The Least Killifish (Heterandia formosa) is prolific, live-bearing, and one of the smallest fish in North America.
Surprisingly, for it being a native fish and very hardy, this species is not a common fish to find at pet stores. This is, in my opinion, a huge loss for this hobby and there are several reasons for that.
The Least Killifish is an excellent choice for nano tanks. They are tiny, omnivores, and very easy to care for. Therefore, Heterandia formosa can be recommended even for beginner hobbyists.
This care guide provides necessary information about the Least Killifish, including how to breed, keep and care for them in a home aquarium.
Quick Notes about Least Killifish
| Name | Least Killifish |
| Other Names | Dwarf topminnow, Mosqu fish or Midget livebearer fish |
| Scientific Name | Heterandia formosa |
| Tank size (optimal) | 10 gallons (~40 liters) |
| Keeping | Easy |
| Breeding | Easy |
| Size | up to 1.5 inches (~ 3.5 cm) |
| Optimal Temperature | 68 – 78°F (20 – 26°C) |
| Optimal PH | 6.5 – 8.0 |
| Optimal GH | 5 – 20 |
| Dwellers | Top-Middle |
| Nitrate | Less than 40 |
| Diet | Omnivore |
| Temperament | Peaceful |
| Life span | up to 1.5 years |
| Color Form | Olivaceous or yellowish brown with black streak |
Etymology of Formosa Killifish
The name Heterandria is of Greek origin, ‘Heteros’ meaning ‘Other or different’, and ‘Andros’, meaning ‘male’.
The name Formosa came from the Latin ‘Fōrmōsus’, meaning ‘Beautifully formed’.
Note: In 1853, the appellation of Heterandria was proposed for this species because of the difference between the sexes.
Distribution of Formosa Killifish
Heterandia Formosa is restricted to low-elevation Coastal Plain areas of subtropical humid climate zones, and the tropical Everglades.
These fish can be commonly found throughout the south-eastern United States, ranging
from at least as far west as the western Mississippi river drainage in West Baton Rouge Parish, LA, throughout the Florida peninsula, and as far north as the Cape Fear River, NC.
They are reported in North Carolina, Georgia, South Carolina, Florida, Alabama, Mississippi, Louisiana, and even Texas.
Natural Habitat of Formosa Killifish
Heterandia formosa occurs almost exclusively in shallow, still (or slow-moving), and heavily vegetated waters such as ponds, canal margins, lakes, swamps, rivers, and streams.
Although this species is most abundant in freshwater, it is occasionally reported also in brackish water (up to 20 ppt.).
Description of Formosa Killifish
Heterandria formosa is a very small fish. The mean standard length of males ranges from about 0.6 to 0.8 inches (1.5 – 2 cm). Fully grown females generally range between 1 – 1.5 inches (3 – 3.5 cm) in length.
Distinguishing characteristics of the Least Killifish:
- The body is compressed and fusiform.
- The head constitutes a little less than ¼ of the entire length.
- The snout is very short.
- The mouth is quite protractile with a low jaw slightly projecting below the upper one.
- The eyes are very large.
- The dorsal fin is higher than long.
- The ventrals are very short, small, and broad.
- The ground color is olivaceous or yellowish brown above.
- A black streak extends from the snout along the middle of the flanks to the caudal fin.
- This fish may have from 6 to 8 brownish-black vertical streaks along the sides.
- A jet black spot exists at the base of the caudal, dorsal, and anal fin, except in the males in which the latter fin has a yellowish tint.
Lifespan of Formosa Killifish
Some studies indicated that in the wild the Least Killifish have maximum longevity between 120 – 150 days.
However, in captivity, under optimal conditions and lower temperatures, these fish generally live for up to 1.5 years.
Note: Temperature greatly affects its lifespan. High temperature increases their metabolic rate and reduces lifespan.
Typical Behavior of Formosa Killifish
Sociality:
The Least Killifish are very social. In nature, they are generally found in large groups. Therefore, in aquariums, it is highly recommended to have at least 6 of them to start. Large groups (15 or more) make them more confident.
It makes their life richer and less stressful.
Ideally, you need to have 3-4 males per 1 female. In their natural habitats, there are also more males than females.
Temper:
The Least Killifish are not aggressive fish. Nonetheless, they are not completely harmless, as some sources mention them to be.
