Freshwater Fish · Tetras

Cardinal Tetra

Paracheirodon axelrodi

Blue-and-red shoaler from soft blackwaterIntermediate

Lifespan: 4-6 years
Size: 1.4-1.8 in
Social needs: Shoaler; keep 10+ together
Time: Low-moderate; daily feeding, weekly water change
Cost: Low-Medium
Diet: Omnivore; micro pellets, flakes, frozen foods

Overview

What is this animal?

The cardinal tetra is a small shoaling characin from the soft, acidic blackwaters of the Rio Negro and upper Orinoco basins in Brazil, Colombia and Venezuela.
Its red stripe runs the full length of the body, distinguishing it from the similar neon tetra and giving a dense shoal a strong visual effect under subdued light.

Many cardinals are still wild-caught, so they arrive sensitive to shipping stress and need stable, mature water.
In a settled, well-cycled tank they are hardy and long-lived, but they are not as forgiving as a true beginner fish.

Temperament

Peaceful, social and active, cardinals shoal tightly when secure and scatter when kept in too-small groups.
A large shoal settles faster, colours up and swims more openly.

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They make good community fish alongside other peaceful soft-water species such as dwarf cichlids, corydoras and small rasboras.
Avoid large, boisterous or predatory tankmates that will eat them or keep them hiding.

A good fit for

Planted soft-water community aquariums
Keepers wanting a colourful display shoal
Those with a mature, stable cycled tank
Blackwater and biotope-style setups

Common mistakes to avoid

Adding fragile wild imports to immature tanks
Keeping too few, causing chronic stress
Hard, alkaline water dulling colour and health
Neon tetra disease spreading through the shoal

Enclosure

Minimum 20 gallons, for a group of 10

Cannot be kept alone. The minimum is 10, and the space above is for that group.

Keep a shoal of at least ten in a tank at least 24 in long holding 20 US gallons or more, filtered and heated to 75-82°F.
They show best in a dim, heavily planted tank with dark substrate, driftwood and tannin-stained water that mimics their native habitat.

They prefer soft, slightly acidic water and dislike high mineral content.

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A fully cycled, stable system matters more than decor.
Provide gentle flow and floating cover so these timid fish feel secure.

Check your enclosure

Supplies

Setup

Tank and stand
Filter
Heater and thermometer
Substrate, plants and decor
Liquid test kit
Dechlorinator and starter supplies

Ongoing

Micro-pellet or finely crushed tropical flake
Frozen daphnia and cyclops
Frozen or live baby brine shrimp
Microworm culture (optional, for fry and conditioning)
Spirulina-based flake

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Fine net for removing leftovers
Small feeding spoon for accurate pinches

Safe substrate: Fine, smooth sand · Smooth, rounded fine gravel (not for loaches or sand-sifting cichlids; sand is better for corydoras) · Bare bottom (quarantine and grow-out only)

Work out the cost

Care routine

Daily

Check temperature and equipment
Feed
Head-count and look everyone over

Every few days

Top up evaporationevery 3 days

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Weekly

Count the school
Test the water
Water change and gravel vacuum
Wipe the front glass only

Every few weeks

Trim and thin the plantsevery 2 weeks

Monthly

Check supplies and reagent dates
Replace or recharge chemical media, if you use any (skip it in a blackwater tank)
Rinse the filter media

Health review: Quarterly school and stability review, every 3 months

Tools

Is your enclosure big enough?Check your measurements against the recommended size What will it cost?Setup, monthly and over its whole life, itemised Is this ammonia reading dangerous?Works out the toxic part from your pH and temperature

Care in depth

Diet

Omnivore; micro pellets, flakes, frozen foods

Cardinals are micro-predators that eat small invertebrates and zooplankton in the wild.
Offer a quality micro-pellet or crushed flake as the staple, rotated with frozen or live daphnia, baby brine shrimp, cyclops and finely chopped bloodworm.

Feed small amounts once or twice daily, only what is taken within a minute.
Their small mouths need appropriately sized food, and varied feeding supports colour and overall condition.

Health

Signs of good health, and the common problems

The main concern is neon tetra disease, an untreatable Pleistophora infection that causes fading colour, lumps and spinal curving; affected fish should be removed promptly.
Ich and fin rot appear when fish are chilled or stressed during acclimation.

