A vastly underrated fish, the Least Killifish (Heterandria formosa) is also the smallest fish found in North America. Native to the Lower Coastal Plain from South Carolina to southern Louisiana (Rosen 1979; Page and Burr 1991), this fish is actually a livebearer, not a true killifish. True killifish are oviparous, meaning that they lay eggs.
Because they are such a small species of fish, Least Killifish can be kept in smaller aquaria. Still, as they prefer schools/colonies, these fish have a minimum tank size of 5 gallons. Given their small size, Least Killifish also prefer tanks with low water movement as they will be battered around by a strong filter. In terms of water parameters, the Least Killifish is relatively undemanding. They will happily live in waters between 66 and 75℉ with normal pH levels (around 7.0).
Small and peaceful fish, Least Killifish prefer single-species tanks. They should not be kept in “community” tanks but may be kept with other small fish such as Endler's Livebearers and aquatic snails. Keep in mind that Least Killifish fry may be consumed by other species of fish (but not by their own kind).
These shy fish do best when kept in small colonies without other fish. They generally shoal (swim together), but mine school on occasion (swim together, facing the same direction). They spend more time in open water when they feel more comfortable and at ease. It may take a few weeks after introduction to a new tank before a group of Least Killifish feels safe enough to spend more time in the open. In addition, the presence of shelter is comforting to them—they tend to hover around large plants and shaded areas. Mine hover around a large Amazon Sword plant (Echinodorus grisebachii) and a decently-sized bunch of Windelov Java Ferns (Microsorum pteropus ‘windelov’). For best results with this fish, keep them in a heavily-planted, single-species tank.
When it comes to feeding, Least Killifish are unfussy, omnivorous eaters. Because of their small size, it is advisable to crush flakes and other larger food items prior to feeding. This small livebearer will accept both live and prepared foods.
A prolific livebearer, Least Killifish are easy to breed. In an aquarium with adequate water parameters and where both sexes are present, breeding is bound to occur. Mature females drop fry regularly as opposed to in defined groups. This is because they undergo superfetation, the “successive fertilization of two or more ova of different ovulations resulting in the presence of embryos of unlike ages” (source: Merriam-Webster Medical Dictionary). Newborn Least Killifish are relatively large and are capable of caring for themselves. In general, adults do not pose a threat to their young and specimens of all ages can peacefully coexist in the same tank.
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Thank you for reading this short care sheet on Least Killifish (published 2/5/17)! For more information, please browse around AquariumKids.com. Feel free to contact me at evanb [at] aquariumkids [dot] com with any questions :)