Eilyos tetra
Hyphessobrycon eilyos
A small, lively South American freshwater fish, valued for the bright red coloration of its rear fins and its peaceful schooling behavior. An ideal resident for Amazonian biotopes.
Water Temperature
23 - 28 °C
pH Value
5.5 - 7.5
Water type
Freshwater
Tank level
Bottom and middle
Adult size
4 cm
Minimum tank volume
>= 80 L
Minimum group
>= 8
GH
2 - 10 dGH
KH
1 - 5 dKH
TDS
40 - 200 ppm
Conductivity
63 - 313 uS/cm
Species description
Geographical Origin and Habitat: Hyphessobrycon eilyos is native to South America, specifically endemic to the upper Paraná River basin in Brazil. It primarily inhabits small rivers, creeks, and slow-flowing forest streams characterized by clear or slightly amber-colored water. It prefers shallow marginal zones with dense aquatic and submerged vegetation, fallen branches, roots, and decaying leaf litter on sandy or muddy substrates, which offer shelter and a rich abundance of microinvertebrates.
Taxonomy and Morphology: It belongs to the class Actinopterygii, order Characiformes, family Characidae. It is a small-sized fish, reaching a maximum adult length of about 4 centimeters. It features a laterally compressed and moderately deep body, typical of the genus Hyphessobrycon. It possesses a well-developed adipose fin positioned on the back near the tail, and jaws equipped with small, sharp teeth adapted for capturing tiny prey in the water column.
Social Behavior: It is a schooling species that must be kept in groups of at least 6 to 8 individuals, preferably more, to reduce shyness and promote natural behavior. It has an extremely peaceful, active temperament and swims primarily in the middle water column. In the aquarium, it coexists harmoniously with other small characins, pencilfish, corydoras, and peaceful fish of similar size, showing no aggression.
Coloration and Sexual Dimorphism: The color pattern consists of a translucent, silvery, or light yellowish-brown base body. The defining feature is the bright red or intense reddish-orange coloration decorating the caudal and anal fins, while the other fins may show orange tints. It presents an indistinct dark humeral spot behind the operculum. Adult females are slightly larger and display a rounder abdomen compared to the slender males.
Care, breeding and tankmates
Aquarium Setup: It requires an aquarium with a minimum capacity of 60 liters. The aquascape should feature dense vegetation along the sides and back of the tank, leaving plenty of open swimming space in the center. Using plants like Limnophila, Vallisneria, or Cryptocoryne is recommended. A dark substrate and the presence of dried beech or oak leaves on the bottom, alongside driftwood and bogwood roots, will help recreate natural conditions and enhance the red coloration of their fins.
Diet and Feeding: It is an omnivorous and opportunistic fish. In the aquarium, it readily accepts a wide variety of prepared foods sized appropriately for its small mouth. The ideal diet should consist of high-quality flakes or micro-pellets as a staple, supplemented regularly with live or fine frozen foods such as newly hatched brine shrimp, cyclops, daphnia, and bloodworms. A varied diet ensures intense coloration and good overall health.
Water Quality: It needs clean, soft, and slightly acidic water. The optimal parameters are a water temperature between 22°C and 26°C, pH between 6.0 and 7.0, general hardness (gH) between 3 and 10 dGH, and carbonate hardness (kH) below 5. Using peat filtration is advisable to simulate the waters of its native biotope. Weekly partial water changes (approx. 10-15%) are essential to keep nitrate levels low.
Compatibility and Cohabitation: It is ideal for South American community aquariums or peaceful community tanks. It coexists perfectly with bottom-dwelling Corydoras, small loricariids, other tetras like neon or cardinal tetras, and angelfish if the tank is large and the tetras are fully grown. Avoid housing it with large, aggressive, or predatory fish (such as large cichlids) that could easily view it as prey due to its small size.
Aquarium Breeding: Breeding is possible and follows the typical pattern of characins. It requires a separate breeding tank with very soft and acidic water (pH 5.5-6.0, gH 1-2), dim lighting, and a mesh grid on the bottom or clumps of Java moss to protect the eggs from parental predation. The pair spawns free eggs that fall to the bottom. Parents must be removed immediately after spawning. Fry hatch in 24-36 hours and must be fed infusoria followed by artemia nauplii.
Risks and Diseases: It is susceptible to common freshwater fish diseases such as White Spot Disease (Ichthyophthirius), fungal skin infections, and fin rot if water conditions are poor. It suffers greatly from stress caused by unstable water parameters or excessive water current. Using a high-quality water conditioner and constantly monitoring water values are the best preventive measures.
Geographic Origin and Distribution
Hyphessobrycon eilyos is native to South America, specifically endemic to the upper Paraná River basin in Brazil. It primarily inhabits small rivers, creeks, and slow-flowing forest streams characterized by clear or slightly amber-colored water. It prefers shallow marginal zones with dense aquatic and submerged vegetation, fallen branches, roots, and decaying leaf litter on sandy or muddy substrates, which offer shelter and a rich abundance of microinvertebrates.
Origin
Tropical oceans and reefsAmazon, Orinoco, and Guianas
Fish profile
- Temperament
- Peaceful
- The temperament of Hyphessobrycon eilyos is overwhelmingly peaceful, making them an exemplary model of a docile community fish. They are not prone to fin-nipping or aggressive territorial disputes, provided they are kept in appropriately sized schools that allow them to establish a natural hierarchy. While males may occasionally engage in harmless sparring displays to establish dominance or impress females, actual physical damage is exceedingly rare. Their gentle nature ensures they will not harass slower-moving tank mates, fostering a tranquil and harmonious aquatic environment.
