Least killifish care guide
Also called: Least livebearer, Dwarf livebearer, Heterandria formosa
The least killifish is one of the smallest fish in the world, a tiny livebearer from the southeastern US. It is peaceful, hardy and breeds all the time, but it must live with equally tiny, gentle fish or it will be eaten.
Least killifish need a tank of at least 10 gallons (38 L), a group of 6 or more, and water at 20–26°C (68–79°F) with a pH of 7–8. They grow to about 2.5 cm (1 in) and are peaceful.
Plan a tank with least killifishSee all tank mates
Least killifish quick facts
| Scientific name | Heterandria formosa |
|---|---|
| Family | Poeciliidae |
| Origin | Southeastern USA, from North Carolina to Louisiana and Florida (most sold are captive-bred) |
| Adult size | 2.5 cm (1 in) |
| Minimum tank | 10 gallons (38 L) |
| Temperature | 20–26°C (68–79°F) |
| pH | 7–8 |
| Hardness (GH) | 5–20 dGH |
| Group size | 6 or more (group) |
| Temperament | Peaceful |
| Swims at | Middle |
| Diet | Omnivore |
| Care level | Easy |
| Lifespan | 2–3 years |
| Safe with shrimp? | Mostly (eats baby shrimp) |
| Safe with plants? | Yes |
Tank setup
A colony of six or more does well in a 10-gallon (38 L) tank, and a single pair needs even less. Fill the tank with fine plants, Java moss and floating plants so adults feel safe and fry can hide. Use a sponge filter or other gentle flow, because they are weak swimmers.
How many least killifish in a tank?
The smallest common tank that suits least killifish is a 10 gallon. Here is roughly how many fit in each size when they are the only fish, based on adult size and waste output. Leave room if you want tank mates.
| Tank | Least killifish (alone) |
|---|---|
| 3 gallon | Too small |
| 5 gallon | Too small |
| 10 gallon | 6–27 |
| 15 gallon | 6–41 |
| 20 gallon | 6–55 |
| 20 gallon long | 6–55 |
| 29 gallon | 6–79 |
| 40 gallon | 6–110 |
| 55 gallon | 6–151 |
| 75 gallon | 6–206 |
| 125 gallon | 6–344 |
What you need for least killifish
Sized for a 10 gallon tank, the smallest common size that suits a group of 6 least killifish. For a different tank or a mix of fish, use the stocking calculator for a list sized to your plan.
Tank and equipment
- 10 gallon aquarium (tank or kit) (opens Amazon in a new tab)Bigger tanks are more stable and easier for beginners. How to choose
- Adjustable heater, 50 W (opens Amazon in a new tab)Holds about 73 °F (23 °C) in a room at 65 °F (18 °C). How to choose
- Aquarium thermometer (opens Amazon in a new tab)Check it daily: heaters can fail stuck on or off. How to choose
- Hang-on-back filter rated 40–60 GPH (opens Amazon in a new tab)Community tank: 4–6 times the tank volume per hour. How to choose
- About 25 lb of aquarium gravel (opens Amazon in a new tab)Enough for a layer about 2 in (5 cm) deep. Smooth gravel suits most community fish. How to choose
- Water conditioner (dechlorinator) (opens Amazon in a new tab)Removes chlorine and chloramine from tap water. Seachem Prime is a popular, concentrated choice. How to choose
- Liquid test kit (ammonia, nitrite, nitrate, pH) (opens Amazon in a new tab)Liquid kits are more accurate than strips. The API Freshwater Master Test Kit is the common choice. How to choose
- Gravel vacuum (siphon) (opens Amazon in a new tab)Cleans the gravel and does the water change in one go. How to choose
- LED aquarium light, about 20 in long (opens Amazon in a new tab)Match the light to the tank length. Basic LEDs grow easy plants. How to choose
Food and extras for least killifish
- Micro pellets or flakes for small fish (opens Amazon in a new tab)Small enough for tetras, rasboras and other nano fish.
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Water parameters
Keep them at 20–26 °C (68–79 °F), pH 7.0–8.0 and moderately hard water of 5–20 dGH. They come from subtropical Florida and the Carolinas, so they cope with cooler water better than most tropical fish. Wild fish also live in slightly brackish water, but plain fresh water is fine.
Best tank mates for least killifish
These popular species share least killifish' water needs and temperament. See the full list of 89 good matches.
Cherry shrimp
3 cm (1.2 in) · 20–27°C (68–81°F)
Nerite snail
2.5 cm (1 in) · 22–27°C (72–81°F)
Guppy
5 cm (2 in) · 22–27°C (72–81°F)
Dwarf gourami
7.5 cm (3 in) · 24–28°C (75–82°F)
Neon tetra
3.5 cm (1.4 in) · 21–25°C (70–77°F)
Panda corydoras
4.5 cm (1.8 in) · 21–25°C (70–77°F)
Bristlenose pleco
12 cm (4.7 in) · 22–26°C (72–79°F)
Bronze corydoras
7 cm (2.8 in) · 21–26°C (70–79°F)
Avoid keeping least killifish with
- Angelfish: Angelfish can eat least killifish: they're small enough to fit in its mouth once it's grown.
- Common goldfish: Common goldfish can eat least killifish: they're small enough to fit in its mouth once it's grown.
- Fancy goldfish: Fancy goldfish can eat least killifish: they're small enough to fit in its mouth once it's grown.
- Oscar: Oscar can eat least killifish: they're small enough to fit in its mouth once it's grown.
- Bala shark: Bala shark can eat least killifish: they're small enough to fit in its mouth once it's grown.
- Blood parrot cichlid: Blood parrot cichlid can eat least killifish: they're small enough to fit in its mouth once it's grown.
Feeding
They are omnivores with very small mouths. Offer crushed flakes, micro pellets, baby brine shrimp, daphnia and other tiny live or frozen foods. Add some plant matter such as spirulina flakes, and let them graze algae on plants and glass.
Behavior
Very peaceful and a little shy. Males stay around 2 cm (0.8 in) and females reach about 3.5 cm (1.4 in), so almost any medium-sized fish can eat them. Keep them on their own or with nano species such as pygmy corydoras, otocinclus, small pencilfish and shrimp. Males may squabble over females, but they rarely hurt each other.
Breeding
Very easy. Instead of one big batch, each female gives birth to one or two fairly large fry a day, spread over 10 days or more. Adults seldom eat their young, so a densely planted tank lets the colony grow on its own. Newborn fry can eat baby brine shrimp and powdered food right away.
Tips from experienced keepers
- Do not keep them with bettas, angelfish or larger tetras. Anything with a big mouth may eat the tiny males.
- Java moss and floating plants are the best fry protection, so you do not need a breeding box.
- Crush flakes to powder. Their mouths are tiny and they ignore food that is too big.
- Cover the filter intake with a sponge so newborn fry are not pulled in.
- In a room that stays above 20 °C (68 °F), many keepers run them without a heater.
Frequently asked questions
Is the least killifish a real killifish?
No. Despite the name, it is a livebearer in the same family as guppies and mollies. Females give birth to live young instead of laying eggs.
How many least killifish can live in a 10-gallon tank?
Their waste output is tiny, so a planted 10-gallon (38 L) tank can hold a colony of a dozen or more. Start with at least six and let them breed slowly.
Can least killifish live with shrimp?
Yes. Adult dwarf shrimp are safe, and they are a popular choice for shrimp tanks. They may eat a few newborn shrimplets, so add plenty of moss for cover.
Sources
We checked these references when writing this page. Values are for healthy long-term keeping, not survival limits. Last reviewed 2026-10-13. See our editorial policy.