fish and need an appropriate group, footprint and substrate. Review the Corydoras care guide and compare the exact species' temperature range. Avoid buying a lone bottom fish merely to "clean" the aquarium. No fish replaces substrate maintenance.

Purple, Coral Red and Golden Pencilfish Compared

Name confusion is a major risk with this product, so compare the species before ordering. Colour can overlap and photographs can be misleading; use accepted names and source records wherever possible.

Trade name	Accepted species	Practical distinction
Purple Pencilfish	Nannostomus rubrocaudatus	The fish on this page; tiny Peruvian species with red-purple colour between three dark bands
Coral Red Pencilfish	Nannostomus mortenthaleri	Separate Peruvian species; commonly associated with a clearer pale belly beneath the stripe pattern
Golden or Beckford's Pencilfish	Nannostomus beckfordi	Larger, more widespread species with a different body pattern and care profile
Dwarf Pencilfish	Nannostomus marginatus	Separate dwarf species historically confused with red imports before formal descriptions

The phrase "Peru Red II" is a trade label for this sourced item, not part of the scientific binomial. It is included here only to help buyers reconcile older stock lists. The public product title uses Purple Pencilfish because Google UK keyword data, current search results and specialist references consistently pair that common name with Nannostomus rubrocaudatus.

Breeding Overview

Purple Pencilfish have been bred in aquariums, but this is an advanced project rather than a reason to expect fry in every community tank. They are small egg scatterers with no dependable parental care. Adults may eat eggs, and the fry require extremely small first foods. Breeding also demands softer, more acidic water than many mixed communities can maintain safely.

A separate mature breeding aquarium is the controlled route. Use dim light, very soft stable water, gentle air-driven filtration and dense fine-leaved plants, spawning mops or a protective egg layer. Condition adults with varied small live and frozen foods. A compatible pair or carefully selected small breeding group may spawn among the fine material. Remove adults if the aim is maximum egg survival.

Egg and fry timing varies with temperature and conditions. Do not rely on a rigid hour count. Prepare infusoria, rotifers or an equivalent genuinely tiny first food before spawning, then move to microworm and newly hatched brine shrimp only when the fry can take them. Clean water is essential, but large chemistry changes can be more damaging than a small amount of carefully managed feeding residue.

Breeding colour is not a sex guarantee at retail size. Mature females usually develop a fuller abdomen, while males display brighter pigment and modified fin features, but social status can mask male colour. This product remains unsexed and is not sold as a breeding pair.

Health, Stress and Troubleshooting

This species' common problems are usually not unique diseases. They are the predictable results of unstable water, poor feeding access, unsuitable companions or cramped social space. Start diagnosis with measured conditions rather than medication.

Sign	First checks	Possible response
Pale colour after arrival	Time since move, light, cover, temperature and aggression	Dim the tank, reduce disturbance and verify water before treating
Hollow belly or weight loss	Particle size, feeding competition, appetite and waste appearance	Separate feeding points; seek aquatic-veterinary advice if decline continues
Torn fins	Male pursuit, fin-nipping tank mates and sharp decor	Improve sightlines or separate the aggressor/victim; correct the cause
Rapid breathing	Ammonia, nitrite, temperature, oxygen exchange and gill irritation	Test immediately, increase safe aeration and correct water quality
Surface clustering	Oxygen, current, heat and whether the group is using its normal level	Do not assume it is normal surface behaviour if breathing is laboured
White spots or rubbing	Recent introductions and a clear diagnosis	Use a suitable quarantine/hospital plan and follow a verified treatment label

Do not medicate a display tank simply because colour has faded. Many treatments affect invertebrates, plants or biological filtration, and some fish are sensitive to dosing errors. Confirm the diagnosis where possible and follow the product label for the actual water volume. If several fish deteriorate despite correct water, or if there are ulcers, severe breathing problems or continuing weight loss, seek advice from an aquatic veterinarian or experienced fish-health professional.

Home Quarantine

A separate mature quarantine aquarium lets you observe feeding, breathing and waste without exposing an established community. Match its water to the planned display and provide cover. Quarantine is a keeper-side biosecurity step; this page does not make unsupported claims about the supplier's treatment or observation process. Keep dedicated nets and siphons where practical, and avoid transferring water between systems.

Arrival and Acclimation Planning

Prepare the aquarium before delivery. Confirm that ammonia and nitrite are zero, temperature is stable, the lid is secure and tank mates are not being fed into a frenzy. Read the site's fish-acclimation guide and the current delivery and Live Arrival Guarantee terms before ordering, because evidence and contact deadlines apply to qualifying claims.