- Diet
- Omnivore
- In their natural habitat, Hyphessobrycon eilyos are micro-predators and foragers that feed on tiny insects, crustaceans, and various zooplankton. Within the aquarium environment, they readily accept high-quality dry foods such as micro pellets and crushed flakes designed for small tropical fish. To ensure their best coloration and overall health, their diet should be regularly supplemented with live or frozen foods like Daphnia, newly hatched brine shrimp, and bloodworms. A varied diet prevents nutritional deficiencies and mimics their diverse natural foraging behavior.
- Life expectancy
- 3-5 years
- In captivity, the lifespan of this species usually ranges between 3 and 5 years, provided the aquatic environment remains stable and stress-free. A well-planted, clean aquarium with constant water changes helps extend the fish's life. Poor management of nitrates and nitrogenous compounds can significantly reduce its biological defenses.
- Adult size
- 4 cm
- This profile records an adult size of 4 cm. Use it to size the permanent habitat and choose compatible furnishings and cohabitants.
- Sex ratio
- School
- While Hyphessobrycon eilyos does not strictly require a specific male-to-female ratio, keeping a mixed group with a slight majority of females is generally recommended. Having more females helps distribute any amorous or territorial chasing behavior from the males, preventing individual females from becoming overly stressed. In larger schools of ten or more, the sex ratio becomes less critical as aggression and attention are widely dispersed throughout the group. Observing a healthy mix of both sexes also allows aquarists to enjoy the more intense, vibrant coloration that males display when showing off to the females.
- Feeding frequency
- Twice daily
- To maintain optimal health and vitality, Hyphessobrycon eilyos should ideally be fed small portions one to two times per day. It is important to only offer an amount of food that the entire school can completely consume within two to three minutes. Overfeeding can lead to poor water quality, algae outbreaks, and potential health issues for these small fish. Providing a fasting day once a week can also be beneficial to clear their digestive tracts and replicate natural food scarcity.
- Bioload
- The bioload produced by Hyphessobrycon eilyos is generally considered to be quite low due to their small adult size and active metabolism. These tiny fish do not generate significant amounts of waste, making them excellent candidates for densely planted setups or nano aquariums when kept in appropriate numbers. However, a proper filtration system and regular water changes are still essential to maintain optimal water quality. If kept in very large schools, their cumulative bioload can increase slightly, but it rarely overwhelms a standard, cycled aquarium filter.
- Flow
- Weak
- Coming from environments ranging from sluggish tributaries to moderately flowing streams, Hyphessobrycon eilyos appreciates a gentle to moderate water flow in the aquarium. They are not built to constantly battle strong, torrential currents, which would quickly exhaust them and induce significant stress. Employing a spray bar or directing the filter outflow towards the tank glass can help diffuse the water movement effectively. Providing calmer resting areas among dense vegetation allows them to retreat and conserve energy whenever needed.
- Jump risk
- Yes
- Like many small, active species, Hyphessobrycon eilyos possesses a moderate risk of jumping out of the aquarium, especially when startled or during the initial acclimation period. Sudden changes in lighting, aggressive tank mates, or sudden movements outside the tank can easily spook them into a leaping reflex. It is highly recommended to keep the aquarium securely covered with a tight-fitting lid or a specialized mesh screen. Floating plants can also be added to the surface to help them feel more secure and deter jumping behaviors.
- Reproduction
- Egg scatterer
- Hyphessobrycon eilyos belongs to the category of egg-scattering fish, meaning they broadcast their eggs freely without providing any subsequent parental care. In a dedicated breeding setup, dim lighting, very soft, acidic water, and an abundance of fine-leaved plants or spawning mops are required to encourage spawning. The adult fish are notorious egg-eaters and must be removed from the breeding tank immediately after the spawning event concludes. Once hatched, the microscopic fry require infusoria or specialized liquid fry food until they are large enough to accept baby brine shrimp.
- Compatibility & tankmates
- Peaceful
- Hyphessobrycon eilyos is an incredibly peaceful and sociable schooling fish that makes a fantastic addition to most community aquariums. They thrive when kept alongside similarly sized, non-aggressive species such as Corydoras catfish, small rasboras, and other docile tetras. It is crucial to avoid housing them with large or predatory fish that might view them as a quick snack, such as larger cichlids. Maintaining them in a group of at least six individuals will help them feel secure, significantly reducing any potential stress or shyness.
- Tank level
- Middle/Bottom
- In the aquarium, Hyphessobrycon eilyos primarily inhabits the middle to upper levels of the water column, rarely venturing completely to the bottom unless searching for fallen food. They are enthusiastic swimmers that appreciate plenty of open swimming space in the center of the tank to dart around in their schooling formations. Providing tall background plants or floating vegetation will give them the upper-level cover they desire without encroaching on their preferred swimming zones. Their mid-water presence makes them an excellent choice for adding active, colorful movement to the central focal point of a planted setup.
Scientific Parameters & Models
FishBase
Window to the official scientific data of the species
Data synced on: 06/15/2026
- Trophic level
- 2.9 ± 0.3 se
- Resilience
- High (less than 15 months)
- Phylogenetic uniqueness (PD50)
- 0.5
- Bayesian Length-Weight
- a = 0.00676, b = 3.12
- 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.
