Least Killifish – Habitat, Care, Feeding, Tank Size, Breeding

Least killifish, which are also called dwarf top-minnows or lesser killifish, is part of the Poeciliidae family. They are not only the smallest livebearer in the world but also one of the tiny fish species in the world.

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The females are just about 1-inch long. Likewise, males might be the smallest vertebrate in the world. Small as they may be, they are lively, social, and friendly. They can also adapt easily to most water types, so they are ideal fish for those with smaller tanks or beginner breeders.

Aquarists have cultured least killifish for their tolerant nature and diminutive size. There are only a few fish that will give you many opportunities than the least killifish. 

So, if you have a soft spot for least killifish, we are going to cover important aspects of this fish, including how to take care of them.   

Least Killifish Natural Habitat

The least killifish are found predominantly in freshwater. However, they can also survive in brackish environments. In their wild, least killifish are usually found in sluggish waterways or stagnant ponds, within marginal plants or heavy vegetation.

They live along weedy margins of plant-filled habitats of the coastal lowlands of Alabama and South Pine Hills. 

Because they are easy to catch, it can be tempting to collect them yourself. But it is important to respect local regulations by obtaining necessary permits.

And if there aren’t local restrictions, you want to have a stock of medicine to deal with any parasites, which are usually associated with wild-caught livebearers.   

Least Killifish Tank Requirements

Because of their diminutive size, these fish can be kept in small aquaria. They can also thrive in most aquatic conditions, but you still need to maintain the minimum tank requirements. If you are keeping them alone, a 5-gallon aquarium can accommodate these fish, as they are tiny.

But if you plan to keep a larger colony or introduce some tank mates, then you would need a bigger tank. In this situation, a 10-gallon tank can be enough to start with. You don’t want to suffocate them too much since these guys aren’t good jumpers. In fact, you can keep them without a lid. 

With that said, I wouldn’t recommend keeping them in a bigger tank because they might find it hard to locate food. Moreover, you would want to pollute your tank with a lot of food. 

Besides size, you also need to take care of the decor. You will probably need a lot of plants. But make sure not to use something like spider wood, which is known to ups the pH level.

You can use limestones, crushed coral, or Texas holey rock to stabilize the pH level. Dragon stones are also a good option since they can provide hiding spots. As for substrate, use sand or river stones. 

The best plants for livebearers are bushy and fast-growing plants, especially if you are keeping a colony. Bushy plants are really helpful to fry because they provide hiding spots.

These plants also help adults least killifish to feel secure and isolate themselves when they are stressed. Good choices of fast-growing and bushy plants are water Wisteria, Guppy Grass, Hornwort, and Java Moss.  

Don’t forget to provide the right lighting. These fish thrive in subdued lighting, but they are opposed to light as most fish species. To subdue the light, you can introduce floating plants. 

Least Killifish Water Conditions

Least killifish love it when they are kept in slow-moving water. So, using a slowly bubbling sponge filter will provide the right environment, especially in small tanks.

They thrive well in room temperature of about 68 – 80 °F (20 – 27 °C), but they can also withstand slightly harsh temperatures ranging from 50 to 90 °F (10 to 32 °C). Since they can live well at room temperature, you might not need to use a heater.

It might only be useful during the winter months. In terms of water hardness, keep it in the range of 5 – 20 dKH. Likewise, maintain a pH level of 6.5 – 8.0. 

Since they are dwarf livebearers, they can withstand a good deal of mistakes. But this should keep you a green light to neglect them.

At least change water for them each week, but you can get away with twice a month, especially if you have a lightly stocked, well cycled, and well-planted aquarium.

If you are using hard alkaline water probably from the tank, you can add some dissolved aquarium salt and a little dechlorinator. Some aquarists claim that livebearers like least killifish can live healthily in salty water.   

While they can survive in brackish conditions, you shouldn’t keep them in those conditions unless you have no other option. The good thing is that least killifish are hardly sick, but if you get them in the wild, you may have to deal with some yuck. 

Least Killifish Diet and Feeding

Least killifish are generally omnivorous, but they aren’t picky as such. In their natural habitat, they feed on aquatic invertebrates such as copepods, ostracods, and cladocerans. They are also known to consume plant material.

So, you can feed them on most fish food. For the best outcome, try feeding them a mix of tiny foods. Live foods in particular should form a larger part of the diet. These foods are not only healthy, but they make your killifish more colorful. 

A good mix of mosquito larvae, brine shrimp, small daphnia, bloodworms, small grindal worms, wingless fruit flies, and other small creatures that can fit into their mouths is an ideal diet for least killifish. 

Occasionally, you can try feeding them with dry foods and plant-based food. If you are giving them flakes, make sure to crash them before feeding.

Basically, anything that is under 80 microns might be good for adult least killifish. For baby killifish, feed them with food under 200 microns. The fry can feed even immediately after they are hatched. 

Least Killifish Tank Mates

Typically, these fish are shy, so they thrive when kept in small colonies by themselves. When you introduce them to a new tank, they might spend a couple of weeks before they can feel safe enough to play in the open.

But once they are familiar with the environment, they spend most of their time in open water and love swimming together. 

Due to their tiny nature, least killifish fry might be consumed by other fish, so try to keep them in single-species tanks. Males are also tiny, so they can be mistaken as a snack by other bigger species. 

With this knowledge, you should be careful when selecting tank mates. In fact, you shouldn’t keep them in community tanks. They might not be able to compete with more aggressive tank mates.

The best tank mates for these small fish are other tiny, non-predatory fish like Elder’s livebearers and aquatic snails. Other suitable tank mates include pencil fish, Otocinclus, dwarf Corydoras.

Least Killifish Breeding

Least killifish are among the species that are easy to breed. The least you can do is keep them in a tank with the right water parameters. Breeding will occur naturally; it is almost impossible to prevent reproduction if both sexes are present in the tank. 

Like most livebearers, the males release the sperm through the anal fin into the female. After that, the fertilized eggs will grow within the female killifish up to the period they hatch. At this point, the free-swimming fry is released into the water. 

In most cases, the young ones are released in bits. A female can release as many as 40 fry into the water for 10 – 14 days. Sometimes, it might be a little longer, even up to 4 weeks.

This species has a slightly different fry production method known as superfoetation. This process involves the development or formation of a second fetus when there is an already existing one in the uterus.

So, it is possible to have fry at different stages of development in the uterus of a female least killifish. On top of this, the egg yolks are usually nutritionally deficient, meaning that the developing fry can be deprived of essential nutrients. As a result, fry are often dropped continually rather than in broods. 

The good thing with least killifish is that their fry are relatively large, meaning they can take care of themselves. Moreover, adult killifish do not pose any threat to the young ones.

So, all ages can coexist peacefully in the same aquarium. Males can show a little aggression, especially when competing for females, but they rarely cause physical damage. 

Another thing to note is that males tend to exhibit inbreeding depression. A generation of sib mating often leads to a reduction in male reproductive performance.

So, it is recommended to start with a larger colony when breeding these fish to take care of the effects of inbreeding.


Thanks for reading this short guide on least killifish. The least killifish are interesting livebearers reproduced through a process of superfoetation. Most people love their muted color, especially the black line and visible stripes on their brown bodies.

These fish are generally friendly, less picky, and resistant to harsh conditions. Also, being such small fish, they can make a very good choice for both novice and experienced fish keepers. A 10-gallon tank is more than enough to keep a small colony. 

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