rm
Passport
- Grows to
- 2 cm (0.8")
- Lives
- 2–3 years
- Temperament
- Peaceful
- Care
- Beginner
- Swims
- Mid-water
- Keep
- In a group of 6 or more
- Water
- 20–26 °C (68–79 °F)
United States
Care requirements
i
On record for Least Killifish: adult size, lifespan, minimum tank, temperature, ph, hardness, minimum group, diet, care level, swim level, tank length.
A figure that is not on record is left out rather than guessed. One marked Suggested comes from keeping knowledge rather than a page.
- Minimum tank 12 L / 3.3 US gal
- pH 7–8
- Hardness 5–20 dGH
- Diet omnivore
- Tank length 30 cm minimum
Care notes written from general aquarium knowledge, not from the cited sources.
About
Least killifish are not killifish. They're livebearers — close relatives of guppies and mosquitofish — and the name is a long-standing misnomer that has stuck.
They're also among the smallest fish in the world. A full-grown female is barely longer than a fingernail and the male is smaller still, which makes them one of very few vertebrates you can keep properly in a genuinely tiny tank.
They're native to the southeastern United States, which makes them a temperate fish. Wild populations sit out cold winters outdoors, and in a tank they do well at the cooler end of what the grid allows rather than needing tropical warmth.
Tank setup
Give your least killifish a small, densely planted, quiet tank. They're too small to compete with almost anything, and in a standard community they disappear — either eaten or simply outcompeted at every feed.
Plant it thickly with fine-leaved plants and moss. Cover is what they live in, and it's where the fry survive.
You may not need a heater, which is unusual and useful. Check the room first, though: an unheated tank only works if it actually stays inside the range on the care grid above, and a cold snap in an unheated room will take it under.
Keep the water hard and alkaline. Like other livebearers they want minerals.
Cover the tank, and use a sponge filter — anything else will draw in fry.
Feeding
Least killifish are micropredators with mouths so small that most prepared food is useless.
Feed baby brine shrimp, microworm, cyclops, small daphnia and finely powdered dried food. Grade everything down further than seems reasonable.
They also graze biofilm and algae continuously, and a mature planted tank supplies a real part of their diet on its own.
Avoid: normal flake and pellets, and avoid any tank mate quick enough to reach food first.
Filtration and flow
A sponge filter and gentle flow. There is no safe unguarded intake at this size.
Keep the water clean and stable with small regular changes. A tiny tank swings quickly.
They're hardy across a wide temperature range, which makes them forgiving of a room that cools overnight.
With plants
Least killifish are entirely safe with plants and need a heavily planted tank to do well.
Moss is the single most valuable addition. It holds food, gives cover, and is where fry spend their first weeks.
Floating plants add shade and make them noticeably bolder.
Health
They're hardy and long-lived for something so small, and problems are rare in a well-planted tank.
Most losses are predation or competition rather than disease. Check who else is in the tank before looking for an illness.
At this size a fish in trouble declines within a day, so watch the colony daily rather than weekly.
Breeding
Their breeding is the genuinely unusual thing about them. Instead of dropping a whole brood at once, a female carries embryos at different stages of development and gives birth to a few fry every day or two over a period of weeks.
That's called superfetation, and it's rare. It means you never see a dramatic birth — just a steadily increasing number of very small fish.
Fry are tiny but not helpless: they take finely powdered food and newly hatched brine shrimp straight away, which makes them far less fiddly than most young this size. A mature planted tank feeds them as well. The adults largely ignore their own, so a colony builds up without help.
Tank mates
Avoid51
- Agassiz's Dwarf Cichlid Agassiz's Dwarf Cichlid grows to 5 cm against Least Killifish at 2 cm, big enough to eat it.
- Angelfish Angelfish grows to 15 cm against Least Killifish at 2 cm, big enough to eat it.
- Bala Shark Bala Shark grows to 35 cm against Least Killifish at 2 cm, big enough to eat it.
- Betta Betta grows to 7 cm against Least Killifish at 2 cm, big enough to eat it.
- Black Skirt Tetra Black Skirt Tetra grows to 6 cm against Least Killifish at 2 cm, big enough to eat it.
Only with care17
- Serpae Tetra Serpae Tetra is noticeably larger than Least Killifish and may bully it or outcompete it at feeding time.
- Amano Shrimp Least Killifish is small enough to leave adult Amano Shrimp alone, but will still take shrimplets.
- Bamboo Shrimp Least Killifish is small enough to leave adult Bamboo Shrimp alone, but will still take shrimplets.
- Blue Bolt Shrimp Least Killifish is small enough to leave adult Blue Bolt Shrimp alone, but will still take shrimplets.
- Blue Dream Shrimp Least Killifish is small enough to leave adult Blue Dream Shrimp alone, but will still take shrimplets.
40 species are not covered at all, so they could not be judged either way.