Least Killifish 101: Care, Diet, Tank Size, Tank Mates & More

The Least Killifish (Heterandria formosa) is a species of livebearer that is appreciated by hobbyists and abundant in nature. This species presents a beautiful contrasting color scheme that brings a wild and imposing aspect to the fish. It is a powerful and resistant fish. This species occurs in a region of North America.

It is a rustic and easy breeding species that is gaining more space within the aquarium hobby, mainly in Europe. However, it is still rarely found in stores or specialized breeders outside the United States.

In this article, all issues involving the Least Killifish and its maintenance in aquariums will be addressed and explained.

Least Killifish swimming in aquarium
Least Killifish (Heterandria formosa) by Brian Gratwicke (licensed under the Creative Commons Attribution 2.0 Generic license)

Species Summary

The Least Killifish is a tropical freshwater livebearer belonging to the Poeciliidae family. It is distributed over a wide area of the United States. 

The species is found near densely vegetated shores, in still or slow-flowing waters; it can even occur in brackish water environments. It hardly inhabits open areas. 

This species, like most killifish and livebearer, reaches an ideal size to be kept in aquariums; in the case of the Least Killifish, its diminutive size is the vital point to be taken into account in its maintenance. 

One characteristic that makes an impact on hobbyists is their rugged appearance, in addition to their unique behavior. They have been popular animals for years and a great fish species for beginners. 

It is still a relatively rare species worldwide despite it’s growing reputation. In recent years it has gained more and more popularity, and has begun to spread to different continents.

Generally speaking, the Least Killifish is a hardy and peaceful fish in aquariums, making it an excellent choice for beginner aquarists.

Least Killifish Care Guide

These fish are incredibly adaptable and can live in community aquariums with other peaceful fish of the same size without much trouble. Like other livebearers, Least Killifish must be kept in groups of at least six individuals of the species. They are found in large schools containing hundreds of them in the wild.

Least Killifish are shy, small, and beautiful fish that will show their best colors and behavior when given a balanced diet in an aquarium suitable for their species.

It is a resistant fish that is easy to care for but does not exclude us from an in-depth understanding of the species, its habits, and behaviors. Like any other species, the Least Killifish has its peculiarities, addressed and detailed in the following topics.

Tank Size

Least Killifish are relatively active, small-sized fish that spend most of their time swimming around the aquarium searching for food or trying to breed. They can be kept in tiny spaces, which makes them perfect for nano tanks; a 15″ x 11″ tank or something around 3.1 gallons is a good size to start with a pair.

Remember that, like most gregarious fish, the more individuals and the more space available, the better for the fish’s well-being.

Tank Mates

They are highly active and peaceful fish; they can present problems when kept in community aquariums because of their size.

Least Killifish are very small, making them an excellent food for larger fish. These fish should be kept with peaceful tankmates of similar size and habit. If you choose to keep it in a community tank it will do better with bottom fish and small delicate fish or even shrimp. Avoid keeping it with larger, territorial and aggressive fish.

Suitable tank mates for the Least Killifish include

Same Species Tanks

I recommend keeping only this species in the aquarium These fish are often kept as a single species in Biotope-type aquarium setups where the species’ original natural environment is imitated.

It is an incredible species to be kept alone in a very spacious aquarium in a large group. The unique behavior of these animals delights the eyes of any spectator.

Water Parameters

They are hardy fish that can withstand sudden changes in parameters and live in a safe wide range of parameters.

This livebearer can live very well in different parameters; the perfect temperature for its maintenance is from 68 to 82 F. The ideal pH range is between 6.5 to 7.5, and the hardness is 5 to 25 H.

This difference between hardness and pH is related to the region of origin of the specimen and demonstrates the variation of parameters during the dry and rainy seasons.

What to Put in Their Tank

As for any other aquatic animal, an aquarium heater and a filtering system are essential to keep the tank with Least Killifish healthy. The filtration system must be well dimensioned, creating a flow as light as possible since this species usually inhabits stagnant or slow-flowing water.

