Often confused with the neon tetra, the cardinal is larger and has a red stripe running the full length of its body beneath the blue. A stunning shoaling fish that prefers warm, soft, acidic blackwater.
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South America (upper Orinoco and Rio Negro basins, Brazil, Colombia, Venezuela)
Origin
New World
Climate
🌴 Tropical
Water type
💧 Freshwater
Family
Characidae
Genus
Paracheirodon
Part of the Tetras
Small, often brightly colored characin fishes popular as peaceful shoaling community aquarium fish. Tetras are kept in groups, appreciate soft, slightly acidic water and planted tanks, and range from tiny nano species to larger schooling fish.
From the minimum an animal needs to be kept humanely, up to the ideal setup. Bigger is almost always better — minimums are floors, not targets.
Photo coming soon
Minimum
Shoal planted nano
20 gal / 76 L planted
Paracheirodon axelrodi schools in groups of 6+ (10+ ideal). A 20-gallon planted tank is the practical minimum, with soft acidic blackwater (pH 5.5–6.5), warm 26–28 °C, and dense plants for cover.
Recommended
Planted Amazon community
29 gal / 110 L long planted
29-gallon long planted community with a shoal of 10–12, driftwood, peat-leached water, and peaceful dither fish. Truly stunning in groups under dim light.
Paolo Neo / Public domain (Wikimedia Commons)
Photo coming soon
Ideal
Amazon biotope shoal
40 gal+ / 151 L+ biotope
Amazon biotope with deep leaf litter, soft acidic water, driftwood, and a large shoal of 15+. Schools sweep across the tank under dim canopy lighting — closest to wild igarapé behaviour.
Life & growth stages
How this animal changes through its life — each stage often has its own care, diet and space needs.
Photo coming soon
Egg
Fish eggs are small, translucent spheres, often laid in clutches on plants, substrate, or in a nest — or carried/brooded by a parent in livebearing and mouth-brooding species. A dark eye spot and the curled embryo become visible inside as development progresses.
Photo coming soon
Fry
Newly hatched fry are tiny and semi-transparent, frequently still carrying a yolk sac that fuels them before they feed freely. They lack full fin structure and adult coloration, staying near cover until they can swim and forage on their own.
Photo coming soon
Juvenile
Juveniles look like miniature adults but with developing fins and muted or different markings; many species shift pattern and color as they mature. Growth is rapid at this stage given clean water and steady feeding.
Adult
Adults show the species' full size, finnage, and mature coloration, and are sexually mature. Many fish develop sex-specific differences in size, color, or fin shape, which can intensify during breeding.
Color & pattern variants
Natural variants occur in the wild; selectively bred (man-made) variants were developed in captivity.
Keep cardinals in a planted aquarium of at least 75-90 L (around 20-25 gallons) so a substantial shoal can be maintained. A dark substrate, driftwood, leaf litter, and dim lighting recreate their blackwater origins and intensify their red and blue coloration. Provide dense planting at the sides with open swimming space in the middle.
Unlike many tank-bred tetras, cardinals are wild-caught in large numbers and do best in warm, very soft, acidic water: temperature around 25-28 C (77-82 F), pH roughly 4.5-6.5, and low hardness. They are sensitive to nitrate and unstable conditions, so a mature, well-cycled tank and consistent water changes are important.
Substrate
Dark fine sand or fine gravel best displays their red-and-blue coloration and mimics the leaf-littered blackwater streams they come from. Adding botanicals like Indian almond leaves enhances the natural look and tints the water.
Equipment & setup
A planted tank of 20+ gallons with gentle filtration, a heater at 76-84F (they tolerate warmth better than neon tetras), and soft, acidic blackwater conditions. Subdued lighting with floating plants makes them feel secure and shows their colors.
Diet
Cardinal tetras are omnivorous micro-predators. Offer a varied diet of small, high-quality flakes and micro-pellets along with regular live or frozen foods such as daphnia, baby brine shrimp, cyclops, and bloodworms in moderation.
Feed small portions once or twice daily. A varied, protein-rich diet improves their already vivid coloration and helps condition them.
Behavior & temperament
Peaceful and shoaling, cardinals should be kept in groups of at least 6-10, where they form a dazzling moving band of red and blue. Larger groups are calmer, more colorful, and more natural in behavior.
They are excellent residents of calm, soft-water community and biotope aquariums alongside other small, gentle species such as small rasboras, pencilfish, and dwarf cichlids. Heavy planting, tannin-stained water, and gentle flow provide security and enrichment.
Health
Because most cardinals are wild-caught, new arrivals can carry parasites and be stressed by shipping; quarantine and slow acclimation are strongly recommended. They are susceptible to ich, velvet, and bacterial infections when chilled or kept in hard, alkaline, or poor-quality water.
They are somewhat more demanding than neon tetras and intolerant of high nitrate and swings in water chemistry. Stable, warm, soft, acidic water plus a mature tank greatly reduces disease risk.
Tips, DIY & hacks
Keep them in shoals of at least 8-10 (more is better) for natural schooling and reduced stress, and use peat or catappa leaves to soften and acidify the water toward their wild blackwater parameters. They are sensitive to nitrates and new-tank conditions, so add them only to a mature, well-cycled aquarium.
