Blue emperor tetra
Inpaichthys kerri
The blue emperor tetra (Inpaichthys kerri) is a small, peaceful shoaling fish from Brazil’s Aripuanã and upper Madeira basins. It is also sold as the royal or purple emperor tetra and is sometimes confused with Nematobrycon palmeri. The adipose fin helps distinguish them: Inpaichthys has one. Keep a group in a warm, stable aquarium with open swimming room and cover. A 60 × 38 cm footprint and about 68 litres is a species-specific guideline; actual stocking also depends on layout, filtration and tankmates.
Plan your aquarium →At a glance
- Temperature
- 24–27 °C
- pH
- 5.5–7.0
- GH
- 1–12 °dGH
- Aquarium length
- At least 60 cm
- Minimum aquarium volume
- At least 68 L
- Adult size
- Up to 3.5 cm
- Group size
- At least 6 individuals
- Temperament
- Peaceful
About this species
Inpaichthys kerri is a small characin in Acestrorhamphidae. Seriously Fish describes a standard length of about 35 mm, while FishBase reports 2.8 cm standard length for a male or unsexed specimen. The sources therefore differ in their maximum or measurement basis. Standard length excludes the caudal fin and is not the same as total length.
Trade names include blue, purple, royal and regal tetra. These names are also sometimes applied to the emperor tetra Nematobrycon palmeri. Check the scientific name when buying. One visible distinction is the adipose fin: present in Inpaichthys, absent in Nematobrycon. Males are generally more colourful; females are deeper-bodied and have a mainly reddish adipose fin, while males have a blue one. Colour also varies with light and condition.
Natural habitat
The species is recorded from the Rio Aripuanã and upper Madeira basin in Mato Grosso, Brazil. Seriously Fish describes forested tributaries with sluggish water. Leaves and other organic matter often stain these waters and contribute to acidic conditions. This describes the reported natural habitats; it does not mean every aquarium must have dark, tannin-stained water.
A natural-style setup can use sand, wood, roots and some safe leaf litter under subdued light. Aquatic plants are not characteristic of the natural habitats described in the species profile. The fish also does well in a well-maintained, densely planted community aquarium. In either style, retain open swimming space and sheltered edges. Use aquarium-safe materials and remove decaying leaves before they foul the water.
Aquarium and layout
Seriously Fish gives a tank of around 60 × 38 × 30 cm, about 68 litres, for a small group. FishBase reports a minimum aquarium length of 60 cm and keeping in groups of five or more. These are starting points from species sources, not a guarantee that every stocking combination fits that volume. Allow enough swimming room and account for filtration and maintenance.
A sand substrate with wood and roots suits a biotope-style layout; a planted aquarium is also appropriate. Provide gentle flow, cover and a steady temperature. This species is not a strong competitor against much larger or very boisterous tankmates. Use a mature filter, feed where the group can eat calmly, and acclimate newcomers gradually to temperature and water chemistry.
Water conditions
Seriously Fish recommends 24–27 °C, pH 5.5–7.0 and 1–12 °dGH. FishBase reports a wider pH range of 6.0–8.0, hardness of 5–12 dH and temperature of 24–27 °C. Because the pH ranges differ, this guide follows the more cautious, acidic to near-neutral range in the species profile. Measure consistently and keep conditions stable; avoid rapid pH corrections.
For breeding, Seriously Fish gives softer water at pH 5.5–6.5 and 1–5 °dGH. Those are breeding conditions, not required daily targets for every community aquarium. Keep temperature steady and prevent waste build-up with suitable filtration and regular maintenance.
Behaviour and group size
The blue emperor tetra is peaceful but does better in a group. FishBase lists five or more; Seriously Fish describes around half a dozen and notes that undersized groups are more likely to nip fins. At least six is therefore a practical, source-based starting point, with enough space and a balanced group. Male displays toward one another are not by themselves a reason to keep the species alone.
Choose calm tankmates of similar size. Small South American tetras, pencilfish, hatchetfish, dwarf cichlids and peaceful bottom dwellers such as Corydoras and Otocinclus are suggested. Avoid much larger or pushy fish that outcompete them at feeding time. Observe feeding and fins after introduction: compatibility also depends on numbers, layout and individual fish.
Feeding
Seriously Fish reports that the species accepts most prepared foods. Also offer small live or frozen foods regularly, such as brine shrimp and Daphnia. A varied diet supports condition and colour. Give small portions that are eaten promptly and check that less assertive fish receive food as well.
No reliable species-specific pellet size was found. Choose food the fish can actually take, observe the group and vary prepared foods with suitable small animal foods. Remove leftovers and avoid overfeeding, which can affect water quality.
Breeding
The species has been bred regularly in the hobby. For raising fry, Seriously Fish recommends a separate, dimly lit spawning tank with fine-leaved plants or spawning mops; mesh can also keep adults away from eggs. Soft, acidic water is used for breeding. Condition the fish with varied small live foods and remove adults after spawning because they may eat the eggs.
The source gives hatching in about 24–48 hours and free swimming after three to four days. These are profile-based guide times; development depends on conditions. Start with very fine food such as infusoria and move to larger foods only when fry can take them. Breeding is not guaranteed and requires stable water and appropriate first foods.
Suitable plants
Java fern
Attach Java fern to wood along the margin for cover while keeping swimming space open. This is a practical aquarium pairing, not a claim of natural co-occurrence.
- Light
- Low, Moderate
- CO₂
- CO₂ injection not needed
Sources & review
This care guide draws on several specialist sources and was last checked on .