Purple Pencilfish (Nannostomus rubrocaudatus)

Plan for a tiny, specialist pencilfish

Purple Pencilfish is Nannostomus rubrocaudatus, a freshwater member of the pencilfish family Lebiasinidae. FishBase records it from Peru and gives a maximum of 2.6 cm standard length. It suits a keeper prepared to maintain a mature, heavily planted aquarium, feed tiny foods and manage the territorial behaviour of rival males within a substantial group.

This is a small, timid fish with more demanding social behaviour than its body size suggests. It is peaceful towards suitable species, yet rival males may defend space and spar during the day. A bright, open nano aquarium gives a subordinate fish few ways to leave another male’s view. The stronger plan is a mature, heavily planted aquarium with gentle filtration, dark visual cover and several broken sightlines.

The trade label Peru Red II also accompanies this animal. Pencilfish is the more precise family-level description, although you can browse the wider tetras collection when comparing other small shoaling fish. Compare adult behaviour, water and group needs rather than choosing by colour or retail category alone.

Understand the animal and purchase unit

One purchase supplies one Purple Pencilfish. Plan the permanent aquarium around the recorded adult measurement, group behaviour and space needed between rival males.

Sex is guaranteed only when the selected option explicitly states male or female. This Purple Pencilfish listing has no sex-guaranteed option.

Build the aquarium as though the eventual group may contain more than one male. Adult males can be less stocky and more colourful than females, with modified anal-fin rays, but colour and body shape are not a purchasing guarantee. Ordering two fish does not create a selected pair, and a larger order does not create a chosen sex ratio.

Make the group decision before checkout

• Choose the final group plan before deciding how many fish you need.
• Aim for 10 or more where the aquarium footprint, planting and filtration can support them.
• Allow several screened territories rather than one uninterrupted open area.
• Keep enough feeding access for timid and subordinate individuals.
• Use the permanent water range from the start.

The gallery images are representative, based on reference photographs of this species, so colours can look slightly different from the live animal.

Check the decisive care numbers

The Seriously Fish species account uses a planning length of 30 mm standard length and gives 24–28°C, pH 4.0–7.0 and 1–5 dGH for aquarium care. For a mixed-sex group, it calls for base dimensions of at least 90 × 30 cm or equivalent, corresponding to approximately 81 litres. That footprint matters because males can be particularly aggressive towards one another.

These numbers work together. An aquarium can hold approximately 81 litres yet have an unsuitable tall, narrow footprint. Water can sit inside the pH range yet remain unsuitable if temperature or hardness falls outside the corresponding range. A suitably sized tank also needs to be biologically mature, gently filtered and arranged with enough cover to interrupt pursuit.

This fish suits a keeper who can

• maintain stable water inside all three stated water ranges;
• provide the full group rather than a token singleton;
• use dense planting, floating cover and wood to create refuges;
• offer tiny prepared foods plus small live or frozen fare every day; and
• screen every companion for adult size, temperament and feeding pressure.

Build space between rival males

A long planted footprint is more valuable than an open display built around the fish’s tiny body length. Rival males may spar regularly, and confined quarters can turn repeated displays into serious injury. For a group that may contain both sexes, start with at least a 90 × 30 cm base or equivalent and approximately 81 litres. Plan for 10 or more fish where the completed aquarium can support the group.

Use heavy planting to split the aquarium into several visual areas. Dark substrate can reduce the exposed feel of the setting, while floating plants soften the upper level. Driftwood branches create further divisions. Dried leaf litter may encourage microbial colonies as it decomposes; those microorganisms can become a secondary food source for fry, while released tannins and other compounds are thought to be beneficial.

Lay out the footprint in three parts

Avoid a single wall of decoration that leaves one dominant open zone in front. Instead, create repeated transitions between cover and visible water. A chased fish should be able to pass behind planting or wood rather than crossing the full aquarium in view. Keep refuges accessible enough that you can still observe body condition and feeding.

Watch the group after the layout settles. Brief side-on displays and short chases can form part of normal territorial behaviour. Repeated pursuit across the whole footprint, fin damage or an individual remaining hidden indicates that the arrangement is not dispersing attention effectively. Add or reposition visual barriers while preserving the required footprint and gentle flow. More plants cannot turn an undersized base into suitable social space.

Test the layout before the group arrives

Stand at the front and look through the aquarium from end to end. If you can see one uninterrupted corridor across the entire 90 cm length, add a plant group or branch that interrupts that view while leaving a feeding route open. Then look from each end panel. A useful arrangement creates several partly screened areas rather than one cave with a single entrance. Check that water still moves around every refuge and that no fish would need to enter a dead end to escape pursuit.

