Taxonomy & naming
Aplocheilus dayi was described by Franz Steindachner in 1892, and the parenthetical authority reflects the subsequent transfer of the species from the original genus of placement to its current home in Aplocheilus. Eschmeyer's Catalog of Fishes recognises the valid combination as Aplocheilus dayi (Steindachner, 1892). The genus Aplocheilus — the Old World panchax group — encompasses the surface-dwelling killifishes of South and Southeast Asia and is placed in the family Aplocheilidae within the order Cyprinodontiformes.
The specific epithet dayi honours Francis Day (1829–1889), the British surgeon and ichthyologist whose comprehensive work on the fishes of India and Burma documented much of the subcontinental freshwater fauna in the latter half of the nineteenth century. Although Day himself recorded fishes from Ceylon (Sri Lanka), this species was described by Steindachner after Day's death, and the dedication is an honorary one. The American Killifish Association IUCN file cites the 2019 Red List assessment as authored by de Alwis Goonatilake, S., Fernando, M., and Kotagama, O.
The common names in use include Day's panchax, Ceylon killifish, and green panchax — the last name reflecting the suffused green iridescence of the male flanks and is shared loosely with other Aplocheilus species, so the more precise Day's panchax is preferred in reference contexts.
Morphology
Aplocheilus dayi is a slender, dorsoventrally flattened killifish with the upturned mouth and flat dorsal profile characteristic of a dedicated surface feeder. FishBase records a maximum standard length of approximately 2 in in aquarium specimens; wild fish may grow somewhat larger, and Aquarium Glaser notes that the species remains considerably smaller than its congener Aplocheilus lineatus, which reaches roughly 3 in. The British Killifish Association gives males as up to 2 inches (approximately 2 in), with females of similar size.
Male colouration is striking: the body is olive to brownish-green above, shading to a yellowish or pale belly, with a variable number of iridescent blue-green to gold scale edges creating a jewelled lateral pattern. The unpaired fins are typically adorned with bright yellow to orange marginal and sub-marginal banding, and the caudal fin may carry a dark central blotch. Females are plainer — generally brownish without the vivid fin colouration — and are rounder-bodied when gravid. Sexual dimorphism is thus clear: the brilliance of the male's flanks and finnage versus the subdued female, a distinction useful for pairing fish in captivity.
Habitat
Day's panchax is endemic to Sri Lanka, occurring in the wet-zone lowlands of the island's south-west. The American Killifish Association records confirmed localities that include Padagoda, Alakanda, Avissawella, Bentota, the Colombo area, Hanwella, Hiniduma, Kalmunai, and the Kelani River system — a distribution concentrated in coastal drainages and their lower tributaries within the wet zone, with some populations extending to higher altitudes inland. The British Killifish Association notes that the species inhabits Ceylon coastal streams, ditches, and brooks but has also been found at higher elevations in similar water bodies.
In character, the typical habitat is a shaded, slow to gently flowing stream or ditch in or near remnant coastal lowland forest — with silt or muddy substrate, overhanging vegetation, and dense marginal or surface plant growth. FishBase additionally records the species from brackish mangrove swamps, and this euryhaline tolerance gives it some capacity to persist in tidal-influenced water near the coast. Water chemistry from the British Killifish Association: approximately 7 °GH, pH 6.6–7.0, and temperature 72–77 °F (72–77 °F), figures consistent with the soft, slightly acidic to near-neutral conditions of Sri Lanka's wet-zone streams.
Aquarium Glaser characterises the species as preferring well-planted conditions with gentle water movement, and the Aquarium Glaser documentation notes a pH optimum of 6.5–7.2. The combination of surface-dwelling habit, preference for dense vegetation, and tolerance of brackish water places this fish in a habitat matrix that is simultaneously ecologically specialised and highly vulnerable to development.
Feeding
Aplocheilus dayi is a surface predator, taking prey at or just below the water's film in the manner of all Aplocheilus panchax. In the wild the diet consists principally of insects — terrestrial and aquatic — along with microcrustaceans, small fish fry, and other invertebrates that are accessible to an upturned-mouthed fish hunting the surface layer. The British Killifish Association confirms this diet axis, listing mosquito larvae, micro-organisms, baby brine shrimp, sifted daphnia, and prepared foods as appropriate items for captive specimens.
In the aquarium the species is straightforward to feed. It accepts live and frozen invertebrates readily — mosquito larvae (both black and white), daphnia, cyclops, and brine shrimp nauplii are all taken with enthusiasm — and most individuals learn to accept quality dried foods in flake or micro-pellet form with some acclimatisation. Feeding at or just below the surface keeps the fish in its preferred strike zone; sinking pellets may be ignored. The pike-like tendency noted by Aquarium Glaser means that very small tankmates or fry present in the same vessel are at risk of predation.
