Cardinal Tetra
Paracheirodon axelrodi
The Cardinal Tetra (Paracheirodon axelrodi) is the undisputed and irreplaceable king of freshwater aquariums. Often confused by beginners with the cheaper and more common Neon Tetra (P. innesi), the Cardinal stands out for a clearly superior chromatic magnificence: the iconic bright fire-red stripe does not stop mid-belly, but runs powerfully for the entire length of the body, from the snout to the root of the tail, parallel to the incredible iridescent neon-blue band. This flashy coloration is an evolutionary adaptation to keep the school together in the dark Amazonian black waters. Unlike the Neon, the Cardinal loves very hot waters, making it the favorite and inevitable companion for majestic aquariums dedicated to Discus.
Water Temperature
25 - 30 °C
pH Value
4 - 6.5
Water type
Freshwater
Tank level
Middle
Adult size
5 cm
Minimum tank volume
>= 80 L
Minimum group
>= 10
GH
1 - 8 dGH
KH
0 - 4 dKH
TDS
15 - 160 ppm
Conductivity
23 - 250 uS/cm
Species description
Geographical Origin and Habitat: The Cardinal Tetra is native to South America, primarily distributed in the Rio Negro and Orinoco river basins across Brazil, Colombia, and Venezuela. It inhabits slow-moving blackwater tributaries characterized by dense overhanging vegetation, leaf-littered sandy riverbeds, and extremely soft, acidic water heavily stained brown by humic acids and tannins.
Taxonomy and Morphology: How to Recognize it from the Neon Tetra (Not to be scammed): In the store they will very often sell you the Neon Tetra passing it off as the Cardinal, which costs more and is more delicate. Look at the RED stripe under the flank: if it starts from the tail and stops mid-belly (leaving the throat white) it is a lowly Neon Tetra. In the Cardinal Tetra the red stripe TOTALLY crosses the body reaching under the eye. Furthermore, the Cardinal becomes significantly larger and stockier as an adult (5 cm / 2 inches).
Social Behavior: Stress-Induced Schooling Effect: Beginners buy them expecting the fish to always swim militarily united in the same direction (Schooling). Actually, in the aquarium this will almost never happen. If the fish are relaxed and without predators in the tank, they will scatter everywhere minding their own business. The compact school forms solely if they are terrified or if there is a large cichlid in the tank constantly threatening them.
Coloration and Sexual Dimorphism: It displays an incredibly bright coloration, with an iridescent electric blue-green longitudinal stripe along the upper body and a vivid scarlet red band running the entire length of the lower body. Sexual dimorphism is subtle: mature females are slightly more robust and show a rounder, deeper belly when gravid, sometimes causing the blue stripe to appear slightly curved.
Care, breeding and tankmates
Aquarium Setup: Black Water and Darkness to See their Color: If you want their colors to shine bright enough to blind you, do not throw them in a clean brightly lit aquarium with white gravel bottom like a waiting room. They will fade out of fear. Their neon color explodes solely and exclusively if the aquarium has a black background, very dark gravel or a carpet of dead leaves on the bottom, and the light is so weak (tea-colored water with peat) as to make the tank look like a pool of shadow.
Diet and Feeding: It is an omnivorous micro-predator. In the aquarium, it readily accepts high-quality flake foods or micro-granules. To keep their colors vibrant and promote long-term health, their diet should be regularly supplemented with small live or frozen foods such as brine shrimp nauplii, daphnia, and bloodworms.
Water Quality: Lovers of Heat (Discus Companions): Unlike the Neon that dies if it stays too hot, the Cardinal comes from the Rio Negro. The water must literally be very hot and stifling (26°C / 79°F and up). It is the only commercial schooling fish that can coexist and tolerate the constant 29°C (84°F) of aquariums with gigantic Amazonian Discus without shortening its life expectancy.
Compatibility and Cohabitation: A peaceful, schooling species that should be kept in a group of at least 10 individuals. Ideal tankmates include peaceful dwarf cichlids (Apistogramma), Corydoras, Loricariids, and other small characins. If kept with Angelfish or Discus, they should be introduced when the larger fish are young to minimize the risk of predation.