Sure, you will notice them chasing each other around sometimes but no damage is usually done.
The main problem is that in community tanks, despite their tiny size, Heterandria formosa becomes fin-nippers. That is why keeping them in the community tank may not be a good idea.
Activity:
The Least Killifish can be very shy at first or when they are in a small group (especially males).
However, their behavior changes completely, once they settle and become numerous. In this case, they become very bold and outgoing. You will see them actively scavenge every possible crevice looking for food.
Generally, they do not swim like crazy but when they do, they are pretty quick.
Also, I am surprised that almost nobody mentions that these fish are good jumpers when stressed or scared.
Placement in Tank:
Adult fish usually stay at the top and middle and bottom part of the tank. They may cruise to the bottom of the tank periodically, however, this is not their favorite place.
On the contrary, fry and juveniles prefer to stay closer to the bottom. Although adults generally do not bully the young, they generally do not swim with adults anyway.
Features:
- Social: Yes
- Activity: Moderate
- Placement: Top and middle dwellers
- Peaceful: Yes
- Nippers: Yes
- Jumpers: Yes
Feeding Formosa Killifish
Heterandria formosa is a diurnal omnivore, selectively feeding on algae, attached zooplankton, and insect larvae in densely vegetated areas.
Although these fish may eat even green algae, they still have a strong preference for high-protein food.
The Least Killifish are ready to eat at any time and in any volume. They have an insatiable and constant appetite.
In the aquarium, the Least Killifish can be fed with a wide variety of meals such as:
- brine shrimp (artemia salina),
- Artemia nauplii,
- daphnia,
- fruit flies,
- mosquito larvae,
- cyclops.
| The Least Killifish go crazy for live baby brine! They absolutely love it. |
They will have no problem taking any small live or frozen food as far as it is small enough to fit into their mouth for mastication and digestion. Therefore, for example, large worms (bloodworms, blackworms, grindal worms, etc.) should be chopped into small pieces and flake food should be crumbled.
The Least Killifish do not require only live foods. They also accept frozen and commercial food as well, such as (links to Amazon):
- Fluval bug bites,
- Hikari first bites,
- TetraMin Crisps,
- any kind of really small crushed-up tiny flake food.
How Often to Feed Formosa Killifish?
If you want to make them happy and avoid any potential cannibalization of the young, the Least Killifish should be fed twice per day.
This is very important for that species.
As I have already mentioned, these tiny fish have a huge appetite. Thus, keeping them full will reduce most of the problems.
How Much to Feed Formosa Killifish?
Stick to the so-called “five-minute rule”. This technique means that fish should consume all the food within 5-7 minutes.
Features:
- Diet Type: Omnivore
- Food Preference: Meat
- Feeding Frequency: Daily
Are Formosa Killifish Plants Safe?
Yes, the Least Killifish are completely plant safe. They will not eat any healthy plants in the tank.
According to the studies, ingested plant material may be mostly a result of foraging on attached zooplankton. This specie does not eat living plant material.
Keeping and Caring for Formosa Killifish
Although the Least Killifish are pretty hardy and can withstand varied ranges of water parameters, we still need to address their core needs!
Important: Make sure that you cycle your tank before bringing these fish home. Once the tank is cycled you need to check the quality of the water using a test kit.
Tank size:
The Least Killifish are small fish and do not require a lot of volume. These fish can be easily housed in a 5-gallon (20-liter) tank.
Ideally, a 10-gallon tank (40 liters) will suit them way better. This volume will be enough even for 20 – 30 fish.
In larger tanks, they will be ‘lost’ and difficult to see. However, the main problem with nano tanks is that it can be very difficult to keep water parameters stable.
Therefore, unless you are already an expert, I would recommend staying away from small setups.
Water parameters:
| Important: If you are buying captive-bred (for several generations) the Least Killifish, I would strongly recommend finding out the water parameters they were bred in.
If you can’t find this information, you will have to take the risk and presume that the breeder kept them in water parameters that are close to the natural ones. Related article: |
Temperature: In the aquarium, the Least Killifish will thrive in warm temperature conditions of 68 – 78°F (20 – 26°С).
In nature, the Least Killifish is eurythermal and is found in waters with temperatures ranging from 44 – 102°F (7 – 39°C). This species is intolerant to cold-seasonal temperatures.
