Species profile
Least Killifish
Heterandria formosa
Peaceful
Easy care
40 L minimum aquarium
2–3.5 cm adult size
Top mid
Minimum group: 8
About this species
General notes for hobby use.
Despite the name it is a livebearer, not a true killifish. Its tiny size makes it vulnerable to ordinary community fish.
Water parameters
Suggested general range for long-term care.
Temperature
18–26 °C
pH
6.5–8
GH
5–25 dGH
KH
Not yet documented
Flow preference
Low–moderate
Data confidence
Verified
Behaviour
General behaviour traits used by TanksConnected compatibility checks.
Minimum group size
Minimum group: 8
Tank zone
Top mid
Territory level
None
Predation risk
Tiny fry
Bioload factor
0.35
Very peaceful. Keep as a group in a calm planted setup; adults can eat tiny fry but are less destructive than many larger livebearers.
Compatibility flags
Quick cautions for common tank-planning issues.
Fin nipper
No
Plant safe
Yes
Shrimp safe
Yes
Snail safe
Yes
Cave breeder
No
Digs substrate
No
Jump risk
Yes
Natural history & care
Research-backed context for long-term aquarium care.
Lifespan
2–4 years
Diet type
Omnivore
Minimum aquarium volume
40 L
Minimum footprint
45 × 30 cm
Accepted scientific name
Heterandria formosa
Alternative names
Least Killifish, Dwarf Livebearer, Mosquitofish (confusing historic use)
Native range
South-eastern United States.
Natural habitat
Dense vegetation in slow-moving streams, ditches, swamps and pond margins. A mature heavily planted aquarium with gentle flow and fine cover suits this tiny livebearer.
Diet & feeding
Small omnivore/micro-predator. Feed crushed quality foods and tiny live/frozen foods such as daphnia, cyclops and Artemia nauplii; biofilm is useful for fry.
Breeding & sexing
Research-backed guidance for identifying sexes and reproductive behaviour.
Sexing
Males are much smaller and possess a gonopodium; females are larger, fuller and lack the modified anal fin.
Breeding
Livebearer with superfetation: females can carry broods at different developmental stages and release small numbers of fry over time. Dense plants greatly improve fry survival.