Taxonomy & naming
Knodus borki was described by Axel Zarske in 2008, with the type locality given as the surroundings of Iquitos, Loreto Department, Peru. The original description was published in Vertebrate Zoology (volume 58, number 2, pages 159–171), which placed the species within the genus Knodus, a mid-sized genus of small Neotropical characids distributed broadly across the Amazon and Orinoco basins.
The species is most closely related to Knodus megalops (Myers, 1929). It differs from that species in having an incomplete rather than complete lateral line, a proportionally larger eye, a deeper body anterior to the dorsal fin, and a longer head relative to standard length — measurements that together place it unambiguously outside the range of K. megalops despite superficial similarity. It is further distinguished from Boehlkea fredcochui, the established 'blue tetra' of the trade, by dentition: K. borki carries only 3–4 maxillary teeth compared with the 11–21 of B. fredcochui, and the two species differ in coloration. The long period during which K. borki circulated in commerce as B. fredcochui illustrates how easily similar-looking small blue characids can be misidentified without close examination.
The genus Knodus belongs to the large, historically complex family Characidae. Catalog of Fishes (Fricke, Eschmeyer & Fong) records the valid name as Knodus borki Zarske, 2008.
Morphology
Knodus borki is a slender, moderately compressed small tetra reaching approximately 2 in standard length. Body depth anterior to the dorsal fin is proportionally greater than in the closely related K. megalops, a character used in the original description to separate the two species. The lateral line is incomplete — another key diagnostic feature distinguishing it from K. megalops.
The coloration gives rise to the common name Blue Tetra: the body carries a blue iridescent sheen, most visible under side-lighting. The fins are generally hyaline to pale. While detailed colour descriptions in the literature are limited, the blue gloss is consistent enough that the fish was long confused with Boehlkea fredcochui, which shares the same general impression. Differences in dentition — K. borki has far fewer maxillary teeth — confirm the two as distinct species when examined closely.
Habitat
Knodus borki is known from the environs of Iquitos in the Loreto region of northeastern Peru, within the western Amazon basin. The surrounding landscape is lowland Amazonian rainforest dissected by rivers, streams, and seasonally flooded floodplain habitats. FishBase classifies the species as benthopelagic, inhabiting freshwater rivers and streams.
Reported water conditions align with the broadly tropical parameters typical of lowland Amazonian tributaries: temperature 72–79 °F (72–79 °F). Specific field data on pH and conductivity at collection sites are not documented in available sources, but Iquitos-area waterways generally range from soft and slightly acidic blackwater to more neutral white-water systems. The species likely inhabits vegetated margins and slower reaches of streams where small characids concentrate.
Feeding
Knodus borki is an omnivore in the manner typical of small Neotropical characids. Natural diet details for this species specifically have not been published, but members of the genus Knodus and closely related small characids generally feed on small invertebrates — aquatic insect larvae, microcrustaceans, and terrestrial insects falling to the water surface — supplemented by algae and plant matter.
In the aquarium the species accepts a range of small foods readily. Prepared foods including fine-grade flake and micro-pellets form a practical base diet; variety and conditioning are improved by offering small live and frozen foods such as Daphnia, cyclops, and baby brine shrimp. Feeding small amounts multiple times daily suits the active foraging style common to small characids.
Mating
Knodus borki is an egg-scattering open spawner with no parental care, following the typical pattern of small characids in the family Characidae. Specific courtship observations for this species are sparse in the literature, but the general breeding mode is shared with close relatives: males pursue females, with spawning occurring among fine-leaved vegetation or in open water near plant cover.
Conditioned adults in good condition will attempt to spawn readily. Hobbyist accounts suggest the species spawns under or among broad plant leaves in the aquarium, indicating some preference for plant cover during the spawning act. As with most small egg-scattering tetras, there is no pair bond and no guarding of eggs or fry by either parent.
Breeding
Breeding Knodus borki follows the standard small-tetra protocol. A conditioning period on varied live and frozen foods brings adults into spawning condition. A separate breeding vessel with fine-leaved plants or spawning mops, shallow water, and subdued lighting provides suitable conditions. The eggs are adhesive and are deposited among plant matter; the adults will consume eggs if not removed promptly after spawning.
Fry are very small at hatching and require appropriately fine first foods. Infusoria or Paramecium cultures are the traditional first food for the newly free-swimming fry; once the fry have grown enough, they accept freshly hatched brine shrimp nauplii. Hobbyist accounts specifically note that the fry of this species require infusoria or Paramecium at the earliest stage. Water quality must be maintained carefully during rearing, as the fry are sensitive to accumulated waste.
In the aquarium
Knodus borki is a peaceful, active schooling fish well suited to a community aquarium. It is best kept in groups of eight or more individuals — a proper school reduces timidity, brings out natural behaviour, and minimises the mild fin-nipping tendency that can emerge in small, cramped groups. The Seriously Fish profile recommends a minimum tank of around 25 US gal (approximately 30 × 15 × 12 inches) for a school.
Water conditions of 72–79 °F (72–79 °F) and a pH of 5.5–7.0 suit the species well, consistent with soft, slightly acidic to neutral Amazonian water. A planted aquarium with open swimming lanes and some shaded areas replicates the structured habitat small characids favour and shows the blue iridescence to good effect under side-lighting. Tankmates should be peaceful and similarly sized; the species is compatible with other small tetras, small rasboras, corydoras, and calm dwarf cichlids. Avoid housing it with significantly larger or more aggressive fish.
Conservation
Knodus borki has not been formally assessed by the IUCN Red List; no conservation assessment is recorded for this species. FishBase carries a notation of Data Deficient (DD), reflecting the limited information available on this species' population size, range boundaries, and ecological requirements.
The Iquitos region of the Peruvian Amazon, while encompassing substantial wild habitat, faces ongoing pressures from deforestation, river modification, and artisanal fisheries. For a species with a seemingly restricted documented range and little published ecological data, the absence of an IUCN evaluation means the conservation picture remains unclear. Most specimens in the hobby are likely wild-caught imports given the species' limited profile, though its small size and standard breeding mode mean captive breeding is straightforward for experienced hobbyists. Further field surveys would be needed to determine whether the species is genuinely range-restricted around Iquitos or more widely distributed in the western Amazon basin.