Important: It does not mean that the temperature in your tank can fluctuate that much in short periods of time! The range of their tolerance is strongly influenced by acclimation temperature. In other words, fish that are acclimated at lower temperatures can extend their lower temperature tolerance further compared to fish acclimated to higher temperatures.
pH: The ideal pH range should be between 6.5 – 8.0.
Hardness: the Least Killifish can live in any water but they will appreciate optimal GH between 5 – 20.
Filtration and Water Flow:
These fish prefer relatively still or slow-moving water. Thus, chose filters where you can adjust the flow (with the lowest possible setting for your tank).
Note: Keep in mind the size of the fish. Some filter intakes can suck this tiny fish in! You will need to use an extra pre-filter in the form of a sponge to prevent them from being sucked in.
Lighting:
No special requirements.
The Least Killifish have a distinct diurnal activity pattern, which is synchronized with the light/dark and feeding cycles.
Lighting should be adapted to the needs of plants in the tank.
Related article:
Substrate:
No special requirements.
Plants and Decorations:
Because of its size, it requires protective cover against predation by most cohabiting fishes. Thus, the natural habitat of Heterandria formosa is most abundant in water with leaf litter, thickly vegetated waters, and near complete vegetative covering on the surface.
This should be replicated in an aquarium.
Plants provide good hiding spots for the Least Killifish. You can use floating plants, fake plants, driftwood, leaves, and small rocks to provide great hiding spots for them.
Water Changes:
When it comes to tank water, you should change it on a weekly basis. Any water that you add to the tank should also be at least dechlorinated.
Note: Dechlorinators are available at any pet store (for example, Seachem Prime – link to check the price on Amazon).
You will have to change 20-30 % of their water every week. However, you may need to remove a different percentage of water depending on your filter and other tank factors.
Acclimation:
Gradually introduce the Least Killifish to the fish tank. Sudden changes in habitat can harm them.
Give them time to acclimatize/quarantine before adding them into the main tank. Do not rush the process! Do it very slowly to prevent any unnecessary stress.
Related article:
- How I Drip Acclimate Shrimp and Why (the principle is the same)
Formosa Killifish – Male and Female Differences
Heterandria formosa is sexually dimorphic and has several easily distinguishable morphological features such as:
- Size. Males are significantly smaller than females.
- Belly. Females have rounded bellies, compared to slim males.
- Reproduction organs. Males have really big gonopodium.
Note: Gonopodium is a modified anal fin used for sperm transfer to females which has internal fertilization and sperm storage.
Breeding Formosa Killifish
The Least Killifish can easily breed in freshwater aquaria. All you need is to keep them in acceptable water parameters and feed them daily.
These fish are superfetative livebearers, they are not egg scatterers.
Note: Superfetation is when a second, new pregnancy occurs during an initial pregnancy.
Maturity:
According to the study, this species is among the world’s smallest vertebrates at maturity.
Females become mature when they are about 30 – 40 days old and range between 0.3 – 0.4 inches (8 – 10 mm) in length. Males mature at a slightly larger size than females 0.4 – 0.5 inches (10 – 14 mm) and take longer to mature (40 – 50 days).
In females, sexual maturity is indicated by the appearance of a small black dot on the anal fin. In males, maturity is indicated by the development of the gonopodium.
Mating:
According to the study, Heterandria Formosa does not have complicated courtship behavior because males can force matings and insemination.
Nonetheless, when given a choice between a large and a small male, females prefer larger male and the repeatability of this preference is high.
Fertilization:
This species exhibits internal fertilization and sperm storage.
Interesting fact: embryo mass in this species increases by as much as 3000%.
Brooding:
Females of this species display internal brooding of embryos and the most advanced levels of superfetation. They are not like normal ovoviviparous livebearers.
Basically, it means that their eggs will be fertilized each day and females can have multiple pregnancies at the same time but because of the time interval of fertilization, each brood differs in the development stage. In other words, they are almost permanently pregnant.
Females of the Least Killifish can potentially have up to 6 broods of embryos at different stages of development! This will result that fry will be born not in one day but spread over a number of days.
They give birth to small clutches (1–8 fry) at intervals of a few days to two weeks.
Note: According to the study, in the natural environment, reproduction typically begins in March and continues through October, resulting in at least 2 – 4 generations per year.
