6 × Cardinal Tetras (Paracheirodon axelrodi) – Captive Bred

Choose cardinal tetras for a mature social aquarium

The Cardinal Tetra (Paracheirodon axelrodi) is a small freshwater member of the Acestrorhamphidae family from South America’s upper Orinoco and Negro River basins. FishBase records a maximum of 3 cm standard length, measured to the base of the tail, and describes shoals occupying the middle water layers. It suits a prepared keeper with a mature, filtered aquarium, measured water and room for a proper social group; it is not a good choice for a newly filled or unstable aquarium.

The accepted scientific name remains Paracheirodon axelrodi, with Eschmeyer’s Catalog of Fishes placing it in Acestrorhamphidae. Its familiar pattern combines a bright blue lateral stripe with extensive red along the lower body. The fish is peaceful and active in midwater, where the movement and colour of a coherent group become the main feature of the aquarium.

Start with a mature aquarium at least 60 cm long and about 60 litres. Our complete Cardinal Tetra Care Guide sets a conservative routine target of 24–27°C, pH 5.5–7 and 1–8 dGH, with at least 10 cardinals where the aquarium has the capacity. These are practical starting values for this species, followed by measured comparison and gradual adjustment rather than a fixed recipe for every water supply.

The species is assessed as Least Concern, but that category does not reduce the keeper’s responsibility to provide stable long-term care. Never release aquarium fish, plants or aquarium water outdoors. Plan the full adult community before adding livestock, including the space, filtration and social needs of every group.

Understand the pack and complete the social plan

One purchase supplies one captive-bred pack of six cardinal tetras. Six fish are fewer than our conservative social minimum of 10, so use this pack as part of a plan that produces a completed group of at least 10 without exceeding the aquarium’s swimming-space or filtration capacity. It can contribute to a carefully assessed established group, or form part of a larger cardinal group planned before introduction.

The pack quantity describes the animals included in the purchase; it is not an adult stocking formula. A nominal 60-litre aquarium is our starting point for 10 cardinals, not permission to add unlimited companions. Check the aquarium’s usable 60 cm length after decor and equipment, count existing occupants, and allow for the adult size and social requirements of every companion.

Sex is guaranteed only when the selected option explicitly states male or female. This 6 × Cardinal Tetras listing has no sex-guaranteed option.

Body shape can vary with maturity, feeding and condition, so build a social group rather than relying on visual guesses about sex. The pack should be assessed as six unsexed fish for stocking purposes. Breeding is a separate specialist project and should not be used as the reason for placing an assumed pair in a community aquarium.

The gallery images are representative of this type of animal rather than photographs of the exact animal supplied, so colours can look slightly different from the live animal.

The gallery shows the red-and-blue pattern and group movement associated with cardinal tetras, but a picture cannot establish the sex, measurements or individual markings of the fish in the pack. Appearance also changes with lighting, background, transport and settling. Judge welfare through breathing, balance, swimming, feeding, fins and water tests rather than expecting every fish to match one frozen image.

Check the plan before purchase

• Group: the completed cardinal group reaches at least 10.
• Space: the mature aquarium is at least 60 cm long and about 60 litres before companions are considered.
• Water: the aquarium can hold 24–27°C, pH 5.5–7 and 1–8 dGH steadily.
• Filter: repeated tests show zero ammonia and zero nitrite.
• Tank mates: every species passes adult-size, temperament and water screening.

Plan aquarium space around the adult size

Use at least 60 cm of aquarium length and about 60 litres as the starting plan for a group of 10 cardinal tetras. Add more room for a larger group or any companions. Litres alone do not describe swimming length, surface exchange, filter capacity or the space lost to wood, plants and equipment, so check the aquarium’s dimensions and usable open water as well as its nominal volume.

