Least Killifish: Ultimate Guide (Care, Diet, Breeding & More)

Least killifish swimming in an aquarium.

Despite the moniker of being the Least Killifish and being the 7th smallest fish in the world, this little fighter is a bundle of energy and curiosity.

The Least Killifish, Heterandria formosa also known as the Lesser Killifish, Dwarf Topminnow or Midget Livebearer truly has a personality greater than its namesake size.

It’s one of the most bred freshwater fish in the world but needs your help to ensure genetic diversity and healthy generations to come.

Their reproductive strategies are pretty unique to most fish and when kept in captivity, you’ll need to follow nature’s game plan to maximize their evolutionary potential.

It won’t cost you an arm and a leg though as they don’t need any special equipment beyond basics. They reproduce prolifically and easily integrate into the tiniest of spaces.

If you ever wanted a micro-scale farm operation this is ideal and certainly one of the easiest to become a successful breeder with.

Today we’ll examine the best practices and some common pitfalls on keeping these highly social creatures whether you are a newcomer, breeder, or seasoned fry-master.

Species Overview

The Least Killifish is not only one of the smallest fish commonly kept by aquarists but in fact one of the world’s smallest living vertebrates. Native to the south-east United States both sexes of these Goliath fish top out at just over an inch.

Generally green-colored with a mid-sectional horizontal stripe, they’re easy to identify once you know what you are looking for.

Tiny fish like these are easy to misidentify and there are dozens of known instances where pet suppliers have erroneously or dubiously sold fry of some other species in error.

CategoryRating
Life Span3-5 Years
FamilyPoeciliidae
Color VarietyGreen to khaki
Care LevelEasy
CompatibilityPeaceful with Many Types of Fish
Food and DietOmnivore
TemperamentDocile
Tank Size (Min.)5 Gallons
Tank SetupRocks and Plants in Freshwater
Size0.8 – 1.4 inches

In addition to having a visual reference, when purchasing these look for two more precise attributes. They almost always have a single deep brown to charcoal black stripe through the center of the body and a dark spot on the dorsal fin.

If they have multiple stripes or mottle, they are either not wearing standard-issue military gear, or they are not Least Killifish.

Least Killifish are from the Heterandria genus and are not true killifish. They may look similar in shape and anatomy but can be distinguished by their unique uniform coloration and diminutive size. They also tend to be less assertive around others preferring to explore shyly, yet happily in peace.

The most interesting facet of the lesser killifish are the number of evolutionary reproductive strategies it has adopted.

Above all they are viviparous or livebearers, giving their albeit very tiny offspring a better shot at survival as fully-formed fry versus defenseless eggs. Beyond that they have a unique way of both mating and gestating that adds to this mystique; more will follow on this when we speak at length about breeding.

Lifespan

In their natural habitat, these tiny fish can live up to 5 years. Unlike their annual cousins, these temperate-climate natives are built to survive over many changing seasons.

It has one of the greatest size-to-longevity ratios of any invertebrate. However, it also suffers from high sensitivity to inbreeding depression whereby genetic diversity leads to decreased size, health, virility, and lifespan of their offspring.

The most common strategy for increased longevity involves ensuring a regular turnover of Least Killifish in your tank. Meaning that you have virulent breeding males in good ratio to females and there is always an influx of new bachelors to the dating game in your tank.

Good breeding practices in an aquarium where their food and environment mimic their natural environment will maximize their longevity.

Appearance

Its green coloration with a single, mid-section, brown to black, horizontal stripe, and dorsal spot make them easy to identify.

Imposters hiding out in a large school of lesser killifish stand out like a Siberian tiger against the freshly laid snow. They’re pretty uniform with the main distinction between sexes being a secondary anal fin dot on the females.

Again given their small stature, juveniles of other fish are commonly mislabelled online and at local pet stores. Some people have taken home common guppies (Poecilia reticulate) or similar juveniles instead of the Least Killifish (Heterandria formosa).

When you’re looking to buy them just be sure to stick with reputable breeders or have credible pictures on hand as a reference to be sure.