Hold the full water range stable

Maintain 24–28°C, pH 4.0–7.0 and general hardness of 1–5 dGH. The species requires stable conditions, so judge the aquarium by the combination of temperature, acidity and hardness rather than selecting one attractive number. Establish a repeatable pattern before the fish enters the aquarium.

Measure the aquarium itself and the water used for maintenance. The water chemistry guide explains what the main values represent, while the water matcher helps compare your measured aquarium with the stated range. Record the actual results rather than relying on a regional description such as soft water.

Work through all three values

Do not average an unsuitable result against two suitable ones. A reading of 8 dGH does not become acceptable because pH and temperature are in range. Likewise, a suitable test result on one day is only useful when the aquarium can hold similar conditions through feeding, maintenance and normal equipment operation.

Check the range again whenever behaviour changes or filtration is disturbed. Rapid breathing, unusual surface use, sitting low in the aquarium or reduced feeding all justify an immediate water test. Use the measured result to guide the next action rather than interpreting colour alone.

Compare readings across a normal week

Take one set of readings before routine maintenance and another after the replacement water has mixed through the aquarium. For example, a tank at 25°C, pH 6.4 and 3 dGH should not be judged only by the pre-change result; confirm that the completed change still leaves every value within 24–28°C, pH 4.0–7.0 and 1–5 dGH. Repeat the comparison until the maintenance method produces a dependable pattern. If one value crosses a boundary after each change, resolve the replacement-water plan before adding the fish.

Mature the filter before introduction

Never add this fish to an immature aquarium. A working biological filter is essential because fish release toxic ammonia. The RSPCA explains that filter bacteria convert ammonia to nitrite and then to the safer nitrate. Ammonia and nitrite are both toxic, so the aquarium must be fully prepared before livestock enters it.

Use the first tropical tank guide to plan the aquarium and mature its filtration before the group arrives. Maturity is not the same as filling a tank and switching equipment on. Confirm with tests that ammonia and nitrite are both 0 mg/L and that the heater and filter can hold steady conditions.

Choose gentle but effective equipment

An air-powered sponge-style filter should be adequate in most cases because it combines biological filtration with gentle movement. At approximately 81 litres, the species guidance uses a turnover of four to five times the aquarium volume per hour and suggests looking for a filter rated around 325–405 litres per hour. Arrange the outlet so the planted middle-to-upper area remains usable rather than dominated by forceful flow.

For an aquarium of this size, the same guidance suggests roughly 100–125 W of heating to maintain 24–28°C when the surrounding room is around 20°C. Treat that wattage as a planning reference tied to those conditions. Confirm that the completed setup actually holds the required temperature throughout its normal daily cycle.

1. Complete the permanent planting, wood and filtration layout.
2. Run and mature the biological filter before adding fish.
3. Test for 0 mg/L ammonia and 0 mg/L nitrite.
4. Confirm 24–28°C, pH 4.0–7.0 and 1–5 dGH.
5. Check that gentle circulation reaches the aquarium without removing sheltered areas.

Increase the aquarium’s livestock load carefully. Beneficial bacterial populations must respond when fish numbers and feeding increase, so do not combine an immature filter with a sudden unplanned rise in waste.

Run a final equipment check

Observe the aquarium with the planting in its finished position rather than testing an empty tank. Confirm that the filter moves the surface and reaches the screened areas without forcing the entire upper level into one hard current. Check the thermometer at more than one point in the normal daily cycle. Finish by testing ammonia and nitrite again at 0 mg/L. A filter rating, heater wattage or elapsed number of days cannot replace those direct checks.

Feed particles small enough to swallow

In nature, this fish is a micropredator that takes tiny invertebrates and other zooplankton. In the aquarium it accepts dried food of a suitable size, but it should also receive daily small live or frozen fare. Suitable examples in the species guidance include Artemia nauplii, Moina and grindal worm.

OATA’s tetras and pencilfish guidance is group-level: offer only what the fish can eat within a few minutes, once a day. It recommends a good-quality flake or granule and also mentions small live or frozen items such as daphnia, small bloodworm and tubifex. For this tiny species, select only pieces that fit the mouth.

Judge the portion by feeding response and remaining food, not by the apparent emptiness of the aquarium. Uneaten food breaks down and releases waste, so stop after a few minutes and reduce the next portion when food remains. Adding more food does not help a timid individual if the pieces are too large or dominant fish take them first.

