**Hyphessobrycon eilyos** is a small and colorful species of freshwater fish belonging to the family Characidae, part of the group commonly referred to as tetras. This species is native to the rivers of South America, particularly in Brazil. The name "eilyos" is derived from the Greek word for "helpless" or "poor," likely referring to the fish's small size and vulnerable appearance in the wild. Despite being less well-known than some of its tetra relatives, Hyphessobrycon eilyos is admired by aquarium enthusiasts for its peaceful temperament and striking coloration.

### Appearance
Hyphessobrycon eilyos is a small, slender fish, reaching a maximum size of about 1.2 to 1.5 inches (3 to 4 cm) in length. Its body is primarily silver or pale yellow, with a subtle iridescent sheen that reflects light beautifully in aquariums. The most striking feature of this species is its deep red or orange patch located behind the gill cover, contrasting vividly against its otherwise light-colored body. Its fins are mostly transparent but may have faint reddish or yellow tinges depending on their condition and environment. The tail fin is forked, giving it a streamlined appearance that is typical of many tetras.

### Behavior and Temperament
Hyphessobrycon eilyos is a peaceful schooling fish, making it an excellent choice for community tanks. Like other tetras, it thrives when kept in groups of at least six or more. When kept in smaller groups, they may become shy or stressed, resulting in less vibrant colors and reduced activity. In a properly sized group, they swim in tight formations, adding a dynamic element to the aquarium.

They are mid-level swimmers and will occupy the central areas of the tank, rarely venturing to the substrate or surface unless searching for food. Their calm and peaceful nature makes them compatible with a wide variety of other small, non-aggressive species.

### Habitat and Tank Requirements
In the wild, Hyphessobrycon eilyos is found in slow-moving rivers and streams with soft, slightly acidic water. To replicate their natural environment in captivity, the following tank conditions should be provided:

1. **Tank Size**: A minimum of 10 to 15 gallons (40 to 60 liters) is recommended for a small school. Larger groups will benefit from more space.
2. **Substrate**: A dark, sandy or fine-gravel substrate will enhance their colors and create a natural look in the tank. Soft substrates are preferable as they are gentler on the fish.
3. **Water Parameters**:
- Temperature: 72-82°F (22-28°C)
- pH: 5.5-7.0 (slightly acidic to neutral)
- Hardness: Soft to moderately soft water (1-12 dGH)
- They prefer well-oxygenated water with a gentle current.
4. **Decor**: Provide plenty of live plants, such as Java fern, Amazon swords, and floating plants like duckweed, to create shaded areas and reduce light intensity. Hyphessobrycon eilyos feels most comfortable in densely planted aquariums with natural hiding spots.
5. **Filtration**: A gentle filtration system is ideal, as these fish are accustomed to slow-moving waters. Strong currents can cause stress, so a sponge filter or a canister filter with adjustable flow is recommended.

### Diet
Hyphessobrycon eilyos is omnivorous, feeding on a wide variety of small invertebrates, plant matter, and detritus in the wild. In captivity, they will readily accept a wide range of foods, including:
- High-quality flakes and micro-pellets
- Frozen or freeze-dried foods such as daphnia, bloodworms, and brine shrimp
- Small live foods like micro-worms or baby brine shrimp

To maintain optimal health and coloration, a varied diet is essential. Offer a mix of plant-based and protein-rich foods to ensure they get a balanced diet.

### Breeding
Breeding Hyphessobrycon eilyos in captivity is possible but requires the right conditions. Like many other tetras, they are egg scatterers, meaning they release their eggs randomly over the substrate and plants without showing any parental care.

To encourage breeding:
1. **Spawning Tank**: Set up a separate breeding tank with a gentle sponge filter and soft, slightly acidic water (pH 6.0-6.5). The temperature should be slightly warmer, around 78-80°F (25-27°C).
2. **Plants and Spawning Mop**: Provide fine-leaved plants or spawning mops for the fish to scatter their eggs over.
3. **Spawning Process**: Condition the adults with high-quality live or frozen foods. When ready, the female will release eggs, and the male will fertilize them. Remove the adults immediately after spawning, as they will eat the eggs if left in the tank.
4. **Hatching and Fry Care**: The eggs typically hatch in 24-36 hours, depending on the temperature. The fry will become free-swimming a few days later. Feed them infusoria or specially prepared fry food until they are large enough to accept micro-worms or baby brine shrimp.

### Common Health Issues
Hyphessobrycon eilyos is generally a hardy species, but they can be prone to stress-related illnesses if water quality is poor or if they are kept in suboptimal conditions. The most common issues include:
- **Ich**: A parasitic infection that presents as small white spots on the body and fins. It is usually caused by stress or temperature fluctuations.
- **Fin Rot**: A bacterial infection that can affect their delicate fins, especially if water quality is poor.

To prevent health issues, maintain high water quality by performing regular water changes, monitor water parameters closely, and avoid overcrowding the tank.

### Tank Mates
Hyphessobrycon eilyos is a peaceful species that does well in community aquariums with other similarly sized and non-aggressive fish. Ideal tank mates include:
- Other small tetras (such as Neon Tetras, Ember Tetras)
- Rasboras
- Dwarf corydoras (such as Corydoras pygmaeus)
- Small peaceful gouramis (like Honey Gourami)
- Small peaceful loaches (like Kuhli Loaches)

Avoid housing them with large or aggressive fish, as these can stress or harm the delicate tetras.

### Summary
Hyphessobrycon eilyos is an attractive and peaceful tetra species that makes a great addition to community tanks. Its striking red or orange markings, combined with its small size and schooling behavior, make it a captivating fish to observe in well-planted aquariums. With proper care, diet, and tank conditions, these tetras will thrive and display their best colors, bringing life and beauty to any freshwater setup.