Seriously Fish recommends a base of at least 60 × 30 cm and about 54 litres. Tropical Fish Co uses the slightly more conservative planning figure of about 60 litres alongside the same 60 cm minimum length. A tall aquarium with a short base does not provide the same horizontal route as a longer tank with an open midwater lane.

Plan around a recorded adult maximum of 3 cm standard length. Standard length ends at the base of the tail, so the complete fish is somewhat longer, but do not convert it into an exact promised total length. Seriously Fish lists 35 mm standard length, a different planning reference. The care specification uses FishBase’s 3 cm maximum consistently across the listing.

Work through the capacity decision

Do not place a young or small fish in a cramped aquarium on the assumption that an upgrade will happen later. The aquarium should meet the adult plan before the group arrives. This avoids repeated moves and ensures the mature biological filter, heater and layout are already working at the intended scale.

A larger aquarium can offer greater environmental stability, but volume does not compensate for poor maintenance or incompatible stocking. Before adding companions, test the water, check the filter’s established capacity and watch whether every cardinal can swim and feed without persistent crowding.

Hold measured water steady within the routine target

Keep cardinal tetras at 24–27°C, pH 5.5–7 and 1–8 dGH for routine aquarium planning. Use an independent thermometer and reliable liquid or calibrated tests to confirm the water itself; a heater dial, postcode or description such as “soft water” is not a measurement. Compare current holding water with the aquarium and make gradual, measured changes.

The references describe different contexts. FishBase records 23–27°C, pH 4–6 and 5–12 dGH for the species’ environment, while Seriously Fish gives aquarium ranges of 23–29°C, pH 3.5–7.5 and 1–12 dGH. Tropical Fish Co’s narrower routine target of 24–27°C, pH 5.5–7 and 1–8 dGH is a conservative house policy inside the broader species-level aquarium range. It is a starting screen, not a reason to force rapid corrections.

Test temperature, pH, GH and KH before moving the fish. GH describes dissolved minerals, while KH affects buffering and how readily pH may change. A low reading is not automatically a problem to correct with an unmeasured dose, and a pH number should never be chased independently of hardness and buffering. The water chemistry guide explains which values to test and why.

Use measurements to choose the next action

• Values already match: keep the routine stable and continue regular testing.
• A small difference appears: compare repeated readings before changing anything.
• A large difference appears: plan gradual adjustment and re-test each prepared batch.
• Tap-water readings vary: measure the water used for each change rather than relying on an old result.
• Mixed species are planned: use the water matcher to compare measured aquarium water with stated guidance, then check space and temperament separately.

Most UK rooms need a thermostatically controlled aquarium heater to maintain 24–27°C steadily through the year. Position and operate it according to its instructions, then verify temperature away from the heater with an independent thermometer. Avoid direct sunlight, cold draughts and abrupt changes during maintenance.

If reverse-osmosis water is used, measure the final remineralised or mixed result. Fixed tap-to-RO ratios are unreliable because starting GH, KH and dissolved content vary. Prepare replacement water consistently, treat tap water with conditioner where used, and keep changes gradual enough to avoid sudden shifts in temperature or chemistry.

Mature the filter and maintain zero ammonia and nitrite

Use a well-established biological filter and confirm zero ammonia and zero nitrite before adding cardinal tetras. The RSPCA explains that a working biological filter supports bacteria that convert toxic ammonia through nitrite into safer nitrate. Clear water alone does not show that this cycle is established.

Prepare the aquarium by testing rather than waiting a fixed number of weeks. The first tropical tank guide sets out the maturation sequence before livestock arrives. Record ammonia, nitrite, nitrate, pH and temperature so a change can be recognised quickly instead of relying on memory.

OATA’s tetra sheet gives maintenance guidance for the group, not a species-specific optimum: ammonia and nitrite should remain at zero mg per litre, while nitrate should not exceed 20 mg per litre above the normal tap-water level. Test at least once per week and test promptly after unusual behaviour, a filter interruption, overfeeding or a substantial stocking change.

