Heterandria formosa, or the least killifish, are probably my favorite livebearer. They are hardy and easy to keep, so I highly recommend them to anyone looking for their first wild livebearer.
With their small adult size and relatively calm behavior, a group can comfortably live in tanks starting from 20 liters/5 gallons. Less than that would also work for a pair or to rear fry, if you're looking to quickly grow your swarm.
Not that you necessarily need to separate the young from the adults. In my experience, Heterandria formosa don't hunt their young.
My group has pretty consistently been around 12 animals strong, mainly because of a low number of females. Which recently resolved itself, there's plenty of young females reaching adulthood now. (April '25)
As for water parameters, anything outside of the extremes is suitable. A pH of around 7-8, with room temperature year round is perfectly fine.
A dense amount of plants is recommended. While peaceful, the large females especially can get a bit rough with each other and the males from time to time. Sight barriers and hiding spots within vegetation will help prevent some ripped fins. The smaller males may also spar, but are generally less intense with their fights.
The male's long gonopodium suggests a sneaky approach to mating. There is no courtship display; instead, the male will follow a female slowly and approach her from below to quickly insert his gonopodium. Quick business.
Heterandria formosa reminds me very much of a micropredator such as Boraras brigittae during feeding. They are most enjoyable to watch when getting fed live food such as artemia naupli, blackworms, and daphnia. Even blackworms multiple times their size are wrestled and consumed. In bite-sized pieces, of course.
They won't decline flakes or granules though, but live food is where it's at. ;-)
Written April 2025
Gallery
The first picture shows a young male with hypopigmentation. He did not make it to adulthood unfortunately. The next two are phenotypically normal males. Note the length of the gonopodiums.