Mating
Day's panchax is a non-annual killifish and a plant spawner — its reproductive mode is fundamentally different from the diapausing annual killifishes of ephemeral African or South American pools. Pairs spawn continuously over extended periods rather than in a single seasonal pulse, depositing small numbers of adhesive eggs each day into fine-leaved vegetation, surface roots, or artificial spawning mops near the water's surface. The British Killifish Association documents a laying rate of approximately 22 eggs per day on average, spread over a spawning period of around 16 days per pairing.
Males court actively, displaying their vivid lateral colours and spreading the dorsal and caudal fins in a broadside threat-and-invitation posture directed at receptive females. Aquarium Glaser notes a pike-like chasing tendency, and it is advisable to provide the female with sufficient cover and visual barriers during courtship — a heavily planted tank with surface vegetation reduces the risk of harassment. There is no elaborate nest building: eggs are simply pressed into plant material by the spawning pair and left. The adhesive coat on each egg anchors it to the plant surface for the duration of incubation.
Breeding
The British Killifish Association gives an incubation period of 11–13 days at 74 °F (approximately 73 °F) for eggs laid in plant mops or willow root tangles. This is a water-incubation process — eggs remain submerged throughout, with no dry or semi-dry diapause phase, in keeping with the non-annual strategy. A recent peer-reviewed captive breeding study (ResearchGate, 2024–2025) investigated embryonic and larval development in detail, including the effect of spawning substrate on hatching success, and found the species amenable to controlled captive reproduction when appropriate plant or mop material was provided.
The standard hobbyist approach is to run a pair or a trio (one male, two females) in a well-planted tank, allow spawning to proceed, then remove eggs on mop strands or plant cuttings to a separate hatching container at roughly weekly intervals. Fry are free-swimming from hatching and large enough to take newly hatched brine shrimp nauplii (Artemia nauplii) and microworm immediately; no infusoria stage is required. Aplocheilus dayi is described by the British Killifish Association as a ready spawner, and pairs in good condition will spawn continuously in appropriate conditions, yielding a steady supply of fry rather than a single large cohort. Water temperature in the rearing vessel should be maintained in the 72–77 °F range; slight warming to the upper end of the tolerance may accelerate development.
In the aquarium
Day's panchax is a rewarding but somewhat specialised aquarium fish. A well-planted species or species-specific tank of 15–20 US gal is appropriate for a pair or small group; the species is not suited to a general community setup because it will target surface-level fish of comparable or smaller size, and its own surface-dwelling habit makes it vulnerable to harassment from boisterous mid-water schooling fish below it. The British Killifish Association recommends soft aeration and a well-planted tank, with surface plants (such as floating vegetation) or dense emergent growth providing the shaded, cover-rich surface zone the species prefers in the wild.
Water parameters should replicate the soft, near-neutral conditions of the Sri Lankan wet-zone streams: pH 6.5–7.0, hardness around 7 °GH, and temperature 72–77 °F (72–77 °F). The tank must be covered — killifishes are accomplished jumpers, and Aplocheilus dayi is no exception; a tight-fitting lid with no gaps is essential. Aquarium Glaser notes that the species benefits from hiding places during spawning, suggesting that dense vegetation throughout the tank (not just at the surface) reduces male-to-female chase stress.
Because the species is IUCN Endangered and endemic to Sri Lanka, captive-bred stock should be sought in preference to wild-caught individuals. The fish is maintained in specialist killifish society collections and is traded within the British Killifish Association and American Killifish Association networks; club fish lists and species auctions are the most reliable source of acclimatised, captive-bred specimens. Keeping and breeding Day's panchax in captivity is thus a genuinely meaningful conservation act for a fish whose wild populations continue to decline.
Conservation
The IUCN Red List assessed Aplocheilus dayi as Endangered (EN) in 2019, in an evaluation authored by de Alwis Goonatilake, S., Fernando, M., and Kotagama, O. The assessment reflects a documented decline in both the extent of occurrence and population size driven by the progressive destruction and degradation of the low-elevation freshwater habitats of Sri Lanka's wet zone — the coastal streams, forest-edge ditches, and marshy areas in which this fish is endemic. Habitat loss through urbanisation and drainage of the coastal lowlands around Colombo and its hinterland, water pollution from agriculture and industry, and the conversion of mangrove and freshwater swamp areas to aquaculture or development represent the primary pressures on the species.
The known localities — Kelani River tributaries, Bentota, Hiniduma, the Colombo coastal strip — are all within one of South Asia's most densely settled and rapidly developing landscapes. This geographic reality means the species is subject to ongoing pressure that will not lessen without specific protective measures targeting the remnant wetland habitat it depends on. The introduction of non-native fish species, including mosquitofish (Gambusia affinis), a globally recognised agent of killifish decline, may present an additional competitive and predatory threat in disturbed lowland streams.
A recent peer-reviewed captive breeding study (2024–2025) focused on Aplocheilus dayi specifically, noting that its conservation biology had not been well studied and making captive reproduction and early development data available to support breeding programmes. Maintaining viable captive populations in specialist collections provides an insurance population against further wild decline and contributes to the baseline knowledge needed for any future recovery programme. The IUCN assessment page (taxon ID 180566, assessment ID 174825349) provides the formal reference point for conservation status.