Identity and search names

Common name
Least killifish
Scientific name
Heterandria formosa
Other names
Dwarf livebearer, Mosquito livebearer, Topminnow, Least livebearer
Category
Freshwater fish
Group
Livebearers
Origin
Slow vegetated waters of the south-eastern United States, especially Florida and neighbouring Gulf and Atlantic drainages
Review outcome
PASSED

Environment and housing

Environment
Freshwater
Care level
Beginner to intermediate
Adult size
Males usually about 2 cm; females commonly 3–3.5 cm total length
Expected lifespan
About 2–4 years
Minimum housing
At least 45 L for a stable colony, with dense plants and protected fry refuge
Temperature
18–26 °C
pH
6.8–8.2
Hardness or minerals
Moderately soft to hard with stable minerals
Salinity
Freshwater; slight natural salinity tolerance does not require routine salt
Lighting
Moderate with dense plants, moss and floating cover
Social structure
Keep a colony with more females than males and extensive refuge from courtship
Placement or swimming zone
All levels, especially among marginal vegetation
Temperament
Peaceful but persistent males can harass females; tiny fry are vulnerable to almost any tankmate
Compatibility
Best species-focused or with very small peaceful fish and invertebrates that will not eat fry
Habitat structure
Mature planted aquarium with moss, fine-leaved plants, floating cover, gentle filtration and stable mineral chemistry

Feeding and nutrition

Diet
Minute crustaceans, insect larvae, algae, biofilm and very fine prepared foods
Feeding frequency
Feed tiny portions once or twice daily and ensure fry receive appropriately small food

Breeding and reproduction

Breeding or reproduction
Females carry multiple broods at different developmental stages and release small numbers of relatively developed fry over an extended period rather than one large synchronous drop
Parental care
No parental care; adults may eat some fry, although dense cover greatly improves survival

System demands

Filtration
Use mature biological filtration sized for the full adult group and feeding load, with intake protection where juveniles, long fins or small fish are present. Provide steady turnover without preventing normal swimming or resting.
Aeration
Maintain strong surface gas exchange and stable dissolved oxygen. Increase aeration during warm weather, medication, transport recovery, breeding crowding or any ammonia or nitrite event.
Maintenance
Test ammonia, nitrite and nitrate, remove waste from feeding and shelter zones, and perform regular temperature- and chemistry-matched water changes before nitrate or dissolved organics accumulate.

Detailed husbandry

Identity scope
This profile applies to Heterandria formosa, a poeciliid livebearer rather than a true killifish. Sexes differ sharply in size, and females carry several broods at different developmental stages through superfetation.
Adult size and space
Males usually about 2 cm; females commonly 3–3.5 cm total length. At least 45 L for a stable colony, with dense plants and protected fry refuge. Plan around adult dimensions, activity, social structure, aggression and waste production rather than sale size.
Social structure
Keep a colony with more females than males and extensive refuge from courtship
Placement or swimming zone
Primary zone: All levels, especially among marginal vegetation. Keep that zone usable for movement, feeding and refuge.
Feeding ecology
Minute crustaceans, insect larvae, algae, biofilm and very fine prepared foods
Feeding plan
Feed tiny portions once or twice daily and ensure fry receive appropriately small food
Water stability
Maintain 18–26 °C; pH 6.8–8.2; hardness/minerals: Moderately soft to hard with stable minerals. Stable oxygenation and nitrogen-cycle control matter more than chasing one exact number.
Adult projected load
Plan capacity for Keep a colony with more females than males and extensive refuge from courtship at the documented adult size (Males usually about 2 cm; females commonly 3–3.5 cm total length) in At least 45 L for a stable colony, with dense plants and protected fry refuge. Include feeding intensity, territorial behaviour and breeding growth rather than counting sale-size juveniles.
Filtration
Use mature biological filtration sized for the full adult group and feeding load, with intake protection where juveniles, long fins or small fish are present. Provide steady turnover without preventing normal swimming or resting.
Aeration
Maintain strong surface gas exchange and stable dissolved oxygen. Increase aeration during warm weather, medication, transport recovery, breeding crowding or any ammonia or nitrite event.
Maintenance
Test ammonia, nitrite and nitrate, remove waste from feeding and shelter zones, and perform regular temperature- and chemistry-matched water changes before nitrate or dissolved organics accumulate.
Habitat structure
Mature planted aquarium with moss, fine-leaved plants, floating cover, gentle filtration and stable mineral chemistry
Compatibility plan
Best species-focused or with very small peaceful fish and invertebrates that will not eat fry
Breeding plan
Females carry multiple broods at different developmental stages and release small numbers of relatively developed fry over an extended period rather than one large synchronous drop Parental care: No parental care; adults may eat some fry, although dense cover greatly improves survival.
Quarantine
Quarantine new fish in a separate cycled aquarium with dedicated equipment. Match temperature and mineral chemistry gradually, confirm feeding and normal respiration, and never add transport water to the established aquarium.
Welfare checks
Maintain the stated adult space and social structure, verify feeding and intervene early when injury, exclusion, weight loss, abnormal respiration or altered behaviour appears. Its tiny size does not make it suitable for an unfiltered jar. Colonies still need stable water, genetic management and a plan for continuous fry production.
Biosecurity and release
Never release fish, fry, eggs, aquarium water, sediment, plants or live foods into drains, dams or natural waterways. Confirm current Australian import conditions and South Australian possession, trade and movement rules before acquisition or transfer.
Main risk
Its tiny size does not make it suitable for an unfiltered jar. Colonies still need stable water, genetic management and a plan for continuous fry production.
Overridable warnings
Its tiny size does not make it suitable for an unfiltered jar. Colonies still need stable water, genetic management and a plan for continuous fry production. Any override must still preserve adult space, social welfare, stable chemistry, oxygen and filter capacity.

Welfare and risk

Welfare checks
Maintain the stated adult space and social structure, verify feeding and intervene early when injury, exclusion, weight loss, abnormal respiration or altered behaviour appears. Its tiny size does not make it suitable for an unfiltered jar. Colonies still need stable water, genetic management and a plan for continuous fry production.
Main warning
Its tiny size does not make it suitable for an unfiltered jar. Colonies still need stable water, genetic management and a plan for continuous fry production.
Biosecurity
Never release fish, fry, eggs, aquarium water, sediment, plants or live foods into drains, dams or natural waterways. Confirm current Australian import conditions and South Australian possession, trade and movement rules before acquisition or transfer.

Sources

Source note

Current identity or scope, husbandry, system-demand, welfare and Australian biosecurity evidence checked; active retired-source links removed; all AquaNexus wording is original.

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