Stable warmth, soft water and slow, careful acclimatisation prevent most losses.
Quarantine new stock, as wild imports can carry parasites, and avoid adding cardinals to an immature tank where swinging parameters weaken them.

Is this urgent?

6 warning signs and how fast to act, 3 of them right away

In a fish tank the patient is usually the water. Test before you treat: dosing a healthy tank for a disease it does not have makes things worse.

Go now

Any detectable ammonia or nitrite in a stocked tankLarge water change now with dechlorinated, temperature-matched water. Stop feeding. Test again in 12 hours. Both are directly toxic. How toxic the ammonia reading is depends on pH and temperature, so the same number can be harmless or lethal.

Now

Fish gasping at the surface with fast gill movement, or crowding the filter outflowIncrease surface movement now (gently in a betta or gourami tank, without creating a strong current), then test the water. Either the oxygen has crashed or the gills are being chemically burned. Both are hours-scale problems. Bettas, gouramis and corydoras rise to gulp air as part of normal breathing; the warning is constant gulping, fast gill movement, or the whole group hanging at the top.

Now

Sudden deaths, more than one fish in a dayTest ammonia, nitrite, nitrate and temperature before doing anything else. Do not add medication yet. Multiple simultaneous deaths usually point at the water or the equipment rather than a pathogen, so rule those out first.

Now

Today

Flashing: repeatedly scraping against decor or substrateTest the water, then look closely for parasites. Irritation is either chemical or parasitic; the test tells you which before you medicate.

Today

White salt-grain spots over the body and finsBegin the appropriate treatment today and check the temperature. The parasite's life cycle is temperature-driven, and it multiplies with every cycle you wait.

Today

Soon

Clamped fins, hiding, or refusing food for more than two daysTest the water and review recent changes. Usually the earliest visible sign of a parameter drifting.

A few days

Never add a new fish to a tank that is not fully cycled, and never treat blind. A test kit is cheaper than the fish.

This is general guidance for a healthy adult, not a diagnosis. An exotics vet who knows your animal always overrides it, and the time to find one is before you need one.

Bedding and substrate

3 safe, 4 with a caveat, 2 to avoid

Substrate in an aquarium is a filter as much as a floor. Most of the biology lives in and on it.

Safe

Fine, smooth sandCorrect for corydoras, kuhli loaches, eartheaters such as rams and acaras, and anything else that sifts or burrows. Barbels wear away on sharp gravel.

safe

Smooth, rounded fine gravel (not for loaches or sand-sifting cichlids; sand is better for corydoras)Rinse thoroughly before use. Grain size should be too large to swallow and too small to trap food. Kuhli loaches need sand to burrow into and rams and acaras sift mouthfuls of it; bronze and panda corydoras manage on smooth fine gravel, but sterbai corydoras need sand.

safe

Bare bottom (quarantine and grow-out only)Legitimate for quarantine and grow-out tanks. Easy to clean, but it holds far less biological filtration, and it is no long-term home for burrowing loaches or sand-sifters.

safe

With a caveat

Crushed coral or aragoniteDeliberately raises hardness and pH. Correct for rift-lake cichlids, wrong for soft-water species.

with a caveat

Aquasoil for planted tanksExcellent for plants, but most active soils soften the water and pull pH and KH down for months, and many leach ammonia for the first weeks. Right for soft-water species; wrong for hard-water animals (livebearers, rainbowfish, goldfish, and snails, whose shells erode in soft, acidic water) unless you test GH and KH and remineralise. Cycle around it; do not stock into it.

with a caveat

Very deep sand bedsAnaerobic pockets form below about 2 inches in still sand. Either keep it shallow or keep it stirred.

with a caveat

Painted or coated gravelUsually fine, but coatings chip. Test a handful in a jar for a week first.

with a caveat

Never

Sharp or angular gravel with bottom-dwellersWears down barbels and leads to infection in corydoras and loaches.

never

Anything containing metal, limestone of unknown origin, or shells in a soft-water tankSlowly alters chemistry in ways that are hard to diagnose later.

never

This is our general list for freshwater fish. We have not built one specific to Tetras yet.

Top resources for more info

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