Quick Stats
| Adult Size | 5–6 cm |
| Minimum Tank | 20 gal |
| Temperature | 16–24°C (unheated) |
| pH Range | 6.5–8.0 |
| Hardness (GH) | 8–25 dGH |
| Difficulty | Easy |
| Temperament | Peaceful (males can be territorial) |
| Diet | Omnivore/Herbivore — algae, flakes, pellets, blanched vegetables, frozen foods |
| Schooling | 1 male + 2–3 females, or 1 alone |
Tank Setup
American Flag Fish (Jordanella floridae) are native to Florida and the southeastern United States. They are one of the few killifish that thrives in cool water — 16–24°C is ideal, making them a perfect unheated-tank fish. A 20 gallon tank is the minimum for a trio (1 male, 2 females).
Water parameters: temperature 16–24°C (unheated in a normal house is ideal), pH 6.5–8.0, hardness 8–25 dGH. They prefer harder, slightly alkaline water. They adapt to a wide range and are unfussy about chemistry as long as the water is clean.
Set up the tank with a dark sandy substrate, driftwood, hardy plants (Java fern, Anubias, Vallisneria), and floating plants for cover. Males establish small territories around plants and decorations, so visual barriers help reduce aggression. Moderate filtration is fine — they don't produce huge waste. A lid is essential (killifish are jumpers).
Tank Mates
American Flag Fish are peaceful community fish with one caveat: males can be territorial with each other and with similar-looking fish. The standard harem setup (1 male, 2–3 females) prevents male-male aggression. Two males in the same tank will fight unless the tank is very large (55+ gallons) with sight breaks.
Compatible tank mates: White Cloud Mountain Minnows (perfect cold-water overlap), Rosy Barbs, Zebra Danios, Cherry Shrimp, Amano Shrimp, Japanese Trapdoor Snails, and other cold-water community fish. They are also compatible with fancy goldfish in a large tank.
Avoid: other male killifish (territorial conflict), long-finned slow-moving fish (male flag fish may nip fins during territorial displays), and tropical species requiring 26°C+. Small shrimp (cherry shrimp fry) may be eaten — adult cherry shrimp are usually safe.
Diet & Feeding
American Flag Fish are omnivores with a strong herbivorous streak. They eat algae (including hair algae and black brush algae that few other fish touch), flakes, micro pellets, blanched vegetables (zucchini, cucumber, spinach), and frozen foods (bloodworms, brine shrimp, daphnia).
Feed once or twice daily. Include vegetable matter in every rotation — a flag fish without algae or vegetables to graze on will become pale and prone to digestive issues. Sinking algae wafers and blanched zucchini are reliable staples.
They are one of the few fish that will genuinely control hair algae and black brush algae (BBA). A group of 3–4 in a 20 gallon tank will demolish a hair algae outbreak within weeks. They won't eat all algae types (they ignore green spot algae on glass), but for hair and BBA they're among the best options available.
Common Health Issues
American Flag Fish are extremely hardy when kept cool. The main health issue is stress from overcrowding — males kept in too-small tanks become aggressive and the loser stops eating, wasting away over weeks. The fix is always more space and more females per male.
They are susceptible to ich when stressed by temperature swings or poor water quality. The standard heat-and-salt treatment works, but keep the treatment temperature under 26°C — flag fish don't tolerate heat well.
Lifespan is typically 2–4 years. They are short-lived for a fish their size (typical killifish lifespan), but they breed readily in captivity so the population persists. The species is farm-raised in Florida for the hobby, so captive-bred juveniles are widely available.
Breeding
American Flag Fish are egg-scatterers and easy to breed. They spawn in planted tanks, scattering adhesive eggs among plants or spawning mops. Unlike many killifish, they do not bury their eggs — they scatter them on plants and substrate.
Sexing: males are larger, brighter (red, white, and blue colouring), and have a larger dorsal fin. Females are smaller and drabber with a dark spot on the side. To breed deliberately, condition a trio (1 male, 2 females) with high-protein frozen foods for a week, then add spawning mops or fine-leaved plants (Java moss, guppy grass).
Spawning happens over several days — females scatter 10–30 eggs per day. Remove the adults after 4–5 days (they will eat eggs and fry). Eggs hatch in 7–14 days depending on temperature. Feed fry infusoria for the first 3 days, then microworms and baby brine shrimp.
Frequently Asked Questions
Do American Flag Fish need a heater?
No — they prefer 16–24°C and thrive in an unheated tank. They are native to Florida and adapted to seasonal temperature swings. Sustained temperatures above 26°C stress them and shorten their lifespan.
Will American Flag Fish eat hair algae?
Yes — they are one of the few fish that will eat hair algae and black brush algae (BBA). A group of 3–4 in a 20 gallon tank will demolish a hair algae outbreak within weeks. They won't eat green spot algae on glass, but for hair and BBA they're among the best options available.
Can I keep two male American Flag Fish together?
Not recommended. Males are territorial and will fight unless the tank is very large (55+ gallons) with sight breaks. The standard setup is 1 male with 2–3 females (a harem), which prevents male-male aggression.
Are American Flag Fish killifish?
Yes — they are a North American killifish (family Cyprinodontidae), native to Florida. Unlike most killifish (which are annual spawners from temporary pools), American Flag Fish are perennial and live 2–4 years. They are one of the few killifish that thrives in a standard community tank.