Size

This is a truly dwarf species rarely eclipsing an inch. Like many other similar fish, the females tend to be about 10% larger than their male counterparts.

Overall it is considered a small-sized, temperate climate, freshwater fish. Although there seems to be some size adaptation by biome to live in increasingly smaller spaces this won’t be discernible to your average hobbyist.

They are however the smallest, readily-available commercial fish you can buy. If you’re stuck in a tiny flat where pets aren’t normally possible you can add the Least Killifish to your list of plausible roommates. They espouse the tiny home spirit and live just as freely with hardly any maintenance at all.

Least Killifish Care

Least Killifish are very easy to care for and make great fish for those just starting out in the art and science of fish breeding. Generally, they are resistant to most diseases and make themselves home in a variety of water conditions. Since they naturally live in backwater pools or slow-moving bodies of water this is pretty easy to simulate.

The most important thing is that they have a habitat as similar to their natural conditions as possible which is conducive to exploration and play. They are very active and need space to explore and engage socially with others.

With the correct balance of aquarium size relative to the number of fish, proper feeding, and regular water and tank maintenance, you’ll all be engaged for years to come.

Tank Size

They are very small fish which makes their relative footprint quite small compared to their larger cousins. A 10-gallon tank for a pair would make a pretty swanky penthouse, but a single specimen would be happy with a half-sized sublet.

Although they are small and pretty easy to house, always plan for contingency space for rocks, vegetation, and other curiosities to include for them to explore.

Water Parameters

These resilient animals and their killifish cousins have been found surviving, if not thriving in some pretty strange, sometimes toxic environments. This is where their size is a handicap as lethal doses affect them proportionately.

It takes very little toxin to kill them due to their size, but they are able to tolerate far greater doses relative to body mass than many fish or animal species.

Despite these small challenges their water parameters are still undemanding, to say the least, handling variable water PH balances, temperatures, and degrees of salinity.

Ideally, you want to aim for a tank between 68-80F, with just slightly alkaline, salt-free water (PH near 7.7) to mimic the ideal conditions they would experience in nature.

What to Put in Their Tank?

The good news is the lesser killifish won’t want a current or need a heater so what you save on these factors can be put towards more stock or replicating their habitat. This also means a bigger budget for cool things to put in their tank.

You’ll want to give them places to explore, hide, mate, or duel if they wish. Incidentally, since they also like to jump you’ll want to source a decent lid before filling the actual tank with goodies. Once you’ve got the basics, add a substrate on soil and rock bed for your tank.

Next, add plentiful plant life to the aquarium; not only will it provide the occasional nosh, but also serve as hiding spots and ornaments for any needed retreat.

Adding sufficient plant life to your aquarium cannot be under-emphasized for the overall health of your fish habitat. By adding a few floating plants you’ll also be mimicking subdued lighting conditions where they would naturally hideout.

Although lesser killifish may eat your plant decor as a food source of last resort, a well-planted aquarium can help maintain both water quality and animal health overall.

You’ll have to clean the tank less often and inadvertently give your killifish the ever-changing seasons it expects from a natural environment.

Common Diseases

Like similar freshwater species, Least Killifish have excellent immune systems, but because of their size can be more prone to certain endemic illnesses.

The most problematic illnesses they suffer come from parasites like tapeworms, fungi, and bacterium like Flavobacterium columnare that causes Columnarius.

Columnarius is a highly contagious, often fatal bacterial infection that is often mistaken for a fungal attack. The most obvious signs are frayed and ragged fins or skin loss. This is particularly acute near the gills leading to poor oxygen uptake and lethargy.

Another problem you may encounter is fluke, a bacterium harbored in snails that can also harm your Least Killifish. The bacteria attack the brain which quickly affects the nervous system. Affected individuals may swim near the surface or inverted as they slowly lose motor control.

All these conditions will be readily visible to you. You’ll know your lesser killifish are sick by observing their physical health and behavior. If there is skin degradation, they aren’t eating, particularly lazy, or floating upside down something is amiss. When in doubt, check it out and speak with a veterinarian or reputable pet store operator.

