Cardinal tetra

Quick facts

Adult size

5 cm

Lifespan

can live up to 7 years; captive average is 4-5 in well-maintained soft-water tanks

Tank zone

mid

Temperament

peaceful

Difficulty

intermediate

Schooling

recommended 6+ (critical minimum 4, thrives at 12+)

Typically wild-caught

yes - acclimate slowly

Water parameters

Temperature

23–27°C

pH

4.5 to 7.0

Hardness

1 to 6 dGH

Tank requirements

Minimum volume

90 L

Minimum length

60 cm

Flow

low

Lighting

dim preferred

Substrate

any

Driftwood

preferred

Hiding spots

needed

Feeding

Diet: omnivore, feeds primarily at the mid.

Similar to neon tetras but slightly less finicky because the mouth is marginally larger. Micro pellets, crushed flake, frozen daphnia, baby brine shrimp, and bloodworm. Feed small amounts twice daily. Cardinals feed in the midwater and lower water column, slightly deeper than neons. Quality food with astaxanthin and carotenoid content improves the red coloration noticeably.

Compatibility

• Classic community fish for soft-water setups. Pairs well with discus (the traditional tank companion), rams, apistos, small catfish, and other soft-water species.
• Less commonly eaten by angelfish than neons because they're slightly larger, but the risk still exists with full-grown angels.
• Avoid hard-water species (African cichlids, livebearers) because the preferred parameters don't overlap.
• Safe with shrimp. Too slow and small-mouthed to catch even juvenile cherry shrimp.

Habitat

Native to the Rio Negro and Orinoco basins in Brazil, Colombia, and Venezuela. Unlike the neon tetra, the red stripe extends the full length of the body rather than just the rear half. Wild cardinals are still caught from the Rio Negro in enormous numbers (the fishery supports local communities and is considered sustainable because the fish are short-lived in the wild). Tank-bred cardinals are also available and tend to be hardier. The species prefers very soft, acidic, warm water (pH 4.5-6.5, temperature 26–30°C), which means they do best in tanks with RO water or naturally soft tap water. Kept in groups of 10+ in a planted tank with dark substrate, they're one of the most visually striking schooling fish available.

Breeding

Difficult. Requires very soft, acidic water (pH 5.0-5.5, conductivity below 50 uS/cm), dim lighting, and a separate breeding tank. Eggs are light-sensitive and fungus-prone. The difficulty of breeding is why wild-caught cardinals still dominate the trade. The fish scatter eggs among fine-leaved plants at dawn. Remove adults immediately. Eggs hatch in 24-36 hours; fry are smaller than neon fry and need infusoria or paramecium cultures. Experienced breeders use RO water with peat filtration in completely dark tanks. It's doable but far more demanding than breeding neons.

Common problems

Cardinal tetra disease (different from neon tetra disease, despite the similar name) is caused by the microsporidian Pleistophora hyphessobryconis and presents the same way: loss of color, muscle cysts, wasting. No cure; remove infected fish. Wild-caught cardinals sometimes carry internal parasites that cause wasting over several weeks; quarantine new arrivals. Cardinals are less tolerant of poor water quality than neons and will be the first fish to show stress in a neglected tank. They need warm water (26–30°C); keeping them at typical community temperatures (24°C) shortens their lifespan.

Bioload

Bioload coefficient: 1.3 (slightly larger than neon at 5 cm but similar slim body; hobby consensus places at ~1.3x neon).

Bioload coefficients are calibrated against the neon tetra as the anchor (1.0). See the methodology page for the formula and how each value was derived.

Further reading

• Stocking ideas by tank size