Cardinal Tetra

Paracheirodon axelrodi

Also known as: Cardinal, Red neon

Water Parameters

• Temperature: 23–28°C
• pH: 4.5–7
• Hardness (GH): 18–125 ppm (1–7 dGH)

Values above are general hardness (GH). For carbonate hardness (KH), the buffer that keeps pH stable, see the water parameters guide.

Care Summary

Widely regarded as the more beautiful and, surprisingly, the more durable of the two famous blue-and-red tetras. Unlike the neon, the cardinal is not subject to the specific neon tetra disease epidemic that weakens farmed neons, and well-kept cardinals routinely outlive them. The trade-off is water chemistry: cardinals are still largely wild-caught from the Rio Negro, and while they tolerate a range of conditions, they need soft, slightly acidic water to thrive long-term rather than merely survive. In the naturally hard tap water of Brisbane, Adelaide, or Perth they are better kept with rainwater or RO blending. At their best in a large school against dark substrate under subdued light, where the iridescent blue line and red belly carry remarkable distance across a room. Part of that soft-water need is a low carbonate hardness (KH), below about 4 dKH (roughly 70 ppm): high KH holds the pH too high for them, and a low KH matters most if you ever try to breed them.

Tankmates

Frequently confused with the neon tetra; the cardinal is the larger fish, with red running the full length of the body rather than only the rear half. That slightly greater size makes it the better-judged choice for an angelfish community, where neons are often eaten. A peaceful schooling fish for groups of at least six, ideally ten or more. The important caveat is water: cardinals come from soft, acidic Amazon blackwater and show their best colour and health in those conditions, which puts them at odds with hard-water species like guppies, platies, and mollies. Adult shrimp are safe; very small shrimp fry may be taken.

Compatible with

• Neon Tetra
• Harlequin Rasbora
• Ember Tetra
• Bronze Corydoras
• Pygmy Corydoras
• Otocinclus
• German Blue Ram
• Angelfish
• Amano Shrimp
• Discus
• Sterbai Corydoras
• Red Phantom Tetra
• Black Phantom Tetra

Avoid keeping with

• Betta
• Tiger Barb
• Fin Nippers
• Large Cichlids
• Guppy
• Molly

Common Problems

• pale colour and poor health in hard, alkaline water
• sensitivity to parameter swings in immature tanks
• ich
• stress in groups smaller than 6

Sources: seriouslyfish.com, fishbase.org · Last updated: 2026-06-20