Ceylon Killifish

Common Name

Ceylon Killifish, Day’s Killifish

Origin and Habitat

Endemic to Sri Lanka, Aplocheilus dayi inhabits shallow, shaded forest streams with silty bottoms and quiet margins, and it also turns up in brackish mangrove swamps where slow water and cover are abundant. In the wild it shares lowland wet zone waters with aquatic plants and overhanging vegetation, and it often occurs alongside Aplocheilus parvus in mangrove edges. Surface cover and calm pockets are important, and the fish spends much of its time near the top, hunting among floating leaves.

Temperature and Water Conditions

Preferred temperature is in the mid seventies Fahrenheit, roughly the low to mid twenties Celsius. This species tolerates fresh water well and can handle slightly brackish conditions in nature. Aim near neutral to slightly alkaline pH with moderate hardness. Stable, clean water and high oxygen are far more important than chasing exact numbers, except when you follow a specific breeding plan. Keep a tight lid, keep surface cover with floating plants, and keep gentle surface movement so the fish feels secure.

Appearance and Size

Adults reach about 9 cm, roughly 3.5 inches. The body is slender and built for surface hunting with an upturned mouth. Both sexes show a dark spot near the rear base of the dorsal fin. Males are slimmer and more colorful with a pointed anal fin, females are fuller when ripe with eggs and carry a rounder anal fin.

Diet in the Wild

A larvivorous predator that picks off mosquito larvae, small insects, and tiny crustaceans near the surface and among plants. This role as a mosquito controller has been noted in Sri Lankan field work.

Feeding in Captivity

Offer a quality floating micro pellet as the staple, then layer in live or frozen mosquito larvae, daphnia, baby brine shrimp, and small flies. They learn flake and small pellets quickly, but a varied menu keeps color and condition high. Feed modest portions, once or twice a day, so food never collects among floating plants.

Behavior and Tank Setup

Surface oriented, alert, and confident when cover is available. Keep either a single male with several females, or a species group in a planted tank with floating cover and gentle flow. Provide a secure lid since they are jumpers. Peaceful midwater neighbors like rasboras and small barbs that ignore the surface work well, and shrimp are not advised with hungry surface hunters. Use fine leafed or floating plants to create shaded lanes and calm pockets.

Breeding

Conditioning and setup: This is a non annual, mop spawning killifish. Condition a group with live foods, especially mosquito larvae, and keep water very clean and well oxygenated. Use a glass breeding tank with floating mops made from acrylic yarn. Spawning usually begins after a brief acclimation period once the fish settle. A sex ratio of one male to two females often produces strong daily egg output, while community groups with multiple males can perform poorly.

Where eggs go: The male courts the female toward the spawning substrate, and adhesive eggs are deposited on the yarn strands. Mops generally outperform bare tanks and sand for egg recovery, and eggs can also be found on fine plants such as Cryptocoryne leaves if present. Collect eggs from the mops every couple of days.

Incubation: Place collected eggs in small jars of tank water with gentle aeration. Keep temperature near 25 C and remove any obviously infertile eggs. Hatching is typically observed around eleven to thirteen days after fertilization at this temperature.

Raising fry: Newly hatched larvae remain tiny and benefit from very clean water and gentle flow. Begin with infusoria or cultured Paramecium for the first days, then introduce newly hatched brine shrimp once the fry can handle it. Feed tiny portions several times daily, keep pre filter sponges clean, perform small daily water changes, and maintain stable temperature. As they grow, separate faster youngsters to prevent crowding and to keep surface space open for feeding.

Quick Facts

• Scientific name: Aplocheilus dayi
• Common name: Ceylon Killifish, Day’s Killifish
• Origin: Sri Lanka, lowland wet zone streams and mangrove edges
• Adult size: about 9 cm, roughly 3.5 inches
• Temperature: mid seventies Fahrenheit preferred
• Wild pH: near neutral to slightly alkaline, tolerates very light brackish conditions in nature
• Temperament: confident surface hunter, secure lid required
• Diet: insect and larval predator, readily accepts quality prepared foods with live and frozen supplements
• Breeding type: non annual egg layer, adhesive eggs on mops or fine plants, hatch around eleven to thirteen days at 25 C under clean, well aerated conditions

Tropical Fish Co. Notes

Aplocheilus dayi has classic surface hunter charisma, it hovers beneath floating leaves, patrols the open patches, and pounces with style. Give it cover and calm surface lanes, keep the water steady and clean, and it rewards you with color, confidence, and lively hunting behavior. For a breeding project, this species is both interesting and achievable. Floating mops, a one male to two female ratio, and patient egg collection will take you far, and the first time you see a cloud of tiny fry rising to meet infusoria, you will know why this Sri Lankan native is a keeper.