Least Killifish (Heterandria Formosa): Care Guide


Common Name(s)Least Killifish
Scientific NameHeterandria Formosa
OriginUnited States
Temperature69-79°F (21-26°C)
SizeMale Size: 0.8 inches
Female Size: 1.2 inches
Minimum Tank Size5 gallons
Food & DietOmnivorous diet
Lifespan3 years
Water pH7.0-8.0
Tank MatesPencil fish, Dwarf Corydoras, Otocinclus, and Freshwater Shrimp.
BreedingLivebearer
DiseaseMay be suseptible to Skin & Gill Flukes, Tapeworms, Bacterial/Fungal Infections, and Columnarius.
Least Killifish
Least Killifish (Heterandria Formosa)

Least Killifish Care

Least Killifish (Heterandria Formosa) is also known as the little killifish, dwarf topminnow, or midget livebearer. It is a fish species native to the southern United States. Coming from the Poeciliidae family, this fish is one of the smallest fish worldwide, and it is in fact the smallest fish that is native to North America. It is one of the few fish that is endemic to the North America. Although they are not the most colorful fish in the aquarium hobby, it is a unique fish that is very interesting to keep. They are calm and docile fish that can breed in your home aquarium.

Least Killifish Temperature

Coming from regions of the country that are warm and tropical, an ideal temperature to keep your tank at will correspond to the Least Killifish subtropical environment. For a climate more natural for this type of fish, it is pertinent to keep temperatures slightly on the warm side. Temperatures of 69-79°F (21-26°C) will suffice.

Least Killifish pH

As with water temperature, PH levels in your water are equally as important. PH is a measure of how acidic/essential water is. For this fish, an ideal PH level is 7.0-8.0. This is ideal for the owner as most water comes in this variation. Always make sure to check your water PH to have your fish thrive.

Least Killifish Size

Although being a pretty hardy fish, the Least Killifish is one of the smallest in the world and the smallest in the United States. With this in mind, your not looking at a fish that will take a lot of room. Males grow to only about 0.8 inches, while females grow to about 1.2 inches.

Least Killifish Tank Size

Least Killifish should be kept in an aquarium tank of at least 5 gallons in size. Since they are small fish, they are great fish for nano tanks. Even in such a small tank, the fish can thrive with a proper setup and regular maintenance.

However, anything smaller, such as a 2.5 gallon aquarium is not recommend for keeping Least Killifish. At this size, it is very difficult to keep a stable water parameter. In addition, there is simply not enough space for the fish to swim, explore, and exhibit their natural behaviors.

While 5 gallons is the minimum tank size, a tank size of 10-20 gallons is generally recommended for Least Killifish. If you have multiple fish, or you wish to breed them, it is important to have an adequate tank size. As a general rule, there should be 1 gallon of water for every 1 inch of fish in the aquarium. A good filtration system with a functioning biological filter is important as well.

Least Killifish Food & Diet

Least Killifish are omnivores that need a varied diet of both plant based foods as well as protein rich foods. They are not picky feeders, but a quality diet is important for their well-being. This is especially true if you wish to breed them.

There are many types of fish food you can feed your Least Killifish, quality flake food should be their staple. In addition to this, they can be fed live fish food such as daphnia, mosquito larvae, and other small invertebrates. Regarding their plant based dietary requirements, this may be satisfied if there is plenty of natural algae growth in the tank. However, if there isn’t enough algae for the fish to graze on, algae wafers can be fed as a supplement.

Least Killifish Lifespan

Least Killifish has a lifespan of approximately 3 years. Providing the fish with the right tank setup and regular maintenance is important for their longevity.

Lower temperatures can contribute to a longer lifespan as well. Lower lifespan slows down their metabolism. However, low temperatures can weaken the immune system of the fish, resulting in a shorter lifespan. Therefore, keeping the temperature within the acceptable range is important.

Heterandria formosa (Least Killifish) Care and Breeding: One of The Tiniest Fish in The WORLD!

Least Killifish Tank Mates

It is recommended that if you are going to own this fish that you keep it with its species. The reason for this is that it is docile due to its size. They should not be kept in community tanks as many other fish species will see them as snacks. The best fish to pair with the Least Killifish will be other small non-predatory fishes. These can be the pencil fish, dwarf corydoras, otocinclus, and freshwater shrimp.

How many Least Killifish can be kept together?

As discussed early, the Killifish is not a great fish if you want to add a lot of species to your tank. However, the amount of fish you add to your tank will depend on a fish’s temperament, shoaling behavior, and social needs. The general guideline is to have one inch of fish per gallon of water. Therefore if you were to add more compatible species to the tank, make sure to increase its size.

Least Killifish and Cherry Shrimp

Being such a petit fish, the owner who wants to mix the Least Killifish with a cherry shrimp should have no problems. It is not generally known to prey on shrimp actively, but it has been documented.

Least Killifish and Betta Fish

You can add both the Least Killifish and the Betta Fish together, but it is not recommended. Betta fish prefer larger tanks and calmer waters, somewhere near 72 degrees, while the Least Killifish likes warmer temperatures of 79-82. Also, another reason you should avoid adding these fish together is that certain species carry diseases not known to the other. This could pose problems.

Least Killifish Tank Setup

With Least Killifish, it will take more time than not to occupy the top level of the tank since they like to jump. To prevent this from happening, make sure your tank has a lid on it. You can also add vegetation to the top for aesthetics. A rather undemanding fish, water parameters do not really matter. However if your someone who likes it just right, aim for a tropical planted setting.

Least Killifish Breeding

Like most fish in the Poeciliids family, this fish is a live bearing species. What this means is that it will keep its eggs inside until they are ready for birth. Insemination takes place when the male uses his anal fin, or gonopodium to deliver sperm to the female. What makes the Least Killifish reproductive strategy different is that it doesn’t release all of its fry at once but over a time period of two weeks or even longer.

Least Killifish Diseases

The good thing for owners of the Least Killifish is that it is not prone to many illnesses. However like any fish it can become sick so care is still needed. Common conditions that you should look to avoid are Skin & Gill Flukes, Tapeworms, Bacterial/Fungal Infections, and Columnarius. All these conditions are visible to the eye and can be prevented by practicing clean habits with your tank.

Where Can I Find Least Killifish for Sale?

Like our previous article on the Rusty Cichlids you can find Least Killifish on several online sites. If you are hesitant to spend money on a third party site you can use EBay and get these fish from a private seller. However above all this there is the aka.org or commonly known as the American Killifish Association. This organization deals with anything Killifish, so it would be wise to start there.

Least Killifish Price

Prices range for the Least Killifish depending on size, color and amount. On eBay if you want one male/2 female trio then you can look at about $16. This is before the shipping fees of about $17. On the site called Get Gills, you can purchase the fish for about $18 with shipping ranging from $19.99 to $29.99.

Where Can I Find Least Killifish in the Wild

As discussed earlier the Least Killifish is found in the southern United States as Far East as South Carolina to as far west as Texas. However if you want to see these small fish it would be smart to look for low streaming freshwater habitats. These can be slow waterways or ponds with minimal or heavy vegetation. Good places would be lowlands of the gulf coast.

The Least Killifish is a great fish to add to your tank not only due to its size and docile behavior but because it is one of the hardiest fish out there. Although not an entirely colorful fish it definitely makes up for that with its wide range of climate and food tolerances.

Fish Laboratory

With decades of collective fishkeeping experience, we are happy to share the fish care tips that we've picked up along the way. Our goal at Fish Laboratory is to keep publishing accurate content to help fishkeepers keep their fish and aquarium healthy.

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