Given the rusticity of the species, it can inhabit different aesthetic styles. The aquarium’s decoration can be important for the species’ well-being. There should be lots of open space left for them to swim freely because it is a very active species.

Densely planted ponds help the population thrive, providing peaceful places to hide, breed and feed; similarly, hidings can be formed with a hardscape. They prefer well-lit aquariums.

Common Diseases

They are exceptionally disease-resistant fish. Keeping the water and the tank quality always in optimal conditions and a good quality diet, your fish should not present any kind of problem.

They will be susceptible to skin diseases and fin corrosion if kept in soft water or with acidic pH.

Food and Diet

This species, like other livebearers, is omnivorous, feeding on plant material and small invertebrates.

Its primary diet is based mainly on its natural environment’s worms, crustaceans, and plant matter. This fish accepts commercial feeds without problems in aquariums, being great eaters. Dry, live, and fresh foods such as daphnia, brine shrimp, bloodworms, mosquito larvae, and others should be offered regularly.

Providing a varied and balanced diet is essential for demonstrating its full potential. In general, they are fish that always seem to be hungry.


In an aquarium with all the correct parameters kept stable and with an ideal diet, Least Killifish can live for around two years.

In nature, these animals are prone to live less, as they are predisposed to diseases, attacks from other animals, and environmental causes.


The Least Killifish has the classic shape of a livebearer, usually olive green in color, with a dark horizontal band across the center of its body. It also has a dark spot on its dorsal fin and females on its anal fin. It’s the stereotype of a “small wild fish.”


Least Killifish can easily reach 1.4 inches in length when well cared for and fed. Usually, the average size we find in aquariums is around 1 inch.

Behavior and Temperament

It is a species of gregarious fish with peaceful behavior and can be kept in a community aquarium with smaller or the same size peaceful fish.

They are active animals, occupying practically all tank areas, and do not compete very well with other fish when feeding. Fights between males are common, but without physical harm to those involved.

The Least Killifish’s behavior can change if they don’t feel comfortable in the tank to be hidden in the vegetation.

Captive breeding is extremely simple, even naturally occurring, and is carried out commercially and in domestic aquariums worldwide.


It is an ovoviviparous species, where the female must be fertilized internally by the male. The eggs develop inside the mother until they hatch, and the chicks are born, already fully developed, eliminating the need to lay eggs somewhere.

Females can store sperm from males to fertilize later and produce fingerlings in installments over short periods. After the gestation period, which can vary from one to two weeks, females birth already formed young (about 40). There is no parental care. They are highly prolific and reproduce quickly. They are easy to breed fish, where spontaneous reproduction can occur in any domestic aquarium, as long as it is well taken care of.

Gender Differences: Male vs Female

There is apparent sexual dimorphism in adult animals. The male has a more developed dorsal fin and a gonopodium (anal fin adapted for reproduction, in the form of a rod).

Females are larger, more robust, and lack the gonopodium, having a standard anal fin.

Least Killifish Fun Facts

  • Considered one of the smallest fish globally, it is the smallest species of fish found in North America.
  • It is one of the few ornamental fish species from North America available in the aquarium hobby worldwide.
  • Although many commonly call it Killifish, it belongs to the poecilid family, which is considered a livebearer.


Leo Nico, Pam Fuller, and Matt Neilson. 2015. Heterandria formosa. USGS Nonindigenous Aquatic Species Database, Gainesville, FL.

Nelson, J.S., E.J. Crossman, H. Espinosa-Pérez, L.T. Findley, C.R. Gilbert, R.N. Lea and J.D. Williams, 2004. Common and scientific names of fishes from the United States, Canada, and Mexico. American Fisheries Society, Special Publication 29, Bethesda, Maryland. ix, p.386

Page, L.M. and B.M. Burr, 1991. A Field Guide to Freshwater Fishes of North America, North of Mexico. Houghton Mifflin Company, Boston. p.432

Schliewen, U.K., 1992. Complete Book of Aquarium Fish. Barron’s Education Series, Incorporated. p.159

Wittenrich, Matthew L., 2007. The Complete Illustrated Breeder’s Guide to Marine Aquarium Fishes.