Least Killifish (Heterandria Formosa) – Species Profile & Facts

Disclosure: I may earn a commission when you purchase through my affiliate links. As an Amazon Associate I earn from qualifying purchases. – read more

Least killifish, or the Heterandria Formosa, is a livebearer fish that is one of the best fish to have in your tank for many reasons. This small freshwater fish is a very friendly fish that will not disturb your tank too much and you can add it to any community tank as well.

In this article, we will take a complete guide of the least killifish. We will take a look at the specifications of the least killifish, what is its natural habitat, what tank conditions it strives in, what tankmates are best for least killifish, how to breed them and also we will take a look at feeding the killifish – what type of food it eats and how often you should feed it.

Generally, the least killifish is a shy fish that will spend most of its time hiding in safe spots such as rocks and vegetation but will also be found swimming in the open water a lot. At first, it might be a bit shy when it is first introduced in the aquarium, but it will build trust over time and become a very nice fish in your tank.

Let us take a look at this complete guide about least killifish and I hope that it helps you with your decision of whether to get one and also to help you keep it happy and maintain it properly.

Least Killifish Habitat


Least killifish is a freshwater fish that is also often found in brackish waters. It is one of the rare freshwater species that are found in the United States, where the natural habitat of the species is.

It can be found all the way from South Carolina, Florida, Georgia to Louisiana. The most often place to find it is Alabama, especially in the blackwater habitats of South Pine Hills and Coastal Lowlands.

It is known to survive in very different conditions in these areas. The temperature of waters in the habitat of the least killifish can vary all the way from 10 degrees Celsius to 32 degrees Celsius (50-90 degrees Fahrenheit).

It is mostly found is slow-moving waters with a slow current where they feed on microcrustaceans and plant material.

It is also highly sustainable in these habitats, as it can feed on water fleas and survive from that alone. This makes them perfect for a wide range of settings in aquariums.

They are regarded as more tropical fish, but they do live in different conditions in the wild, so it is not obligatory that they have strict conditions.

Least Killifish Water Parameters

As stated above, the least killifish can survive in some very different water conditions. However, to keep the fish healthy, you should aim to provide the killifish with the right water environment and water conditions.

This will enable them to live longer and also be happier in your tank in general. Although they do lean to more tropical settings, the temperatures, and the ph can vary slightly for least killifish; they are quite adaptable and hardy fish.

The least killifish is not a very demanding fish in terms of water parameters. The ideal temperature of the tank water should be around 66 to 75 degrees Fahrenheit (18 to 24 degrees Celsius). But it can survive in different water temperatures, varying from 50 degrees Fahrenheit to 90 degrees Fahrenheit.

In terms of ph of the water, the ideal ranges for acidity should be about 7.0 to 8.0 ph, but it is not exclusive to that. Some waters they live in the natural habitat often vary in acidity due to degrading plants. But the ideal ph should be about neutral to slightly basic.

Feeding Least Killifish

Feeding Least Killifish

Feeding Least Killifish (sourceCC BY-SA 4.0)

Although the least killifish are deemed to be omnivores, which means that they do tend to eat all types of food. However, they are more inclined towards eating meaty foods, so they can also be considered to be carnivores.

The least killifish will happily eat crushed flakes and similar foods and will not be too fussy about it. Ideally, you should keep the food varied. Least killifish do like very much to eat newly hatched brine shrimp, daphnia, mosquito larvae, bloodworms, and more similar foods. You can also feed the flakes, although it is better to keep feeding them meaty foods.

In terms of feeding patterns, you should aim to feed your killifish once or twice a day. But do not overfeed them. If you have shrimp in your tank, you might want to be careful because least killifish can attack them and prey on them in numbers. For this reason, it is advisable to feed them shrimp. Especially the brine shrimp often, because this is the favorite food of the least killifish.

Other considerations for feeding are the black worms, white worms, and also the least killifish will likely feed on algae, although this does not represent a large portion of the diet of the fish. For feeding the fry, they prefer to eat newly hatched brine shrimp and micro worms. This will enable them to grow into healthy fish.

Breeding Least Killifish

Breeding Least Killifish

Breeding Least Killifish (source)

Least killifish are known as being quite easy to breed. It all depends on certain specifications and water conditions, but in general, they are considered as very avid breeders. They are livebearers, which means that the females carry eggs in their bodies. They spawn and drop the fry from their bodies at random rates.

You do not need to put too much intake when it comes to breeding the killifish. All you need to provide is perfect tank conditions and male and female killifish in the same tank. Then breeding will occur naturally.

When the fry is spawned, you do not need to worry about the parents consuming it; however, if you do have a community tank, then it is best to develop the fry in a separate tank to prevent them from getting eaten.

Breeding is quite easy, but some people have reported that they have been unable to develop their own killifish; it might be because of bad tank conditions. The females cannot carry a large amount of fry due to their small size.

Least Killifish Tank Mates

Least Killifish are known as small creatures that will not get in the way of other fish; in fact, they are very peaceful fish in relation to other peaceful fish in the tank. So, you should aim to pair the least killifish with other small and peaceful fish.

The fish that are best compatible with the least killifish are corydoras, otocinclus catfish, endler’s livebearers, guppies, freshwater shrimp, all types of snails, sunfish, mollies and other similar types of fish.

One important thing to keep in mind with the least killifish is the fact that they might be intimidated by larger and more aggressive fish in the tank. Which is only natural due to their size. Do try to not keep them with such fish in your tank. Or you might end up having least killifish that are squeezed out of their habitat. Or even potentially hurt or eaten.

They tend to be generally shy fish, so they might be hiding when you introduce them to the tank mates a lot; they will develop a good relationship with them over time, though, and they are perfectly fine with tank mates.