Aquarium Breeding: Extremely difficult to achieve in home aquariums. It requires a dedicated breeding tank with dim to no lighting, very soft, acidic water (pH 5.0 - 5.5, GH < 2, conductivity under 30 µS/cm). They are egg scatterers and do not care for their spawn. Parents must be removed immediately after spawning to protect the light-sensitive eggs.
Risks and Diseases: Sensitive to sudden changes in water chemistry and rapid drops in temperature, which can easily trigger Ich (white spot disease). While generally hardier than P. innesi, it can still contract Neon Tetra Disease (NTD) if exposed to infected food or dead tankmates.
Geographic Origin and Distribution
The Cardinal Tetra is native to South America, primarily distributed in the Rio Negro and Orinoco river basins across Brazil, Colombia, and Venezuela. It inhabits slow-moving blackwater tributaries characterized by dense overhanging vegetation, leaf-littered sandy riverbeds, and extremely soft, acidic water heavily stained brown by humic acids and tannins.
Origin
Amazon, Orinoco, and Guianas
Fish profile
- Temperament
- Peaceful
- They are highly gregarious and peaceful schooling fish. They feel most secure when surrounded by their own kind. Without a school, they become highly stressed and timid.
- Diet
- Omnivore
- They are omnivorous micro-predators in the wild. In the aquarium, they accept crushed flakes and micro-pellets. Supplement with live or frozen daphnia for optimal health.
- Life expectancy
- 5 years
- In captivity, the Cardinal Tetra typically lives between 3 and 5 years, though under exceptional care they can occasionally reach up to 7 years. In their natural habitat, environmental pressures and seasonal dry periods often limit their lifespan to just a single year. Maintaining very soft, acidic, and biologically clean water is essential to promote the longevity of this small and beautiful species.
- Adult size
- 5 cm
- The catalog's scalar value is 5 cm. The FishBase SPECIES table (v25.04 snapshot) source reports a maximum adult measurement of 2.51 cm for this species, recorded as standard length (SL). The taxonomic record consulted is Paracheirodon axelrodi (v25.04); male or unsexed record. These figures are not interchangeable: the first is the profile's editorial reference, whereas the second is a published maximum under a specific measurement convention; differences among sources can reflect sex, population, life stage and measuring method.
- Sex ratio
- School
- Sex ratio is unimportant in a decent sized school. Keeping a mix of males and females is completely natural. Always keep them in groups of at least six.
- Feeding frequency
- Twice daily
- Feed them small amounts once or twice a day. Provide only what they can consume within two minutes. Overfeeding can quickly pollute their small environment.
- Bioload
- These small tetras have a very low bioload. They generate minimal waste in the aquarium. Standard filtration is usually more than enough.
- Flow
- Stagnant
- They prefer gentle to moderate water flow. Harsh currents can exhaust these small fish and cause stress. A sponge filter creates an ideal slow-moving environment.
- Jump risk
- No
- Reproduction
- Egg scatterer
- They are egg scatterers providing no parental care. Breeding requires very soft, acidic water and dim lighting. Parents will consume their own eggs if not separated.
- Compatibility & tankmates
- Verify
- They are exceedingly peaceful community fish. Keep them only with similarly sized, non-aggressive species. Large predatory fish will easily swallow them whole.
- Tank level
- Middle
- These tetras swim in the middle levels of the water. They appreciate open swimming areas and dense vegetation. They rarely venture to the absolute surface or bottom.
Scientific Parameters & Models
FishBase
Window to the official scientific data of the species
Data synced on: 06/15/2026
- Trophic level
- 3.3 ± 0.39 se
- Resilience
- High (less than 15 months)
- Phylogenetic uniqueness (PD50)
- 0.625
- Bayesian Length-Weight
- a = 0.01122, b = 3.07
- VulnerabilityLow vulnerability (10 / 100)
Image gallery
Licensed images linked to the species or, when marked, to the closest representative taxon.
Related species
Recommendations grouped by close relatives, companion choices, and similar care.
