Ceylon Killifish
Aplocheilus dayi
Aplocheilidae
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About
We already carry this fish's better-known cousin as the golden wonder killifish (Aplocheilus lineatus), and the two get muddled in shops for a good reason: the British Killifish Association's own pamphlet says of the male dayi that "the colouration of the male is most striking and can rival the brilliance of Aplocheilus Lineatus". Even Francis Day, the man the fish is named after, got it wrong — Wikipedia notes he "first reported this fish, although he identified it as Aplocheilus panchax". Three things separate them. Range: A. lineatus runs through southern India and Sri Lanka, while FishBase calls A. dayi "endemic to Sri Lanka" — and lists it as IUCN Endangered, assessed in 2019, which very few aquarium killifish are. Marking: Wikipedia gives the field character plainly — "Males and females have a black dot at the rear end of the base of the dorsal fin." Size: FishBase's size field gives 9.0 cm TL maximum and 7.0 cm common length against 10 cm for lineatus — but its Biology field on the same page adds "10 cm max TL" for dayi as well, so the honest reading is that the two are the same size, not that dayi is the smaller. The 1968 BKA pamphlet, working from aquarium fish rather than museum ones, puts both sexes at around two inches. What it is not, and this is the point of the killifish shelf, is an annual fish. FishBase states it in as many words — "Not a seasonal killifish. Is easy to maintain in the aquarium" — and the BKA's figure makes the same case from the other end: "Under normal feeding maturity will not be reached until ten to twelve months." A fish that takes most of a year to grow up is not a fish with a one-season clock. Compare the bluefin notho, which is on its second generation by then. In character it is a pike in miniature. The BKA describes the male as "very much the 'little pike' in shape and character": a flat-headed, upturned-mouthed surface hunter that hangs motionless under floating plants and takes anything small that moves. FishBase's diet line — "feeds on small insects, insect larvae and fish fry", and the species is "cultivated as a larvicidal fish" — is the whole compatibility argument. Nothing under about 4 cm belongs in the same tank. One practical caution the BKA is unusually frank about: "Rather easily kept if from aquarium stock, but not so easy if from newly imported. Importers of this fish suffering quite large losses." Its summary calls the species "a tricky customer to keep in good health" while conceding that "aquarium bred fish are easier to manage than those from the wild". Buy tank-bred, which is also the right answer for an endangered island endemic.
Care
One male with two or three females in a long, quietly furnished tank with floating cover — the BKA reports that "the male if in good shape will prove a most ardent chaser of the female and slight damage may occur if sufficient hiding places cannot be found", so a single female takes the whole of his attention. Water soft to medium and near neutral: the BKA keeps them "in fairly soft water of about 7 G.H. and with a pH of from 6.6 to 7.0", at 72-77 °F (22-25 °C), while FishBase gives 20-25 °C. Almost no flow — "the water should not be subject to any great disturbance by aeration or by filtration" — subdued lighting, a well planted tank "with a good covering of floating plants", and a tight lid, because this is a surface fish that hunts things landing on the water. No source consulted prints a minimum tank size for A. dayi; this entry gives it 90 litres over an 80 cm base, one step below the 110 litres our A. lineatus entry carries for a 10 cm fish. Tankmates must be too big to swallow. It tolerates mild brackish water — FishBase records it from mangrove swamps — but does not need salt.
Diet
A surface ambush predator on insects, larvae and fry. Live and frozen mosquito larvae, daphnia, bloodworm and Artemia, plus floating pellets. Introduce dried food early: the BKA warns that fish which never learn it may refuse it for life.
Tankmate compatibility
Gets along with (134)
Possible with care (224)
Avoid (60)
Computed by the AquaSage engine for a spacious reference tank — always check the full verdict for your exact setup in the calculator.
Telling males from females
What works for this species, most reliable first.
- Colour and patternStrong · from about 6 months, and only on a settled fish
In rainbowfish, killifish and several barbs the male carries the colour — deeper and more saturated, intensifying further when he displays.
- Extended finsStrong · from about 8 months, and later in a suppressed male
On many cichlids and killifish the male's dorsal and anal fins draw out into trailing points as he matures, while the female's stay rounded.
Also in these lists
Similar species
Same category and the closest water parameters — if this species isn't quite it, start here.
Sold in Lithuania
These Lithuanian shops listed this species in their public catalogue (checked August 8, 2026). Stock moves — ask before you travel.
These links are unpaid: we get nothing for them and ask for nothing.
Frequently asked questions
- What tank size does a Ceylon Killifish need?
- A Ceylon Killifish needs an aquarium of at least 90 litres (23.8 US gallons) with a footprint at least 80 cm (31.5 in) long — for this species, swimming length matters as much as raw volume.
- What water parameters does a Ceylon Killifish need?
- Ceylon Killifish does best at 20–25 °C (68–77 °F), pH 6–7.5 and a general hardness of 3–12 °dGH.
- What group size does a Ceylon Killifish need?
- Ceylon Killifish is a social species — keep a group of at least 3 so natural behaviour emerges and stress stays low.
- Is the Ceylon Killifish suitable for beginners?
- Ceylon Killifish sits in the middle of the difficulty scale — manageable for a careful beginner who has done the reading, but it has specific needs worth researching first.