The cardinal tetra is a small shoaling characin from the soft, acidic blackwaters of the upper Amazon and Rio Negro.
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.
Housing
Keep a shoal of at least ten in 75 litres (20 gallons) or more, filtered and heated to 24-28C.
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.
A fully cycled, stable system matters more than decor.
Provide gentle flow and floating cover so these timid fish feel secure.
Diet
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
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.
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.
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
Is your enclosure big enough?
Minimum volume75 litres (20 US gallons)
CompanyCannot be kept alone. The minimum is 10, and the space above is for that group.
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, or crowding the filter outflowIncrease surface agitation immediately, then test the water. Either the oxygen has crashed or the gills are being chemically burned. Both are hours-scale problems.
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 point at the water or the equipment, not at a pathogen.
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.
What it really costs
Setup, monthly, first year and across a whole life, itemised and with the electricity worked out from the actual wattage. Published figures for the same animal disagree by a factor of four, so the calculator shows ranges rather than picking one.
Intermediate
Beginner
Beginner
Intermediate
Intermediate
Intermediate