Blue emperor tetra
Inpaichthys kerri
An Amazonian tetra of regal beauty: the male displays an intense metallic blue with a blue adipose fin — a diagnostic detail distinguishing it from the female, who has the same fin in orange-red. Native to tributaries of Brazil's Rio Madeira basin. In schools of 8–10+ on dark substrate with dense plants and dim lighting, the males' blue reaches hypnotic depth. In groups that are too small it may develop fin-nipping tendencies. Breeding of intermediate difficulty.
Water Temperature
24 - 27 °C
pH Value
5.5 - 7
Water type
Freshwater
Tank level
Middle
Adult size
3.5 cm
Minimum tank volume
>= 60 L
Minimum group
>= 6
GH
1 - 12 dGH
KH
1 - 6 dKH
TDS
25 - 240 ppm
Conductivity
39 - 375 uS/cm
Species description
Geographical Origin and Habitat: Rio Aripuanã Basin (Brazilian Amazon). Populates the slow-flowing inlets shaded by the dense rainforest, on bottoms of sand and fallen branches.
Taxonomy and Morphology: Blue Emperor Tetra (Inpaichthys kerri). The only member of its genus. Often confused with Nematobrycon palmeri, it is distinguished by the presence of the dorsal adipose fin (absent in palmeri).
Social Behavior: Very lively schooling fish and formidable swimmer. Males engage in continuous, harmless fights parrying with spread fins to dominate the attention of the females of the school.
Coloration and Sexual Dimorphism: Cobalt Blue. Males (more slender) show a bright bluish lateral band on a purplish/lavender background; females are stockier, yellowish-gray, with a black band. The blue lights up only in acidic water.
Care, breeding and tankmates
Aquarium Setup: 80 cm (32 inches) tank. Requires ample space for horizontal free swimming in the center and a dense barrier of plants on the sides. Submerged roots and dim light will make their otherwise dull livery explode.
Diet and Feeding: Voracious micropredator. Hunts insects in mid-water. Excellent eater of live daphnia, bloodworms and brine shrimp. Immediately accepts fine dry granules for characins.
Water Quality: Tolerates a medium GH, but to show the "Blue Emperor" reflection strictly requires a pH below 6.8 and water filtered with active granular peat.
Compatibility and Cohabitation: Excellent community fish. Unlike some nippy tetras, it ignores its companions (does not bite long fins). Ideal with Apistogramma, Corydoras and bottom Loricariids.
Aquarium Breeding: Egg scatterer. In the morning, the couple side by side releasing non-adhesive eggs in the moss (Taxiphyllum). Once the parents are removed (they would eat them), the eggs hatch in 24 hours.
Risks and Diseases: Visual Stress Pallor. If kept in bare tanks, too illuminated (without floaters) or on blinding white gravel, the fish deactivates the chromatophores, resulting in mouse gray for its entire life.
Geographic Origin and Distribution
Rio Aripuanã Basin (Brazilian Amazon). Populates the slow-flowing inlets shaded by the dense rainforest, on bottoms of sand and fallen branches.
Origin
Amazon, Orinoco, and Guianas
Fish profile
- Temperament
- Peaceful
- They are a peaceful, energetic schooling fish that must be kept in groups of eight or more. In proper numbers, they will interact actively with one another, establishing a fascinating pecking order. In numbers too small, they may nip fins out of stress.
- Diet
- Omnivore
- They are unfussy omnivores that will readily accept a wide variety of foods. Offer a staple of high-quality flakes or micro-pellets. Supplement this regularly with live or frozen foods like daphnia, bloodworms, and brine shrimp.
- Life expectancy
- 3-6 years
- In captivity, this fascinating characin reaches a lifespan ranging between 3 and 6 years. Their longevity is strongly supported by a mature aquarium with low organic accumulation and regular water changes. Providing quality food and maintaining a relaxed environment reduces stress and prevents diseases, ensuring the fish live out their full lifespan.
- Adult size
- 3.5 cm
- The catalog's scalar value is 3.5 cm. The FishBase SPECIES table (v25.04 snapshot) source reports a maximum adult measurement of 2.8 cm for this species, recorded as standard length (SL). The taxonomic record consulted is Inpaichthys kerri (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
- Group
- Males are noticeably more slender and display a much more intense, solid blue coloration. Females are plumper with a more subdued brownish-blue tint. Keeping them in a mixed group allows males to display their best colors while showing off.
- Feeding frequency
- Twice daily
- Feed them small pinches once or twice a day, ensuring all food is consumed within a few minutes. They are active feeders that primarily eat from the mid-water column. Avoid overfeeding to prevent compromising water parameters.
- Bioload
- Low
- As a small species, the Blue Tetra contributes a very low bioload to the aquarium system. Standard biological and mechanical filtration easily processes their waste. Regular partial water changes keep the water quality optimal for their vibrant colors.
- Flow
- Moderate
- They thrive in a moderate water flow that simulates the gentle currents of their natural forest streams. A well-placed filter outlet or small powerhead will encourage active swimming behaviors. Calm areas should also be available for resting.
- Jump risk
- No
- Reproduction
- Oviparous
- They are egg scatterers that require a separate breeding tank with soft, acidic water and fine-leaved plants. The parents offer no care and will consume the eggs if given the chance. Remove the adults immediately after spawning is complete.
- Compatibility & tankmates
- Peaceful
- They are generally peaceful but can sometimes become slightly nippy if not kept in a large enough school. They do well with other similarly sized tetras, rasboras, and corydoras. Avoid keeping them with slow-moving fish possessing long, flowing fins.
- Tank level
- Middle
- They are highly active swimmers that predominantly occupy the middle regions of the aquarium. The tank should provide extensive open swimming areas along the front and middle. Dense planting around the back and sides provides necessary cover.
Scientific Parameters & Models
FishBase
Window to the official scientific data of the species
Data synced on: 06/15/2026
- Trophic level
- 3.1 ± 0.4 se
- Resilience
- High (less than 15 months)
- Phylogenetic uniqueness (PD50)
- 0.5625
- Bayesian Length-Weight
- a = 0.01738, b = 3.05
- VulnerabilityLow vulnerability (10 / 100)
Image gallery
Licensed images linked to the species or, when marked, to the closest representative taxon.
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