Least Killifish
Heterandria formosa
Poeciliidae
FishDifficulty: ●●○○○Data: well-established
About
The least killifish is one of the smallest fishes in the world and, despite its common name, is a true livebearer of the family Poeciliidae rather than an egg-laying killifish. Native to slow, densely planted freshwaters from North Carolina down to Louisiana and Florida, it is a subtropical species that tolerates cool, even unheated rooms. Females reach only about 3.5 cm and males barely 2 cm. Extremely peaceful, hardy and undemanding, it is a perfect occupant for a quiet, well-planted nano tank, but its minute size makes it easy prey for larger tankmates.
Care
Keep a small group of 6+ in a mature, densely planted nano tank with gentle flow and neutral to hard, alkaline water. Tolerates cool unheated temperatures. House only with tiny, non-predatory tankmates.
Diet
Micro-omnivore with a tiny gape; feed crushed flake, micro-pellet, baby brine shrimp, daphnia and cyclops, and let it graze biofilm and soft algae.
Tankmate compatibility
Gets along with (31)
Amano ShrimpAssassin SnailBladder SnailBloodfin TetraBlue Velvet ShrimpButterfly Hillstream LoachCherry ShrimpDenison BarbDoctor FishDwarf Orange CrayfishFreshwater ClamGhost ShrimpGlowlight DanioGold BarbJapanese Trapdoor SnailLeopard DanioMalaysian Trumpet SnailMedaka RicefishMystery SnailNerite SnailOdessa BarbPearl DanioPlatyRamshorn SnailRosy BarbRubber Lip PlecoScarlet BadisScissortail RasboraSiamese Algae EaterWhite Cloud Mountain MinnowZebra Danio
Possible with care (173)
Acei CichlidAfrican Butterfly CichlidAgassiz's CoryAgassiz's Dwarf CichlidAuratus CichlidBamboo ShrimpBanjo CatfishBetta (Siamese Fighting Fish)Black Moor GoldfishBlack Neon TetraBlack Phantom TetraBlack Ruby BarbBlack Skirt TetraBlackline RasboraBleeding Heart TetraBlood Parrot CichlidBlue AcaraBlue Dolphin CichlidBlue Tiger ShrimpBoesemani RainbowfishBolivian RamBorleyi (Redfin Hap)Borneo SuckerBrevis Shell DwellerBrichard's TeleogrammaBrichardi (Fairy Cichlid)Bristlenose PlecoBronze CorydorasBuenos Aires TetraBuffalohead CichlidBumblebee CichlidBumblebee GobyCelebes RainbowfishCelestial Pearl DanioCheckered BarbCherry BarbChinese Algae EaterChocolate CichlidClown PlecoCobalt Blue ZebraCochu's Blue TetraCockatoo Dwarf CichlidColombian Ramshorn SnailComet GoldfishCommon GoldfishCongo Dwarf CichlidCongo TetraConvict CichlidCylindricus CichlidCyprichromisDemasoni CichlidDogtooth CichlidDojo (Weather) LoachDwarf Chain LoachDwarf Flag CichlidDwarf GouramiDwarf Neon RainbowfishDwarf Pea PufferDwarf Petricola CatfishElectric Yellow LabElephantnose FishEllioti CichlidElongate MbunaEmerald CoryEmperor TetraEndler's LivebearerEven-spotted SynodontisFantail GoldfishFestivum (Flag Cichlid)Firemouth CichlidFive-bar CichlidFlame TetraForktail RainbowfishGlass CatfishGlowlight TetraGolden Dwarf CichlidGolden JulieGolden PencilfishGolden TetraGreen Neon TetraGuppyHarlequin RasboraHead-and-Tail-Light TetraHoney GouramiHongi CichlidJohanni CichlidJulii CoryKenyi CichlidKeyhole CichlidKissing GouramiKribensisLambchop RasboraLemon CichlidLemon TetraLittle CorydorasMacmasteri Dwarf CichlidMaingano CichlidMarlieri CichlidMasked JulieMollyMoonlight GouramiMultifasciatus Shell DwellerNeon TetraOcellatus Shell DwellerOranda GoldfishOtocinclusPanda CorydorasParadise FishParaguay EartheaterPeaceful BettaPeacock CichlidPearl