Are Least Killifish true killifish?

No, they are actually livebearers belonging to the family Poeciliidae, making them close relatives of Guppies and Mollies rather than true killifish.

Do I need a heater for Least Killifish?

It depends on your room temperature. They are a sub-tropical species that prefers water between 15°C and 26°C. If your home stays within this range year-round, an unheated tank is perfectly fine.

Why is my female fish only having one or two babies at a time?

This is completely normal and is a process called superfetation. Instead of dropping dozens of fry at once, the female develops multiple fry at different stages and gives birth to a few every couple of days.

Can I keep Least Killifish in a community tank?

They are best kept in a species-only nano tank or with very small, peaceful tank mates like dwarf shrimp and small snails. Most standard community fish are large enough to eat them or will easily outcompete them for food.

What is the best filter for a Least Killifish tank?

A sponge filter is the best option. It provides gentle water flow that won't blow these tiny fish around, offers a surface for them to graze on biofilm, and is completely safe for their microscopic fry.

FreshwaterTropicalBeginner

Least Killifish

Heterandria formosa

A diminutive, fascinating livebearer native to the southeastern United States, the Least Killifish is an ideal candidate for heavily planted nano aquariums.

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Ideal Conditions

Temperature: 15 – 26 °C
pH range: 7 – 8
Hardness: 5 – 20 dGH
Adult size: 1.5 – 3 cm
Nano tank: 20 litres+
Lifespan: 2 – 3 years
Diet: Omnivore/Micro-predator
Water flow: low
Preferred setup: Heavily planted with dense cover like Java moss and floating plants

About the Least Killifish

Despite its common name, the Least Killifish (*Heterandria formosa*) is not a true killifish but rather a member of the livebearer family (Poeciliidae). It holds the distinction of being one of the smallest vertebrates in the world, with males rarely exceeding 2 cm in length. Due to their tiny proportions and subtle, intricate markings, they are best appreciated in dedicated nano setups where their natural behaviours can be observed without the threat of larger tank mates.

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Where Least Killifish comes from

Native range

Southeastern United States, Gulf Coast

Countries / regions

United States

Habitat

Sluggish, heavily vegetated waters including swamps, drainage ditches, and the margins of slow-moving rivers.

They have one of the smallest native ranges of North American livebearers but are exceptionally abundant within it.

Similar species

Endler's Livebearer
Poecilia wingei
Clown Killifish
Epiplatys annulatus
Mosquitofish
Gambusia affinis

We’re adding more full species guides as the catalogue grows.

Compatible tank mates

Reviewed guidance based on this species' recorded traits.

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Cherry Shrimp
Neocaridina davidi
Least Killifish are too small to pose a serious threat to adult shrimp, and they share a preference for planted tanks.
Great
Pygmy Corydoras
Corydoras pygmaeus
A peaceful, similarly sized bottom-dweller that will not compete with or predate on the Least Killifish.
Great
Ramshorn Snail
Planorbarius corneus
An excellent cleanup crew companion that helps maintain the pristine conditions required in nano tanks without disturbing the fish.
Great
Endler's Livebearer
Poecilia wingei
Though peaceful, male Endlers are very active and may outcompete the slower, shyer Least Killifish for food.
Compatible with caution
Angelfish
Pterophyllum scalare
Angelfish grow large enough to easily swallow an adult Least Killifish whole.
Not recommended
Betta
Betta splendens
Bettas may view these tiny fish as a snack or act aggressively towards them in a confined nano space.
Not recommended

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