Heterandria formosa – A Tiny Livebearer

This little fish is native to the Southeastern United States. It is commonly called “The Least Killifish” or “Mosquitofish”. It’s main claim to fame lies in it’s extremely small size, full grown males are less than an inch long, females are larger, but not by much.

Although commonly called “Least Killifish” Heterandria are not really killifish. They are a livebearing fish like guppies and mollies. Other nicknames for this little fish are “hets” and “skeeter fish”.

As you can see the colors are mostly shades of gray, but they are attractive. A male is pictured above, on his way after the female kind of out of focus above him. The males and females look about the same and usually have a dark shiny gravid spot around the stomach. There is another female at the bottom center of the picture, and a Killifish (Lucania Parva) to the left. (they look very simular to Heterandria) I haven’t been able to catch him yet without destroying the plants. Females drop their young 2 or 3 at a time daily for several days, and seem to be continuously pregnant. Although they don’t guard their young, parents hardly ever try to eat them, so the fry are safe to raise in the same tank.

Heterandria Pool
Typical habitat of Heterandria formosa.
In this pond they share water with Blue Fin Killies,
Ellasoma zonatus (Pygmy Sunfish) and Ghost Shrimp.
© Clint Norwood

Heterandria formosa are very hardy. They prefer water that is close to neutral pH and medium hardness, but they will adapt to most aquarium conditions without a problem. Temperatures between 68-85°F (20-30°C) are fine as long as temperature changes are gradual. They will eat just about any of the common fish foods like little pigs and are one of the few fish whose fry will prosper on nothing more than common flakes. Although as with any fish a varied diet with live or frozen foods will make them happier. “Skeeters” rival the common guppy in ease of keeping. Get a pair and you’ll soon have a tank full.

I have found that Heterandria make wonderful community tank members with other non-agressive fish. They are not agressive or bothersome to other fish at all. I curently have them in some species tanks where they add interest to an otherwise inactive tank. Heterandria are very versatile, always active and fun to watch. One of the smallest aquarium fish available you can house up to 10 in a one gallon tank. Another benefit is their wonderful appetite, put some hets in a tank of finicky fish and they will teach the other fish how to eat.

In case you haven’t figured it out, Heterandria are one of my most favorite fish.