Watch the whole group eat

• Confirm that small and subordinate fish take particles, not merely approach them.
• Check body condition as well as colour.
• Use food that remains accessible in the middle-to-upper region where the species tends to feed.
• Change particle size or presentation when repeated strikes fail to result in swallowing.
• Keep the daily ration small enough that filtration is not burdened by leftovers.

Use the first few feeds as a practical test

Place a small amount where the group can see it and watch for actual swallowing. If several fish strike repeatedly without taking a particle, use a finer size at the next feed rather than increasing the portion. If one individual hangs behind cover while the others eat, present part of the same measured ration nearer that refuge. End the feed after a few minutes, remove obvious remains and note which food size produced the cleanest response across the whole group.

Use a repeatable weekly routine

Carry out a partial water change of 25% at least once every week. Test the water at least weekly and keep ammonia at 0 mg/L and nitrite at 0 mg/L. Nitrate should not rise more than 20 mg/L above the normal level in the tap water used for the aquarium.

Apply the nitrate rule to your baseline

If the tap-water nitrate result is 10 mg/L, a rise of 20 mg/L places the upper group-guidance boundary at 30 mg/L. If the tap result is 25 mg/L, the corresponding boundary is 45 mg/L. Test both waters rather than treating one fixed nitrate number as suitable for every household.

When filter media needs cleaning, rinse it lightly in aquarium water removed during the partial water change. Do not clean it under the tap, because chlorine or chloramine may kill part of the beneficial bacterial population. Avoid replacing all established biological media during routine maintenance.

Connect test results with behaviour. Any measurable ammonia or nitrite needs prompt attention, especially when fish breathe differently, stop feeding or change swimming position. Pause further livestock additions while the biological filter and water quality are brought back to the required condition.

Keep planting useful as cover while leaving enough access to inspect the group and remove waste. Maintenance should preserve the broken sightlines that control male conflict rather than clearing every dense refuge at once. After work is complete, confirm that filtration is running, surface movement has resumed and the fish can return to sheltered areas.

Close the maintenance loop

Write down the tap-water nitrate result beside the aquarium result each week. A tap baseline of 10 mg/L makes 30 mg/L the stated upper comparison boundary, while a baseline of 25 mg/L makes it 45 mg/L. After the water change, check that the filter has restarted normally and that the group resumes controlled swimming. If ammonia or nitrite is above 0 mg/L, stop treating the routine as complete: investigate filtration and waste before adding any more livestock.

Prepare the first month before arrival

Complete the aquarium before choosing an arrival date. Confirm mature filtration, 0 mg/L ammonia, 0 mg/L nitrite, stable 24–28°C, pH 4.0–7.0 and 1–5 dGH. The final planting and wood should already create several broken sightlines, allowing the fish to settle without an immediate major redesign.

Read the shop’s delivery and Live Arrival Guarantee policy before the parcel arrives.

Follow the arrival steps in order

1. Be present to receive the parcel so live fish are not left unattended.
2. Start filming before the sealed parcel is opened, keep the outer box visible and continue in one take as it is opened and the fish is shown inside the sealed bag.
3. Open and inspect the parcel on the same day it arrives.
4. Float the bag for 20–30 minutes before releasing the fish.
5. Never pour the bag water into the aquarium.

Keep the aquarium calm and observe breathing, swimming and body condition as the fish begins using the planted middle-to-upper area.

Use clear checkpoints

Healthy fish should have clear, bright eyes, undamaged fins, intact scales and an appropriate swimming response. Watch for hanging at the surface, sitting on the bottom, erratic swimming, darker or paler colour, increased hiding, altered aggression, unusual gill movement, white spots, fluffy growths, loss of scales, weight change or reduced feeding.

If any of these changes appears, test water quality first and contact us for health guidance. Record the measured temperature, pH, hardness, ammonia, nitrite and nitrate together with the observed change. That combination is more useful than colour alone, because a small timid fish may become paler after disturbance without showing the same pattern as a fish that is breathing rapidly or refusing food.

Add further livestock only after the aquarium remains stable under its existing feeding and waste load. Slow, planned stocking gives beneficial filter bacteria time to respond as waste and feeding increase.

Keep a short settling record

During the first week, note whether every fish joins the group, reaches food and returns to cover without continual pursuit. At the first weekly review, compare those observations with ammonia and nitrite at 0 mg/L and with the recorded temperature, pH and hardness. By the first month, the key decision is whether the aquarium works under its normal feeding and maintenance routine. Delay any further stocking if one fish remains excluded, several fish miss food or the water readings do not remain stable.