Use this weekly routine

1. Test ammonia, nitrite, nitrate and temperature before maintenance so the result reflects the aquarium’s current condition.
2. At least once every week, carry out a partial water change of 25% and siphon accumulated solid waste where needed.
3. Treat replacement tap water with conditioner to remove chlorine or chloramine before adding it.
4. Match replacement temperature and avoid abrupt chemistry changes.
5. Check the filter for blockages and confirm that water is circulating across the aquarium.

If filter media needs cleaning, do not run it under the tap. Rinse it lightly in aquarium water removed during the partial water change, which reduces the loss of established bacteria. Do not replace all established biological media at the same time unless the equipment manufacturer’s safety instructions require a specific action.

Stocking raises the amount of waste the filter must process. Add fish only when the aquarium has capacity, and continue testing as the bacterial population adjusts to increased feeding and waste. If ammonia or nitrite appears, stop treating the calendar as proof of maturity: test again, reduce avoidable waste, check filter operation and protect the livestock while the cause is addressed.

A planted aquarium still needs filtration and testing. Plants can use nitrogen compounds, but they do not turn an immature filter into a mature one or prevent every water-quality problem. Stable results, routine maintenance and direct observation work together.

Build cover around an open midwater swimming route

Create a long, open route through the middle of the aquarium, with cover around the sides and rear. Cardinal tetras occur mainly in shoals in middle water, so they need room to spread, turn and regroup. Plants, aquarium-safe wood and shaded edges can provide security without filling every part of the swimming area.

A natural-style arrangement can use sand, driftwood roots, branches and dried leaf litter. Seriously Fish notes that decomposing leaves can encourage microbe colonies and release humic substances, while plants and wood create a more sheltered appearance. Use only aquarium-safe materials, remove anything that decays excessively or traps waste, and leave access for siphoning and inspection. Natural peat is unnecessary and its collection is environmentally damaging.

Keep the current gentle enough for small tetras to hold position without continuous effort, while maintaining circulation and surface movement. Watch the fish rather than judging flow only from a filter rating. A group compressed into one sheltered corner, repeatedly swept away from food or forced to swim continuously against the current needs a layout or flow adjustment.

Match observations to actions

Use subdued or moderated lighting with a consistent day-and-night cycle. A controlled experiment discussed in the store care guide found colour fading under excessive illumination, but pale colour can also accompany transport, illness or water problems. Treat a colour change as a prompt to test and observe, not as proof that lighting is the only cause.

A dark background may make the red and blue pattern easier to see, but decor is not a treatment and colour intensity is not the sole measure of welfare. Breathing, balance, feeding, group position and intact fins give a fuller picture.

Feed a small complete food once daily

Feed a suitably small complete food as the staple, once a day, in an amount the group consumes within a few minutes. Spread particles across the shoal so every fish has access, and reduce the next portion if food repeatedly reaches the substrate or filter uneaten. Cardinal tetras have small mouths, so particle size is as important as the food name.

Wild feeding helps explain the emphasis on small animal prey. Walker’s study of 80 wild cardinal tetras across 14 Rio Negro basin sites found predominantly small invertebrate material, especially microcrustaceans and chironomid larvae. FishBase also records worms and small crustaceans, while Seriously Fish describes a broader natural menu that includes small invertebrates, crustacea, filamentous algae and fallen fruit.

In the aquarium, use a reputable fine complete flake or micro-granule that the fish can swallow cleanly. Add safe, suitably small live or frozen variety such as Daphnia or small bloodworm where appropriate. Variety complements the complete staple; a single frozen food is not a complete long-term diet.

Check access before increasing the portion

• Food is repeatedly spat out: reduce particle size and confirm that it softens or sinks at a usable rate.
• Only the fastest fish feed: distribute small amounts across a wider area rather than making one crowded feeding point.
• Food remains after a few minutes: remove recurring leftovers and offer less next time.
• A fish stops feeding: test water and observe breathing, balance, body condition and social pressure.
• New companions take everything: reconsider feeding locations and compatibility instead of simply adding excess food.