On arrival, follow the current instructions supplied for livestock. Avoid leaving a transport bag floating for an extended period without understanding the water-chemistry implications. Dim the aquarium and offer cover. Do not expect full purple-red colour immediately. Give the fish time to establish normal breathing and group position before offering a very small meal.

For the first week, check the group at each feed. Count fish, observe whether each one swallows food and inspect the body line. A tiny pencilfish can lose condition before a casual observer notices. Keep maintenance gentle and avoid rearranging the entire aquarium while the social hierarchy settles.

Buying Purple Pencilfish in the UK

Use the live product control for current stock, price and purchasable quantity. Those values can change and are deliberately not hard-coded into this care guide. Each quantity represents one unsexed fish, not a pair, selected sex or exact gallery specimen. Plan and budget for the complete group before buying a single fish.

Eligible first orders can use the active code WELCOME10 for 10% off, subject to the current checkout and promotion terms. Qualifying livestock deliveries are covered by the Live Arrival Guarantee only when the published delivery, evidence and contact-time conditions are followed. The guarantee is not a substitute for confirming that your aquarium is mature and compatible.

Purple Pencilfish FAQs

Is the Purple Pencilfish the same as the Coral Red Pencilfish?

No. Purple Pencilfish is the most consistent common name for Nannostomus rubrocaudatus. Coral Red Pencilfish usually refers to Nannostomus mortenthaleri, a separate species. This page sells N. rubrocaudatus.

Is it a tetra?

It belongs to the order Characiformes, but its family is Lebiasinidae, not the tetra family Characidae. "South American tetra" may appear as a supplier category, but pencilfish is the more precise customer-facing group.

How many Purple Pencilfish should I keep?

Plan for at least eight, with ten or more preferred in a suitable larger aquarium. Group size alone is not enough: males also need length, plants and broken sightlines.

What tank size do they need?

A practical starting point is a mature aquarium about 60 cm long and roughly 60-80 litres. A carefully designed 50-litre setup can work, but tiny open nano tanks are poor choices for male rivalry and stable soft water.

What temperature is best?

Use a stable 24-26°C as an everyday target within a broader 23-27°C care range. Match all tank mates and avoid sudden changes.

What pH and hardness do they need?

Aim for pH 5.5-7.0 and soft water around 2-8 dGH. Stability matters more than forcing the aquarium to the most acidic edge of a published range.

Are Purple Pencilfish suitable for beginners?

They are better classed as moderate care. A beginner who can cycle a tank, test soft water, feed tiny foods and manage a calm group can succeed, but they are not an ideal first impulse purchase.

What do Purple Pencilfish eat?

Use a complete micro-granule or finely crushed flake, supported by small frozen or live foods such as cyclops, daphnia and baby brine shrimp. Confirm every fish can swallow the particles.

Are they peaceful?

They are peaceful towards carefully chosen neighbours, but males can be competitive and sometimes forceful with one another. Provide space, cover and more than one feeding point.

Can they live with shrimp?

They are not guaranteed shrimp-safe. Adult shrimp may coexist in a planted tank, but tiny shrimplets and other minute invertebrates fit the natural micropredator diet.

Why has my fish lost its red colour?

Stress, recent transport, hierarchy, bright light, poor cover, underfeeding or unstable water can all reduce colour. Check breathing, appetite, body condition and test results before considering treatment.

Can you select males and females?

No. The fish are sold unsexed. Mature males are often brighter and females fuller, but age and social status make casual sexing unreliable.

Will the fish look exactly like the gallery?

No. The gallery is representative. Colour, stripe width, sex, body shape and fin details vary, and the exact dispatched individual is not selected from the pictured fish.

How long do Purple Pencilfish live?

This page does not state a fixed lifespan because robust species-specific evidence is limited. Longevity depends on source, water stability, diet, social stress and individual health.

Does the Live Arrival Guarantee apply?

It applies to qualifying livestock orders only when the current delivery, evidence and contact-time terms are followed. Read the delivery page before ordering.

Is there a first-order discount?

Eligible first orders can use WELCOME10 for 10% off while that promotion remains active, subject to checkout terms.

Evidence and Further Reading

This listing's identity and size boundary were checked against Eschmeyer's Catalog of Fishes, FishBase and the 2009 original species description. Aquarium planning values were cross-checked with established species profiles and kept conservative where published ranges differ. Scientific references identify the species; the live Shopify controls remain the authority for current price, stock and purchasable options.

Continue Planning

Browse the live-fish shop only after writing down the complete group and tank plan. Compare the care-guide library for exact companion requirements, and use the buying guides to plan equipment and maintenance. For this species, the best purchase is not the most colourful single fish; it is a correctly identified group placed in a stable aquarium designed around their behaviour.