Least killifish with shrimp
The least killifish (Heterandria formosa) is not a true killifish. It is a tiny livebearer from the southeastern United States, related to guppies and mosquitofish. Seriously Fish calls it the smallest known livebearing fish, and it lives in the heavily vegetated edges of still or slow water. Shrimp and Snail Breeder says it ranges from North Carolina through Florida to Louisiana. Other names include dwarf livebearer and dwarf topminnow.
Least killifish at a glance
| Least killifish (Heterandria formosa) | |
|---|---|
| Adult size | Males about 0.6–0.8 in; females 1–1.5 in (Shrimp and Snail Breeder); Seriously Fish lists 20 mm (0.8 in) |
| Temperature | 68–79 °F (Seriously Fish); 68–78 °F (Shrimp and Snail Breeder) |
| pH | 7.0–8.0 (Seriously Fish); 6.5–8.0 (Shrimp and Snail Breeder) |
| Hardness | 5–20 dGH (both) |
| Tank | A pair in 12 × 8 × 8 in (Seriously Fish); 10 gallons ideal for a colony (Shrimp and Snail Breeder) |
| Lifespan | Up to about 1.5 years in aquariums (Shrimp and Snail Breeder) |
| Temperament | Peaceful, but can nip fins in community tanks (Shrimp and Snail Breeder) |
Will least killifish eat shrimp?
- Adult shrimp: no. A fish under 1.5 inches cannot swallow a grown cherry shrimp.
- Shrimplets: yes, often. Shrimp and Snail Breeder says these fish search every nook for food, will find shrimplets and molting shrimp even in planted tanks, and that keeping them with dwarf shrimp is risky. Seriously Fish still lists freshwater shrimp among suitable tankmates. Expect fewer babies to survive than in a shrimp-only tank.
- Molting shrimp: a soft, freshly molted shrimp may get picked at. Plenty of cover helps. See shrimp dying after molting.
Matching the water
This is where least killifish shine. Seriously Fish lists 68–79 °F, pH 7.0–8.0 and 5–20 dGH. Cherry-type shrimp tolerate 65–85 °F, pH 6.2–8.0 and GH 4–8 (The Shrimp Farm). By our comparison, about 70–76 °F, pH 7.0–7.8 and GH 5–8 fits both, often with little adjustment of hard tap water. Many tanks need no heater in a warm room. This is not a fish for soft, acidic Caridina tanks. See shrimp water parameters and shrimp tank temperature.
Tank size and setup
- Size: Seriously Fish says a pair can live in a 12 × 8 × 8 in tank. Shrimp and Snail Breeder suggests 10 gallons for a colony and warns that very small tanks are harder to keep stable.
- Heavy planting: both sources stress dense vegetation and surface cover. Floating plants and thick moss also shelter shrimplets. See java moss for shrimp and best plants for a shrimp tank.
- Minimal flow: a gentle sponge filter, or a sponge over the intake. Shrimp and Snail Breeder notes that intakes can suck in these tiny fish. See shrimp-safe filters.
- Lid: Shrimp and Snail Breeder says they jump when stressed. See do shrimp need a lid.
Feeding both
Least killifish are omnivores with a big appetite. Seriously Fish suggests small live or frozen foods such as brine shrimp and Daphnia, plus algae or spirulina-based food. Shrimp and Snail Breeder recommends feeding twice a day and crumbling flakes small. A well-fed group is less likely to hunt shrimplets (our inference; no guarantee). Drop sinking shrimp food in a planted corner too. See best shrimp food.
A colony, not a pair
Seriously Fish says the species breeds very easily. Gestation is about four weeks, and females carry several broods at different stages, so they drop a few fry every few days. Shrimp and Snail Breeder suggests starting with at least six fish and says well-fed adults usually ignore their own fry. Expect the fish colony to grow along with the shrimp, and plan where extra fish will go. Males chase each other but rarely do damage (Seriously Fish).
Don't confuse it with mosquitofish
Seriously Fish notes the least killifish is often confused with the more aggressive Gambusia mosquitofish, which shares some common names. Buy by the scientific name. Since it is native to the US, check your state's rules before collecting wild fish, and never release aquarium fish (our note).
How to add least killifish to a shrimp tank
- Shrimp first. Let the colony build up and settle in moss before the fish arrive (Practical Fishkeeping).
- Start small. Add six fish, then watch shrimplet numbers for a few weeks.
- Keep a backup. If a particular shrimp line matters, keep some in a separate tank.
- Quarantine the fish separately. Practical Fishkeeping says many fish medications are not safe for invertebrates. If fish get sick, ask a fish veterinarian or an experienced store about treatment that won't harm the shrimp. See how to quarantine and is copper toxic to shrimp.
Common mistakes
- Assuming a tiny fish is harmless to baby shrimp.
- Underfeeding, which makes them hunt harder.
- Adding them to a soft-water Caridina tank.
- Buying a Gambusia by mistake.
- Leaving the tank open.
Track it
Log temperature, pH, GH and TDS on the free Water Parameter Log printable so you can keep the tank in the range both species share.
Frequently asked questions
Will least killifish eat cherry shrimp?
Not adults. They are too small. Shrimp and Snail Breeder warns they find and eat shrimplets and sometimes molting shrimp, so expect fewer babies to survive.
Are least killifish good for a shrimp tank?
They are a good water match, since both like hard, slightly alkaline water, and Seriously Fish lists freshwater shrimp as tankmates. The trade-off is lost shrimplets.
What water do least killifish need?
Seriously Fish lists 68 to 79 °F, pH 7.0 to 8.0 and 5 to 20 dGH. With cherry shrimp, aim for about 70 to 76 °F, pH 7.0 to 7.8 and GH 5 to 8.
Do least killifish need a heater?
Often not, in a room that stays around 70 °F or warmer. Seriously Fish lists 68 to 79 °F, so check your room temperature.
How big do least killifish get?
Males reach about 0.6 to 0.8 inches and females 1 to 1.5 inches, according to Shrimp and Snail Breeder.
Are least killifish real killifish?
No. Despite the name, Heterandria formosa is a livebearer in the guppy family that gives birth to live fry.
Sources: Seriously Fish, Heterandria formosa (least killifish) (smallest known livebearer; vegetated margins of still or slow water in the southeastern US; 20 mm SL; 68–79 °F, pH 7.0–8.0, 5–20 dGH; a pair in 12 × 8 × 8 in; heavily planted, minimal flow; small live or frozen food plus algae or spirulina; good tankmates include freshwater shrimp; males competitive but rarely damaging; about four-week gestation with superfetation; often confused with Gambusia); Shrimp and Snail Breeder, Least killifish care guide (range NC to Florida and Louisiana; males 0.6–0.8 in, females 1–1.5 in; 68–78 °F, pH 6.5–8.0, GH 5–20; 10 gallons ideal; up to 1.5 years; feed twice daily; jumpers; intake guards; can nip fins; risky with dwarf shrimp, finds shrimplets and molting shrimp; at least six fish); The Shrimp Farm, Red rili shrimp tank set-up and care (cherry-type Neocaridina: 65–85 °F, pH 6.2–8.0, GH 4–8); Practical Fishkeeping, What can I keep with shrimp? (build the shrimp colony first; many fish medications unsafe for invertebrates); Aquarium Co-Op, Cherry shrimp tank mates (small meat-eating fish go after shrimp). The shared water zone and the feeding inference are our own.