Home / Fishes / Killifish and livebearers / Least killifish
No federal permit to keepLeast killifish
Heterandria formosa
Fishes › Killifish and livebearers (Cyprinodontiformes) › Poeciliidae (Livebearers)
Heterandria formosa (known as the least killifish, dwarf topminnow, mosqu or midget livebearer) is a species of livebearing fish within the family Poeciliidae.
Care at a glance
Species-specific aquarium care
- Tank: At least 24 in long and 12 in front to back for one adult (adults commonly reach about 0.8 in / 2.1 cm; the largest recorded was 1.4 in).
- Water: Freshwater (tolerates brackish).
- Temperature: 68–79°F (range recorded for this species).
- pH: 7–8.
- Hardness: 9–19 dGH.
- Layout: Uses both the bottom and open water: give open swimming space plus cover near the substrate.
- Diet: Eats a varied diet: mix quality prepared foods with frozen and live foods. FishBase classes it as a omnivore.
Calculated from this species' own FishBase record: tank size from its typical adult length and swimming zone, water values from its recorded ranges. The group guide below fills in general care.
Care guide
Species care notes
- Water: a fairly wide range, from slightly acidic to slightly alkaline, 68–79 °F, pH 7–8, 5–20 dGH.
- Behaviour: temperament peaceful; a shoaling fish best kept in a group of six or more; not a community fish.
- Difficulty: Easy (Seriously Fish rating).
Short excerpts from the species profile:
- Setup: “A gently-filtered, heavily planted setup suits it best.”
- Diet: “Unfussy and omnivorous, it will accept most foods offered.”
- Tankmates: “Not a community fish in the general sense, although it's very peaceful.”
From the Seriously Fish profile for this species, which has the full habitat, maintenance, diet, behaviour and breeding write-up 167
Livebearers (guppies, mollies, platies and swordtails)
Applies to most species in this group. Habitat: Warm streams, ponds and brackish waters. Individual species can differ, so compare with the species numbers above and the linked sources.
Housing & setup
- Planted tanks with gentle flow; mollies appreciate harder, slightly brackish water.
Diet
- Flake foods with plant matter, plus live or frozen foods.
Social life
- Peaceful community fish; keep more females than males.
Health watch
- They breed constantly; plan for fry or keep single-sex groups.
Understanding the animal
About this animal
About this animal
This is the same family that includes familiar aquarium fishes such as guppies and mollies. Heterandria formosa is not as commonly kept in aquaria as these species.
Quoted from the Wikipedia article “Heterandria formosa” (CC BY-SA 4.0) 174
In the wild
In the wild
From this species’ own FishBase record (habitat, bottom type, water and breeding data), not its family. Good care copies these conditions.1
| Where it lives | Lives in freshwater and brackish water, in rivers and streams, lakes, estuaries, mangroves. Uses both the bottom and open water. Adults inhabit heavily vegetated standing to slow-flowing fresh and brackish water. They feed on worms, crustaceans and plant matter. |
| Water it lives in | Freshwater / brackish water, 68–79 °F (20–26 °C), pH 7–8, hardness 9–19 dH. |
| Climate | Tropical climate. |
| Behavior to plan for | Breeding: internal live bearers, maternal parental care. |
What this means for care
- Keep water within its wild range, 68–79 °F.
- It tolerates or needs brackish water; check salinity needs before setup.
- Parents guard eggs or young, so expect territorial behavior when breeding.
Native climate (measured)
Native climate (measured)
| Warm-season highs | about 91°F (up to 92°F in the hottest sampled areas) |
| Cool-season lows | about 42°F (down to 36°F in the coldest sampled areas) |
| Average temperature | 65–73°F across sampled records |
| Relative humidity | 71–73% on average (driest month about 68%) |
| Rainfall | about 1350 mm a year, spread fairly evenly |
Measured at 237 wild occurrence records sampled against WorldClim 2.1 climate data (1970–2000 averages). Air conditions in the range are ecological context, not enclosure settings: microhabitats such as burrows, leaf litter or basking rocks can be cooler, warmer or more humid.
From the sources
From the sources
Quoted from the matching sections of reputable sources for this exact species. Specialist databases are used first; Wikipedia (community-edited, CC BY-SA 4.0) only fills gaps. Other sources are short credited quotes.
| Overview | Heterandria formosa (known as the least killifish, dwarf topminnow, mosqu or midget livebearer) is a species of livebearing fish within the family Poeciliidae. This is the same family that includes familiar aquarium fishes such as guppies and mollies. Heterandria formosa is not as commonly kept in aquaria as these species. Despite the common name "least killifish", it belongs to the family Poeciliidae and not to one of the killifish families. — Wikipedia (community-edited) |
| Where it lives | Home ranges Population dynamics Growth parameters Max. ages / sizes Length-weight rel. Length-frequencies Mass conversion Recruitment Abundance Life cycle Reproduction Maturity Maturity/Gills rel. — FishBase |
| Wild diet | Heterandria formosa primarily eats aquatic invertebrates such as worms and crustaceans. They also eat plant matter. — Wikipedia (community-edited) |
| Reproduction | A truly viviparous fish, embryo derives nearly all of its nourishment from the mother. — FishBase |
| Appearance and size | Maximum length 3.6 cm tl male/unsexed. — FishBase |
| Biology | Adults inhabit heavily vegetated standing to slow-flowing fresh and brackish water. They feed on worms, crustaceans and plant matter. A truly viviparous fish. — FishBase |
Data and studies
The numbers
The numbers
Each bar is what one source reports. Where several sources weigh in, the dashed gold band marks their average.
Adult length
Common length to maximum recorded length. Measured as total length. Fish in home aquariums often stay smaller than wild records.
Water temperature
Range the species tolerates in nature; aim for the middle.
Sources for this sheet
- FishBase (Froese & Pauly, eds.), version 24.07 Peer-reviewed researchThe global scientific database on fishes, maintained by a consortium of research institutions; every value is referenced.
- Seriously Fish species profiles Experienced keeper or breederLong-running aquarium reference written and peer-checked by experienced fishkeepers and ichthyologists; each profile covers habitat, tank size, water, diet, behaviour and breeding.
- Merck Veterinary Manual: Management of aquarium fish Zoo, university or veterinaryThe Merck Veterinary Manual: a standard veterinary reference written and reviewed by veterinary specialists.
- Merck Veterinary Manual: Types and features of aquatic systems Zoo, university or veterinaryThe Merck Veterinary Manual: a standard veterinary reference written and reviewed by veterinary specialists.
- Merck Veterinary Manual: Nutritional diseases of fish Zoo, university or veterinaryThe Merck Veterinary Manual: a standard veterinary reference written and reviewed by veterinary specialists.
- AnAge: The Animal Ageing and Longevity Database (Human Ageing Genomic Resources) Peer-reviewed researchCurated, referenced longevity and life-history records for thousands of species; maximum lifespans are often from zoo animals.
- Wikipedia (species article) Community referenceFree encyclopedia; useful overviews with their own citations. Intro text is quoted under CC BY-SA 4.0.
- iNaturalist (captive observations) Community referenceCommunity observation platform run by the California Academy of Sciences and National Geographic; observations marked captive show animals living in homes, zoos and aquariums.
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