At a glance the quick version
Looks how to spot one
Tiny scaled livebearer
A very small, slender fish. Males stay around 2 cm and carry a long gonopodium, while females grow to about 3 cm with a rounder belly.23
Personality what they are like
A gentle, shy little fish that suits a quiet species tank or a nano setup with other very small, peaceful animals.3
Daily activity how busy a day looks
Low; spends its time among dense plants rather than swimming long distances.13
Grooming and care the hands-on bits
Easy to care for. Small, frequent feeds of tiny foods and a well-planted tank are the main needs.3
Home where they are happiest
A small aquarium from about 30 x 20 x 20 cm suits a pair. Water 20-26 °C, pH 7.0-8.0, moderately hard (about 5-20 dGH). Dense planting mirrors its weedy, still-water home.31
Food basics what goes in the bowl
Omnivore. Tiny live or frozen foods such as brine shrimp and daphnia, plus crushed flakes with some plant content.31
Company who they like around
Kept in small groups; tankmates should be equally tiny and calm, such as pygmy corydoras, otocinclus or dwarf shrimp.3
The story where it all began
Despite its common name, this fish is a livebearer in the guppy family rather than a true killifish. It has long been kept by hobbyists for its tiny size and was listed among the world's smallest fish in the 1991 Baensch Aquarium Atlas.23
Fun facts for the dinner table
- 1
It is the smallest fish species found in North America.2
- 2
Mothers nourish their developing young directly, so the embryo gets nearly all its food from the mother.1
- 3
Instead of one big brood, a female releases a few fry at a time over 10-14 days, as embryos at different stages develop together (superfetation).2
- 4
Newborn fry are large enough to eat brine shrimp nauplii straight away.3
Recognised by the official bits
- IUCN Red List
- Least Concern (assessed 2012)1
- Taxonomy
- Family Poeciliidae (a livebearer, despite the common name)2
The care rhythm a typical week and year
A typical home-care rhythm for a Least Killifish. A starting point, not a vet's advice.
- FeedingSmall amounts once or twice a dayevery day
- Temperature checkevery day
- Water changeAbout 25% of the waterevery week
- Water testevery week
- Glass cleanevery week
- Filter rinseRinse in old tank water, never tap waterevery month
Owner questions the things everyone asks
How often should I feed my Least Killifish?
Feed your Least Killifish small amounts once or twice a day. Furry Diary's starting plan suggests daily feeding with tiny portions to match their small size.3 Pet Wiki plan
What does a Least Killifish eat, and what should it never eat?
They are omnivores and enjoy tiny live or frozen foods like brine shrimp and daphnia, plus crushed flakes with some plant content. Never feed large or hard foods that could harm their tiny mouths.31
How much space does a Least Killifish need?
A small aquarium from about 30 x 20 x 20 cm suits a pair. Dense planting is important to mirror their natural weedy, still-water home.31
What temperature, light or water does a Least Killifish need?
Keep water at 20-26 °C, pH 7.0-8.0, and moderately hard (about 5-20 dGH). Provide gentle light and dense plants for a comfortable environment.31
How often should I clean a Least Killifish's home?
Furry Diary's plan suggests a weekly 25% water change, weekly glass cleaning, and a monthly filter rinse in old tank water. Test water weekly to keep conditions stable. Pet Wiki plan
Can a Least Killifish live with others?
Yes, they are gentle and shy, best kept in small groups with equally tiny, calm tankmates like pygmy corydoras, otocinclus, or dwarf shrimp. Avoid larger or boisterous fish.3
How long does a Least Killifish live?
Lifespan varies with care; ask a good breeder or exotic-pet vet for specific estimates. With proper conditions, they can live several years.
Can I handle a Least Killifish?
Handling is not recommended as they are very small and delicate. It's best to observe them in their planted tank and avoid stress.31
Written from the sources on this page, a few checked guides (VCA, ASPCA, RSPCA) and Pet Wiki's own starting care plan. A friendly guide, not a vet's advice.
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Western MosquitofishWhere this comes from
Every fact on this page has a numbered source. Spotted an error? [email protected].
- FishBase: Heterandria formosa fishbase.seused for origin, size, environment, diet, IUCN, reproduction
- Wikipedia: Least killifish en.wikipedia.orgused for names, colour, history, facts
- Seriously Fish: Heterandria formosa seriouslyfish.comused for tank size, water, diet, behaviour
Checked 2026-09-30. Text is written in our own words from these sources.