Fry:
Unlike many other livebearers, the Least Killifish ordinarily do not prey on larvae of their own species. However, it also depends on how hungry they are.
That is why if you are planning to breed these fish it is IMPERATIVE to feed adults live brine shrimp at least 2 times a day! When they are well-fed, adults simply ignore the young.
Even though Least Killifish fry can eat the same foods as adults, it is better to focus on:
- microworms,
- vinegar eels,
- finely crushed flakes, etc.
Formosa Killifish and Suitable Tankmates
It can be very difficult to find good neighbors for these fish. Larger fish can eat them. As for the peaceful and small fish species, there is a risk that the Least Killifish will nip on their fins and tails.
Thus, the ideal situation for the Least Killifish is a small species-only tank. It will also significantly improve their chances to breed.
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Fish
If you still decide to keep this species in community tanks, keep in mind that all tank mates should have at least a gentle temperament and enjoy the same water parameters (for example, Endlers, Pygmy Cory, Danio Rerio, Guppy, etc.)
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Dwarf shrimp
Keeping the Least Killifish with dwarf shrimp is risky. Sure, you may have heard or read somewhere, that they are way too small to bother any shrimp. This is not completely so.
Their small size should not mislead you into thinking that they cannot do any harm to the shrimp. Yes, they can and, in most cases, they will do it eventually.
The Least Killifish are very inquisitive fish. They will look into every nook and cranny of your tank. Even if you have lots of plants and other places to hide, they will find shrimplets and molting shrimp.
There are many reports of people gradually losing their shrimp colony other the period of several months. So, you should understand the risks.
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Freshwater snails:
They are compatible with any freshwater snail.
Avoid:
- Large and/or aggressive, and/or boisterous fishes.
- Keep them away from all types of crayfish and most types of freshwater crabs.
- Even small African Dwarf Frogs will definitely try to catch them whenever it is possible.
Related article:
In Conclusion
The least Killifish is a fun, miniature, prolific, and live-bearing fish. This species can be an ideal candidate for small planted tanks because of its hardiness and ease of care.
Unfortunately, because of its relatively dull coloration, it is not particularly popular in the hobby, which is a pity, because this disadvantage is compensated by its interesting behavior and unique reproduction.
References:
- Belk, Mark C., and Charles Lydeard. “Effect of Gambusia holbrooki on a similar-sized, syntopic poeciliid, Heterandria formosa: competitor or predator?.” Copeia (1994): 296-302.
- LOFTUS, F., ANDJ. A. KUSHLAN. 1987. Freshwater fishes of southern Florida. Bull. Flor. St. Mus., Biol. Sci. 31:147-344.
- Leips, Jeff & Rodd, Frieda & Travis, Joseph. (2013). The adaptive significance of population differentiation in offspring size of the least killifish, Heterandria formosa. Ecology and evolution. 3. 948-60. 10.1002/ece3.509.
- Baer, Charles F.. “Population structure in a south-eastern US freshwater fish, Heterandria formosa. II. Gene flow and biogeography within the St. Johns River drainage.” Heredity 81 (1998): 404-411.
- Soucy, S., and J. Travis. “Multiple paternity and population genetic structure in natural populations of the poeciliid fish, Heterandria formosa.” Journal of Evolutionary Biology16, no. 6 (2003): 1328-1336.
- Mukherjee, Shomen, et al. “Perceived risk of predation affects reproductive life-history traits in Gambusia holbrooki, but not in Heterandria formosa.” PLoS One2 (2014): e88832.
- Doyle, Cathleen M., Paul L. Leberg, and Paul L. Klerks. “Heritability of heat tolerance in a small livebearing fish, Heterandria formosa.” Ecotoxicology3 (2011): 535-542.
- Girard, C. F. (1859). Ichthyological notices. Proceedings of the Academy of Natural Sciences of Philadelphia. 11: 56-68. ;v. 11:91.
- Ala-Honkola, Outi, Laura Tuominen, and Kai Lindström. “Inbreeding Avoidance in a Poeciliid Fish (Heterandria Formosa).” Behavioral Ecology and Sociobiology64, no. 9 (2010).
- Aspbury, A.S., Basolo, A.L. Repeatable female preferences, mating order and mating success in the poeciliid fish, Heterandria formosa. Behav Ecol Sociobiol51, 238–244 (2002).