News
SpeciesScientists Discover Mysterious New Fish in the Red Sea – and Have Never Seen It AgainOceanEye to eye with Britain's hidden sharks at SharkAdemy - OceanographicRiversIn October, biologists surveying a New York harbor for invasive fish pulled up a bright yellow one. It wasn't an invasive species at allOceanShedd Aquarium Sends Shark Eggs to Bolster Wild PopulationsSpeciesA rare fish has been found in Lake Ontario, North America, whose entire body is golden. It is a nati..OceanThe Great Sea Reef Is Fiji’s Wildest Dive, and a New Frontier for ConservationOceanEckerd College professor contributes to designating Biscayne Bay an Important Shark and Ray AreaSpeciesRio Verde Grande Survey Identifies Fifty-Three Fish Species Facing Flow ReductionsFisheriesAdams River Salmon Run in Thompson-NicolaIndiaHilsa production dropped 26% in five years, fisheries minister tells parliamentSpeciesScientists Discover Mysterious New Fish in the Red Sea – and Have Never Seen It AgainOceanEye to eye with Britain's hidden sharks at SharkAdemy - OceanographicRiversIn October, biologists surveying a New York harbor for invasive fish pulled up a bright yellow one. It wasn't an invasive species at allOceanShedd Aquarium Sends Shark Eggs to Bolster Wild PopulationsSpeciesA rare fish has been found in Lake Ontario, North America, whose entire body is golden. It is a nati..OceanThe Great Sea Reef Is Fiji’s Wildest Dive, and a New Frontier for ConservationOceanEckerd College professor contributes to designating Biscayne Bay an Important Shark and Ray AreaSpeciesRio Verde Grande Survey Identifies Fifty-Three Fish Species Facing Flow ReductionsFisheriesAdams River Salmon Run in Thompson-NicolaIndiaHilsa production dropped 26% in five years, fisheries minister tells parliament
EveryFish
Flagfish

Flagfish

Small Florida pupfish with a US flag pattern

The flagfish is a small killifish native to Florida freshwater habitats. Described in 1879, it is known for its unique appearance and role in controlling algae in aquariums.

Share on WhatsApp

  • Endemic to the Florida Peninsula
  • Males resemble the US flag in colour
  • Lays one of the lowest egg counts
  • Effective consumer of hair algae
  • Described by Goode and Bean in 1879

What it is

This robust fish reaches six centimetres in length and features a blunt snout resembling that of a bulldog. Its fins are rounded and set far back on the body, sitting next to the tail fin. Dominant males display a striking pattern of dark rectangles and red stripes that evoke the United States flag. Females and younger fish are less colourful, showing olive bodies with turquoise scales and faint markings.

Where it lives

The species is found only in the Florida Peninsula. It lives south of the St. Johns and Ochlockonee River drainage basins. These fish prefer shallow waters with plenty of plants, such as marshes, canals, and ditches. While they primarily inhabit freshwater environments, records show they can also survive in slightly brackish conditions.

How it lives

Flagfish are omnivores that hunt small invertebrates and zooplankton. They also graze on algae and plant matter, making them effective at controlling hair algae in tanks. Their behaviour includes complex interactions during feeding and breeding, though they do not form large schools in the wild.

Breeding and life cycle

Females lay very few eggs, often just twenty over several days, a record low for fish. Courtship involves the female nipping the male to encourage pursuit, followed by a backward swim and a male backflip. The male typically guards the eggs against intruders, although he may sometimes consume them or the young.

Fish and people

Aquarists value this species for its ability to eat stubborn hair algae. It is a popular choice for planted tanks due to its small size and specific dietary habits. The common name refers to the male's patriotic colouring, which distinguishes it from other killifish in the hobby.

Questions people ask about Flagfish

Why is it called the flagfish?

The name comes from the dominant male's body pattern. It features a dark rectangle and red stripes that look similar to the stars and stripes of the United States flag.

What do flagfish eat in an aquarium?

They are omnivores that feed on small invertebrates and zooplankton. They are particularly useful for eating algae and plant material, including hard-to-remove hair algae.

Do flagfish care for their young?

Males often guard the eggs after spawning and attack nearby fish. However, some observations suggest they may also eat their own eggs or offspring, indicating variable parental care.

Good reading and sources

Where this page comes from

The facts on this page come from the Wikipedia article "Flagfish", written by its contributors and used under the CC BY-SA 4.0 licence, and from its Wikidata record (CC0). EveryFish rewrote the text in its own words, with AI assistance, and compares every page with the article to make sure no passage is copied. It may be reused under the same CC BY-SA 4.0 licence with a link back.

It may contain mistakes, and figures change over time. Nothing here is investment or buying advice. If something is wrong, tell us. Last refreshed 2026-11-15.

Like Flagfish

All killifish and livebearers
🇻🇪 Venezuela and 4 moreGuppy Small tropical livebearer known for bright male colours and rapid reproduction.Livebearer · Rivers and lakes · 1859 🇲🇽 Mexico and 1 moreGreen swordtail A hardy live-bearing fish popular in home aquariums.Livebearer · Rivers and lakes · 1848 🇺🇸 United States and 1 moreSailfin molly Small livebearing fish with a large dorsal fin in males.Livebearer · Rivers and lakes · 1821 🇧🇷 Brazil and 1 moreLargescale four-eyes Surface-dwelling fish with split eyes for dual vision.Livebearer · Rivers and lakes · 1758

More from Cyprinodontidae

All of them
🇺🇸 United StatesDevils Hole pupfish A tiny fish found only in one cavern in Nevada.Other · Rivers and lakes · 1930 Tecopa pupfish 🇺🇸 United StatesTecopa pupfish Extinct subspecies of killifish from California hot springs.Livebearer · Rivers and lakes · 1948 Catarina pupfish 🇲🇽 MexicoCatarina pupfish A tiny Mexican spring fish that vanished in 2014.Other · Rivers and lakes · 1972 🇺🇸 United States and 2 morePupfish Small killifish known for surviving in extreme isolated waters.Livebearer · Rivers and lakes

Photo credits on this page

The small pictures above come from Wikimedia Commons under free licences. Some are cropped to fit.

The weekly digest

Every Monday: the fish of the day, new pages on EveryFish, and the conferences and anniversaries of the week ahead.