Overfeeding creates waste that can raise ammonia and nitrite and increase nitrate. Do not use extra food as a shortcut to faster growth, and do not assume visible algae supplies a complete diet. Review the amount after adding fish, changing temperature or altering the filter because each can change feeding response or waste processing.

Body condition should remain steady without pinched bellies or progressive swelling. A single missed response is an observation; repeated refusal, weight loss, rapid breathing or isolation requires water-quality checks and closer health assessment. Avoid diagnosing a disease from appetite alone.

Receive and acclimatise the fish without delay

Prepare the mature aquarium and confirm its readings before the parcel arrives. The current delivery and Live Arrival Guarantee policy describes hand-bagging in pure oxygen, an insulated poly box and a heat pack when weather calls for it. Someone must be present to receive live fish, and the parcel must be opened and inspected on the day it arrives.

Follow the arrival sequence

1. Receive the parcel in person so live fish are not left unattended.
2. Start filming before the parcel is opened, with the parcel still sealed and the outer box visible.
3. Open it in one continuous take and show the fish while they are still inside the sealed bag.
4. Open and inspect the parcel on the day it arrives.
5. Float the bag for 20–30 minutes before releasing the fish.

Keep the continuous unboxing video with your order number. The video should show the sealed parcel, the opening and the fish still inside the sealed bag before anything is disturbed.

Separately from that arrival sequence, check the receiving aquarium’s temperature, ammonia and nitrite before release. OATA advises zero ammonia and zero nitrite when fish are introduced, and warns that adding livestock too quickly can exceed the capacity of the established bacterial population. Do not solve a failed water test by releasing fish and hoping the filter catches up.

Use the first week to confirm settling

• On release: observe balance, breathing and whether the fish can hold position in the current.
• At feeding: confirm that small particles reach all parts of the group and remove recurring leftovers.
• Each day: look for persistent isolation, surface hanging, bottom sitting, erratic swimming, clamped or damaged fins and abrupt colour change.
• After any concern: test ammonia, nitrite, nitrate, pH and temperature before assuming a behavioural cause.
• After the first maintenance: compare readings before and after the change to detect an abrupt difference in replacement water.

Transport can temporarily affect colour and shoal position, so use several welfare signs together. A fish that remains unable to balance, breathes with obvious effort, develops visible damage or fails to join the group needs prompt assessment. Keep a record of water results and observations so changes can be compared accurately.

Respond to health changes with water tests first

Test ammonia, nitrite, nitrate, pH and temperature when behaviour or appearance changes. A settled cardinal tetra should maintain balance, breathe without obvious effort, hold its fins normally, move with the group and respond to suitable food. Persistent surface hanging, bottom sitting, erratic swimming, rapid or unusually slow gill movement, spots, fluffy growths, wounds, swelling, weight loss or isolation require investigation.

RSPCA fish-health guidance places water-quality checks at the start of the response because several problems can produce similar visible signs. Check the heater, filter flow, recent maintenance, new livestock, feeding and social pressure. Do not name a disease from colour loss, a bent posture or reduced appetite alone.

Quarantine new fish in a separate mature system with dedicated equipment. The MSD Veterinary Manual recommends at least 30 days in its professional aquarium-fish biosecurity guidance, with longer periods where circumstances require. A quarantine aquarium still needs stable temperature, established filtration, tests, shelter and daily observation.

Use a clear escalation sequence

1. Record the visible change and when it began.
2. Test water and confirm heater and filter operation.
3. Review recent additions, maintenance, feeding and aggression.
4. Separate a fish only into a suitable mature system where doing so reduces risk.
5. Seek diagnosis before choosing salt, heat or medication.