Keeping the bacterial, fungal and other dangers in control can also be done by practicing diligent cleaning of your tank and regular water changes.

While having a clean healthy tank is your best bet at prevention if they do get sick, always get the therapeutic medications or treatments recommended by animal professionals.

What Do Least Killifish Eat?

If you have an extra space at your table your Least Killifish is probably happy to join. They will eat almost anything; anything they can fit in their mouth that is.

The diet of the Least Killifish normally consists of microcrustaceans and plant materials, so best to give them the food they would find at home versus your holiday leftovers.

Stick with the standards: aquatic insects, plants, and micro-organisms. Pet store-bought aquatic snacks (crustaceans, larvae, and worms) can be interspersed with small fish flake food to formulate an overall-balanced and varied diet.

They’re not picky but at least try and make it a challenge or interesting for them to hunt or gather their food.

Behavior & Temperament

If you have a young, aspiring aquarist at home this is a perfect fish to start with. Though it can thrive or at least tough through neglect their gentle nature and amicability make them an easy addition to any live-bearer community tank.

Lesser killifish are docile at most times and spend their day exploring the full column of their homes. They tend to avoid the surface except to feed since in nature that is when they are most vulnerable. If they spend long periods at the surface be sure to check for illnesses as this is not common behavior.

Generally, they will mix with others and only become mild-to-assertively territorial during mating disputes. Like many fish, they can become stressed in the absence of food or a shortage of plausible mates. Whenever plausible try and keep up with both pressures to avoid unnecessary drama.

Least Killifish Tank Mates

Your lesser killifish will neither harass nor predate other tank mates. They avoid boxing outside of their weight class and given their small stature are not going to bully anyone. That said it is important that they can be perceived as prey or food by others.

Herbivorous fish work best for this reason and the less territorial the better. The most commonly kept killifish companions are:

  • Dwarf rasboras
  • Smaller killifish (like the Bluefin Killifish)
  • Japanese rice fish
  • Pencil fish
  • Pygmy sunfish
  • Rosy loach
  • Other pygmy herbivorous fish

Although bottom-feeders and cleaners like dwarf corydoras or cleaner shrimp can certainly be added they will compete with your lesser killifish for algal growth.

You’ll never have an easier time cleaning the tank with these janitors around, just know they will compete for some of the same food sources.

Breeding

Lesser killifish breed prolifically and rarely need encouragement to reproduce. When they do decide to mate, you’ll see that they’ve adopted some highly-unusual reproductive adaptations.

Nearly invisible to the human eye, the male Least Killifish will approach the female and then swish his anal fin ever so so gently in her direction. Special tubular spines located on this fin quietly deposit the sperm in only a few swishes of the tail.

As with other live-bearers, early development of your new lesser killifish clan will happen inside their mothers. After a brief gestation of a few days, the gravid (pregnant female) will begin to slowly release her fry. Kind of like a time-release medication she’ll release the brood only a few at a time over several weeks.

This strategy ensures that if in nature some were born during an inopportune time like heavy storms or when predators are plentiful, only a few offspring would be lost at that time. This is also due to their overall size limitation. It is only possible for your female lesser killifish to keep so many developed offspring internally.

This means all of them are not ready at once through concurrent development rather than collective growth called superfetation. Through all of their specialized reproductive adaptations, they breed both prolifically and easily, but there is an evolutionary trade-off to their unique strategies.

They easily suffer inbreeding depression and their actual genetics weaken in the absence of new lesser killifish mates. Just be certain to turn over your stock regularly to keep them diverse and the flames of love aglow.

A final word on breeding, please do not just go out and grab your stock from any water hole if you reside in their natural range. Parasites have often been associated with wild-caught live-bearers and can add unwarranted trouble to an otherwise healthy population.

Furthermore, you don’t really know how healthy these new additions are so best not to add poor genetic diversity.

Final Thoughts

If you thought that care and upkeep might deter you from keeping fish as pets, to breed, or simply enjoy we hope this article has helped to dissuade any concerns.

Least Killifish are easy to keep, low maintenance, and highly affordable, a great combination for a beginning breeder or just an aquarium enthusiast!

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