Screen every companion before mixing

Purple Pencilfish is peaceful towards suitable neighbours but is not an automatic community choice. Its tiny size and timid nature make adult body size, aggression, feeding pressure and water overlap decisive. A calm label on its own is not enough; compare how the proposed companion behaves when mature and at feeding time.

Supported companion categories

Diminutive, non-aggressive characids may work when their water, adult group and feeding behaviour match. Smaller callichthyid or loricariid catfishes can also be considered under the same screen. These are categories for assessment, not universal pairings.

Apistogramma and other dwarf cichlids may share a suitable plan because the pencilfish tends to inhabit the middle-to-upper region and does not actively predate their fry. The completed aquarium still has to meet the cichlid’s own adult territory, water and social requirements as well as the pencilfish group’s footprint.

Avoid fish large enough to swallow it and avoid aggressive, boisterous or forceful-feeding neighbours. Similar size is useful, but similar size does not resolve conflicting temperature, hardness or temperament.

For a stocking and compatibility assessment, contact us with the aquarium dimensions, measured water, established livestock and proposed group numbers. Named combinations beyond the supported categories depend on those details and on the adult behaviour of every species involved.

Manage shoaling and territorial behaviour

This pencilfish is gregarious, but it shoals rather than forming a permanently tight school. Settled fish may spread through planted areas while maintaining contact with the group. Rival males can hold territories and spar during daylight, so the group needs both social choice and room to leave a confrontation.

Aim for 10 or more fish in a spacious aquarium. Group size spreads interaction, but it does not replace the need for at least a 90 × 30 cm base or equivalent when sexes are mixed. Because the purchase does not provide a chosen sex ratio, arrange every group as though several rival males may develop.

Compare three stocking plans

• Species-focused group: an ideally 10+ group in a mature, heavily planted aquarium gives the clearest control over feeding and companion pressure.
• Quiet mixed aquarium: add only screened, diminutive and non-aggressive categories after allowing the full adult space required by every group.
• Short or open nano aquarium: do not rely on tiny body size to offset the lack of retreat distance and broken sightlines.

Do not keep one or two fish merely to avoid male rivalry. The animal is gregarious, and too small a social group creates a different welfare problem. Start with the complete aquarium plan: adequate footprint, mature filtration, dense cover, suitable water and enough feeding access for the intended group.

Answer the key buying questions

How big does a Purple Pencilfish get?

FishBase records a maximum adult size of 2.6 cm standard length for Nannostomus rubrocaudatus.

What size aquarium does a Purple Pencilfish need?

Use a mature, heavily planted aquarium with gentle filtration; for a mixed-sex group, plan around 81 litres with a base of at least 90 × 30 cm or equivalent.

How many Purple Pencilfish should I keep together?

Keep an ideally 10 or more in a spacious aquarium with dense cover and broken sightlines because rival males may spar.

Is a Purple Pencilfish suitable for beginners?

It suits a prepared keeper who can mature the filter, maintain 24–28°C, pH 4.0–7.0 and 1–5 dGH, feed tiny foods and manage a substantial planted group.

What does a Purple Pencilfish eat?

Offer suitably small dried food plus daily small live or frozen fare such as Artemia nauplii, Moina or grindal worm; limit each daily portion to what is eaten within a few minutes.

What temperature and pH does this fish need?

Keep it at 24–28°C and pH 4.0–7.0 with general hardness of 1–5 dGH; these are the Seriously Fish species aquarium ranges.

Is it aggressive or likely to nip fins?

It is peaceful towards suitable other species, but rival males may spar, chase and become particularly aggressive towards one another in confined quarters.

Which kinds of animals can it live with?

Consider diminutive, non-aggressive characids and smaller callichthyid or loricariid catfishes after matching water, adult group size, space and feeding access; avoid fish large enough to eat it and aggressive or boisterous companions.

How does it arrive alive in the UK?

Tropical Fish Co hand-bags live fish in pure oxygen and uses an insulated poly box, with a heat pack when the weather calls for it. Be present to receive the parcel, film every delivery continuously from the sealed parcel as it is opened with the fish still inside the sealed bag, float the bag for 20–30 minutes, and never pour the bag water into the aquarium.

References for identity and care

• FishBase: Nannostomus rubrocaudatus — scientific identity, family, distribution and maximum standard length.
• Seriously Fish: Nannostomus rubrocaudatus — species aquarium dimensions, water, diet and behaviour.
• OATA: How to look after tetras and pencilfish — UK group husbandry and maintenance guidance.
• RSPCA aquarium environment guidance — biological filtration and waste conversion.
• Tropical Fish Co delivery policy — packing, receipt, acclimatisation and Live Arrival Guarantee terms.