Avoid blind treatment. Different diseases, injuries and water-quality failures can look alike, and an unsuitable medicine or dose can add further stress. For diagnosis and treatment planning, the OATA fish-vet directory provides a route to UK veterinary help.

Clean nets and equipment between systems and avoid moving water, filter media or decor from quarantine into the display without a biosecurity plan. Continue observing the rest of the group because one visible case may not be the only affected fish.

Plan for long-term care without attaching a fixed lifespan promise. Stable water, suitable food and social housing support welfare, but age at purchase, health history and husbandry vary. Daily observation and weekly records are more useful than expecting every fish to follow one age estimate.

Keep display care separate from a breeding project

Plan this pack as part of a healthy social display rather than an assumed breeding pair. Cardinal tetra breeding is possible but difficult, and a community aquarium is not a controlled spawning system. The absence of eggs does not indicate poor health, while a rounded body does not establish reproductive condition or sex.

Seriously Fish recommends a dedicated, very dimly lit breeding aquarium with fine-leaved plants, spawning mops or a protective mesh. Its breeding guidance uses soft, acidic water at pH 5.5–6.5, 1–5 dGH and around 26.5–29°C, with gentle sponge filtration. These are breeding-project values, not replacements for the routine display target of 24–27°C, pH 5.5–7 and 1–8 dGH.

FishBase reports that eggs hatch in 24–30 hours and fry become free-swimming after three to four days in captivity. Those timings describe reproduction under suitable conditions rather than a promise for a home attempt. Fry require their own safe environment, appropriately small food and stable water before any breeding project begins.

Decide whether the project is ready

• A separate mature aquarium is operating and measured.
• Water can be adjusted gradually without changing the community display.
• Eggs and adults can be managed separately.
• Food suitable for very small fry is prepared before spawning.
• There is a responsible long-term plan for any juveniles.

Do not create abrupt pH, hardness or temperature changes in a stocked display to trigger spawning. Keep written measurements, change one controlled factor at a time and stop if adult welfare deteriorates. A stable display group remains the appropriate objective when the separate breeding requirements cannot be met.

Screen every companion at its adult size

Avoid any fish large enough to eat cardinal tetras. OATA’s rule for smaller tetras is direct: they should not be kept with fish capable of eating them. Apply that assessment at the companion’s adult size rather than judging a juvenile that is temporarily similar in length.

Choose peaceful companions with overlapping temperature, pH and hardness requirements. Seriously Fish describes cardinal tetras as generally peaceful and identifies similarly sized characids, gasteropelecids, lebiasinids, smaller callichthyid or loricariid catfishes and non-predatory small-to-medium-sized cichlids as potentially suitable categories. Each exact species still needs its own water, adult-size, social and space assessment.

Use caution with adult angelfish, territorial fish and persistent fin-nippers. The store care guide does not treat adult angelfish as guaranteed companions because a larger fish can prey on a small tetra. Territorial pressure can also exclude cardinals from feeding or open water even when direct injury is not immediately visible.

Shrimp need a separate risk assessment. Adult Amano shrimp or larger Neocaridina may coexist in a suitably planned aquarium, but shrimplets and other small vulnerable shrimp can be eaten. Dense planting can reduce exposure but cannot guarantee survival. Their mineral requirements must also overlap with the water selected for the cardinals.

Use this five-part screen

1. Predation: compare the companion’s adult body and mouth size with a 3 cm standard-length tetra.
2. Water: find a routine temperature, pH and hardness suitable for both species.
3. Space: include the full adult group and territory requirements of every animal.
4. Temperament: exclude predators and assess territorial or fin-nipping behaviour.
5. Feeding: make sure faster animals do not prevent the cardinal group from eating.

Regional origin or a generic “community fish” label is not enough. Two South American species can require different water, space or behaviour management. Likewise, two small fish are not automatically compatible if one needs harder water, a different temperature or a separate social group that the aquarium cannot hold.

