Mosquito fish - Gambusia affinis

Scientific name: Gambusia affinis

Common name: Mosquito fish

Family: Poeciliidae

Usual size in fish tanks: 4 - 7 cm (1.57 - 2.76 inch)

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Recommended pH range: 6.8 - 8

Recommended water hardness: 4 - 19°N (71.43 - 339.29ppm)

0°C 32°F30°C 86°F

Recommended temperature range: 21 - 26 °C (69.8 - 78.8°F)

The way how these fish reproduce: Pseudo-Livebearer

Where the species comes from: North America

Temperament to its own species: peaceful

Temperament toward other fish species: aggressive to smaller

Usual place in the tank: Middle levels

General Information

The western mosquitofish (Gambusia affinis) is a small livebearing poeciliid native to North America—the Mississippi River basin and Gulf-slope drainages from the U.S. Gulf Coast south-west to Tampico, Mexico. It has been introduced worldwide for mosquito control, often with harmful ecological effects. Females reach up to ~7 cm TL; males ~4–5 cm TL.

Food & Feeding

Omnivorous micro-predator: takes insect larvae (including mosquitoes), small crustaceans, worms, and grazes biofilm/algae. In aquaria use quality flakes or micro-pellets as staple and rotate frozen/live daphnia, brine shrimp, cyclops, bloodworms (in moderation). Provide some plant matter (spirulina) and feed small portions 1–2× daily.

Sexing

Males are smaller and possess a gonopodium (modified anal fin); females are larger and often show a gravid spot.

Breeding

True livebearer (viviparous), not a “pseudo-livebearer.” Females can store sperm and exhibit superfetation. Gestation is typically ~16–28 days depending on temperature; broods commonly number 20–60 fry. Adults may predate fry—dense plants/mops improve survival.

Lifespan

Usually ~1–2 years in the wild; up to ~3 years reported in captivity with excellent care.

Tank Requirements & Water Parameters

Footprint & cover: active surface-oriented fish; provide open surface lanes with floating cover and plants.
Water: very tolerant; aim for pH 6.8–8.0, hardness moderate–hard, temperature 21–26 °C in home aquaria (species tolerates cooler/warmer outdoors but stability matters).
Filtration: moderate flow, high oxygenation; they handle light brackish conditions as well.

Compatibility & Tank Mates

Not community-friendly despite size. Mosquitofish are notorious fin-nippers with intraspecific aggression (especially males). Keep either single-sex groups with space and cover, or avoid mixing with delicate/slow tank mates. Do not house with tiny or long-finned species.

Behaviour & Usual Place in the Tank

Surface to upper-middle levels: they patrol the topwater, darting for prey and oxygen-rich zones; drop lower when threatened. Labeling them strictly “middle” is inaccurate.

Conservation & Legal/Release Note

Introduced populations often harm native fishes and amphibians; many regions regulate their release/stocking. Never release aquarium fish to the wild.

Short Description

Gambusia affinis is a hardy, surface-dwelling North American livebearer that breeds readily and tolerates a wide range of conditions—but its aggression and invasive potential make it a poor choice for community tanks and a species that should be kept responsibly.

Q&A

Native where? U.S. Mississippi basin and Gulf-slope drainages to Tampico, Mexico (North America).
How big? Females up to ~7 cm TL; males ~4–5 cm TL.
Reproduction type? Livebearer with ~16–28 day gestation; broods often 20–60 .
Community safe? Generally no—fin-nipping and conspecific bullying are common.