GouramiPenguin TetraPeppered CorydorasPolka Dot LoachPort CichlidPowder Blue CichlidPurple Emperor TetraPygmy CorydorasRabbit SnailRainbow CichlidRainbow SharkRanchu GoldfishRed EmpressRed RainbowfishRed Whiptail CatfishRed Zebra CichlidRed-Tail SharkRedhead CichlidRegan's JulieReticulated Hillstream LoachRosy Red MinnowRosy TetraRummynose TetraRusty CichlidRyukin GoldfishSailfin MollySalt and Pepper CorySand-sifting XenotilapiaSaulosi CichlidSerpae TetraSeverumShubunkin GoldfishSilver Tip TetraSimilis Shell DwellerSkunk CorySparkling GouramiStar SapphireSterbai CorydorasStriped KribensisSulawesi Cardinal ShrimpSunshine PeacockSveni EartheaterSwordtailT-Bar CichlidTaiwan Reef CichlidTelescope GoldfishThai Micro CrabThick-lipped GouramiThreadfin AcaraThreadfin PetrochromisThreadfin RainbowfishThree-spot GouramiTiger BarbTurquoise RainbowfishUmbrella Dwarf CichlidUpside-down CatfishVampire ShrimpVenezuelan CoryWrestling HalfbeakX-ray TetraYoyo LoachZebra Mbuna
Avoid (67)
Adonis PlecoAfrican ButterflyfishAltum AngelfishAngelfishBala SharkCalvus CichlidCardinal TetraCheckerboard CichlidChili RasboraChocolate GouramiClown KillifishClown LoachCommon PlecoCompressiceps CichlidCrystal Red ShrimpCuckoo CatfishDemon EartheaterDiamond TetraDiscusDuboisi TropheusDwarf RasboraElectric Blue AhliEmber TetraEndlicheri BichirFeatherfin SqueakerFigure-8 PufferFrontosaGerman Blue RamGoby CichlidGolden Wonder KillifishGreen DiscusGreen Spotted PufferfishGreen TerrorHeckel DiscusIridescent SharkJack DempseyJaguar CichlidJewel CichlidKoiKuhli LoachLivingstoniMalawi EyebiterMalawi HawkMarble HatchetfishOrnate BichirOscarPeacock BassPictus CatfishPike CichlidRed DevilRed TerrorReticulated Flying FoxRostratusRoyal PlecoSailfin PlecoSalvini CichlidSilver ArowanaSilver MoonySpotted HoploStrawberry RasboraStriped Raphael CatfishTexas CichlidTropheusUaru (Triangle Cichlid)Venustus (Giraffe Cichlid)Wolf CichlidZebra Pleco
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.
- The gonopodiumDefinitive · from about 4 weeks old
Look at the anal fin, underneath and just behind the belly. In females it stays a broad triangular fan. In males it narrows into a rod and angles forward — that rod is the gonopodium, a rebuilt fin rather than an organ that appears from nowhere.
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Frequently asked questions
- What tank size does a Least Killifish need?
- A Least Killifish needs an aquarium of at least 20 litres (5.3 US gallons) with a footprint at least 30 cm (11.8 in) long — for this species, swimming length matters as much as raw volume.
- What water parameters does a Least Killifish need?
- Least Killifish does best at 15–26 °C (59–78.8 °F), pH 7–8 and a general hardness of 5–20 °dGH.
- What group size does a Least Killifish need?
- Least Killifish is a social species — keep a group of at least 6 so natural behaviour emerges and stress stays low.
- Is the Least Killifish suitable for beginners?
- Yes — Least Killifish is one of the easier species to keep and forgives typical first-tank mistakes, as long as the aquarium is cycled and maintained.
Sources
Water & tank
Temperature15–26°C
Optimal temp.20–24°C
pH7–8
Hardness (GH)5–20°d
Carbonate (KH)3–12°d
Adult size3.5 cm
Min. volume≥ 20 L
Min. tank length≥ 30 cm
Group size≥ 6
Swimming zoneMid
TemperamentPeaceful
Water typeFreshwater
FlowLow
Lifespan~2 yrs
Try in the calculator