Least Killifish Life Span

How long the least killifish live is dependent on more things: natural predisposition to diseases of a certain fish, genetic properties, but also, more importantly, the living conditions and the quality of life you provide them. It depends on you as well how long the fish will last in your aquarium.

In general, the least killifish can live for up to 5 years. However, this number can vary significantly; it can be even as low as 2 years if the catch a disease or do not live in proper conditions.

For this reason, try to provide them with quality conditions and buy them only the high-quality foods and do not feed them flakes only. Try to keep variety in their diet.

This will prevent diseases and stop health deterioration, but also it will keep them happier, which will enable you to see them in their best shape.

Least Killifish Adult Size

Least Killifish are very small fish. When they are born, they are very small. This makes them more fragile, and it is thus better to keep them in a separate tank. When the least killifish grow up – this process can take to up to a few months – they can grow to up to 1.5 inches.

The sizes vary, but it is generally the case that females tend to be larger than the males; the females reach up to about 1.5 inches in size, while the males grow to only about an inch.

So you can distinguish the males and females by their sizes alone. As we said they are very small fish, and they will not get along with larger fish in your tank. In fact, they can quickly become a quick snack for a larger, predatory fish.

Make sure that it has appropriate tank mates that will get along well with the least killifish and will not threaten them.

In terms of the number of killifish in your tank, they do prefer to live in groups; this means that you should keep more killifish together in your tank, possibly 4, 5 or even more. Make sure that you have males and females mixed, preferably more females than males to ensure better breeding conditions.

Least Killifish Aquarium Size

If we consider the size of the killifish, then it is probably best to say that a 5-gallon tank is the absolute minimum tank requirement for the killifish. A more appropriate size for the killifish would be a 10-gallon tank, especially if you intend to keep more of least killifish together.

But they are not limited to smaller tank sizes like 5 and 10 gallons. They will live and strive perfectly well even in a larger aquarium, but for this, they should be kept in larger numbers to keep them comfortable, but also the tank conditions should be perfect for the killifish.

Firstly, there should be no larger and predatory fish in the tank that could feed on the killifish; secondly, the tank conditions and water parameters should suit the killifish.

In terms of the aquarium settings, there should be plenty of hiding spots for the killifish due to their shy nature. Try to keep plenty of vegetation in the tank. Cryptocorynes would be a good idea, also java moss and java fern. Another consideration for tank set up is to have a closed lid over the tank without small holes, as the killifish can jump through them.

Are Least Killifish Rare?

In general, in the natural habitat, least killifish are considered to be “least concern” according to the IUCN Red List of Threatened Species. So they are quite often found in North American freshwaters and brackish waters of Alabama and neighboring counties. They are considered as the smallest freshwater species in the United States. Which is why they might be considered as special or unique.

But this feature alone does not make them a rare species; yes, they have this unique trait of being the smallest, but they are not inherently rare as some other fish species are.

In the fish keeping world, they are not often seen in aquarium setups and home tanks, which is why they might be considered rare by some people. In terms of their numbers they are not rare as such, but in terms of their occurrence in home aquariums, they can be considered a rarity.

To conclude, they are not rare in their natural habitat and are considered as “least concern”, but in fish keeping world, they are quite rarely found in home aquariums and tank setups. We could say that it is rare in some aspects such as their size, though.

Can You Crossbreed Least Killifish with Guppies?

Some people don’t like crossbreeding, and some love it to come up with new species that have been crossbred. The answer is that you possibly can crossbreed the least killifish with guppies. But you have to maintain the right tank conditions to do so.

For people that are into crossbreeding fish, then you certainly can try to do that with least killifish and guppies. The results you will see for yourself; some will breed, while others will not. You can expect mixed results when you try to do this. It has reportedly worked for some people, while some people have never had results of doing this.

Where Can You Buy Least Killifish?

You can buy the least killifish now at your local butcher shop. No, just kidding. You can probably buy least killifish at every shop that sells fish or a specialized animal shop with fish.

Whether they will have them or not is another matter. But most of them tend to have them. But you might be out of luck with some as they are not the fish that are the most demanding in the fish keeping world. Your best bet would be to try a specialized fish shop to see if they have least killifish.

Are the Least Killifish Aggressive?

No, least killifish are not aggressive at all. In fact, they are known as some of the friendliest species out of all species out there. You cannot expect any trouble whatsoever if you decide to own these fish. And they will get along very well with other friendly and small fish in your tank.

In fact, they are known as quite shy fish. Especially at the beginning or when you introduce them to new fish. The only time they can turn aggressive is when you have small, lone shrimp in your tank and they can turn on him in numbers, but this does not occur very often.

They are peaceful species, but do try to keep them away from larger fish that are predators; they can quickly become prey to those fish and get eaten. Otherwise, they are peaceful and shy fish that will make your tank more interesting for sure.


Least killifish is a very good species to have for beginners. These small fish are very peaceful, do not require much maintenance and are very good breeders as well.

These are the reasons to keep least killifish in your tank. Not only that, but they are also good-looking, and are very adaptable too.

Originally found in freshwaters of the United States, especially in the county of Alabama. The least killifish is a very interesting fish because it is considered as the smallest freshwater fish in all of the United States.

To keep them happy, you should take proper care of them. And pair them with the right tank mates for them. Hopefully, you have learned all about the least killifish in this article and you know the specifications for keeping them. Good luck and have fun with the least killifish.

Author Image Fabian
I’m Fabian, aquarium fish breeder and founder of this website. I’ve been keeping fish, since I was a kid. On this blog, I share a lot of information about the aquarium hobby and various fish species that I like. Please leave a comment if you have any question.
Leave a Comment

Your email address will not be published. Required fields are marked *