For a mixed plan, contact us with the aquarium dimensions, measured water, mature stocking list and intended group sizes. The assessment should cover every species together rather than approving one pair of names in isolation.

Protect the group from social and feeding stress

Keep at least 10 cardinal tetras under our conservative care policy. FishBase gives a floor of five, Seriously Fish recommends at least eight to 10 and OATA says tetras should be kept in groups of at least five, with more being better. Tropical Fish Co uses 10 as the practical minimum for this species where total aquarium capacity allows.

This pack contains six fish, so it does not by itself meet that 10-fish plan. Six added to an established compatible group of four would produce 10; six added to an established group of six would produce 12. In either case, confirm the aquarium can support the completed number before combining fish. Arithmetic does not replace filtration, swimming-space or quarantine decisions.

Cardinals are generally peaceful rather than territorial display fish. Watch for persistent chasing, damaged fins, exclusion from food or one fish remaining apart from the group. These observations can indicate social pressure, but they can also accompany water problems or illness, so test the aquarium before changing stocking.

Read group behaviour in context

Do not divide the intended social number across different tetra species. Five cardinals and five unrelated tetras are not the same social plan as 10 cardinals, because each species forms its own group. If the aquarium cannot hold complete groups for two schooling species, choose one complete group and give it the available space.

Answer common cardinal tetra questions

How big do Cardinal Tetras get?

They reach a recorded maximum of 3 cm standard length, measured to the base of the tail. Plan the aquarium around that adult figure.

What size aquarium do 6 × Cardinal Tetras need?

Use a mature, filtered aquarium at least 60 cm long and about 60 litres for a completed group of 10, with more room for companions or a larger group.

How many Cardinal Tetras should live together?

Keep them in a group of at least 10 under Tropical Fish Co’s conservative care policy, provided the aquarium has enough filtration and swimming space.

Are Cardinal Tetras suitable for beginners?

They suit prepared keepers who understand cycling, testing and gradual water adjustment, but not a new or unstable aquarium. Tropical Fish Co classifies their care as intermediate.

What do Cardinal Tetras eat?

Feed them a suitably small complete flake or micro-granule once daily, with safe small live or frozen variety such as Daphnia or small bloodworm.

What temperature and pH does this fish need?

Keep it at 24–27°C, pH 5.5–7 and 1–8 dGH under Tropical Fish Co’s conservative routine target, using measured water and gradual adjustments.

Is this fish aggressive or likely to nip fins?

It is generally peaceful. Keep a proper group and watch for persistent chasing, damaged fins or feeding exclusion while also checking water quality.

Which kinds of animals can it live with?

Potential companions include similarly sized characids, gasteropelecids, lebiasinids, smaller callichthyid or loricariid catfishes and non-predatory small-to-medium cichlids after full water, space and temperament screening.

How should I receive and acclimatise this fish?

Start filming with the parcel still sealed before opening or floating the bag, and continue until the fish are visible inside the sealed bag. Inspect it the same day, then float the bag for 20–30 minutes before release.

Check the care references

• FishBase: Paracheirodon axelrodi — identity, distribution, recorded environment, biology, group floor and maximum standard length.
• Seriously Fish: Paracheirodon axelrodi — aquarium dimensions, water ranges, layout, feeding, compatibility and breeding guidance.
• OATA: How to look after tetras and pencilfish — group care, feeding, maintenance and introduction guidance.
• RSPCA: Aquarium environment — biological filtration and waste processing.
• Eschmeyer’s Catalog of Fishes — current scientific name and family placement.
• IUCN freshwater fishes information brief — conservation and ornamental-trade context.
• Walker: The food spectrum of the cardinal tetra — wild stomach and gut contents.
• RSPCA: Pet fish health — warning signs and water-quality-first checks.
• MSD Veterinary Manual: Management of aquarium fish — quarantine and biosecurity.
• OATA: Find a fish vet — UK veterinary directory.