Guide to Killifish

Stunning red killifish swimming in an aquarium.

Mirko_Rosenau / Getty Images 

Almost any warm water climate in the world has a species of killifish somewhere or fossil record that a species of killifish once existed there. The species vary in coloration and finnage, and though they are closely related to live-bearing tooth carps, they are egg-laying fish; however, egg-laying in a way unlike any other fish on the planet. Speculation has circulated for years about where these strange colorful short-lived little fish came from or why they evolved everywhere there is warm water. Some theories have been floated around as to where they came from, but you should focus on the science today and the facts known about this wonder of nature.

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    Chart showing eight different types of killifish.

    The Spruce / Thomas Reich

    The killifish or egg-laying tooth carps are very closely related to the live-bearing tooth carps (guppy, Molly, Platy, and swordtail) even though they have very different methods of reproduction. The killifish is very widely distributed from the United States through Central America to South America as far as Argentina. In the Old World, they come from most of Africa, from the Far East including Japan, with a few species in the Middle East and even in southern Europe.

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    Natural Environment

    Swampy area with trees and water where killifish live.

    The Spruce / Thomas Reich

    The popular aquarium species have two widely different habitats. There are those that come from areas of tropical rainforest, where they live in pools, swamps, and streams. There is a thick forest that screens the water from the sun so that these fish prefer temperatures on the lower side at an average range of 72 F to 75 F/22 C to 24 C and low lighting. Fish coming from these types of habitat usually spawn by laying their eggs in the roots of floating plants (egg-hangers or top-spawners). Most of the common Aphyosemion species and ‘panchax’ species spawn in this way.

    The other and more popular killifish live in ponds on the tropical savannahs and very often the ponds dry up in the dry season. These fish bury their eggs (egg-buriers or bottom-spawners). As the pond dries up, the parents die but the eggs buried an inch or so (two or three centimeters) deep under the mud survive and hatch within a few hours of the next rains coming back in the wet season months or sometimes even a year later.

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    Annual Fish

    Two killifish swimming in an aquarium

    The Spruce / Thomas Reich

    These killifish species are often called annual fish because they may live one season only. These species have bright vibrant colors and beautiful ​finnage but very short lifespan. Though beautiful for a brief and splendid period, they quickly begin to fade, wither and die. It is not the fault of the hobbyist when this happens, even under perfect conditions, it is the nature of things. The lifespan of this fish is short, and the fact that a dry season never comes in the aquarium does not change that.

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    Killifish in Aquariums

    Two killifish, one black and one orange, swimming in an aquarium

    The Spruce / Thomas Reich

    The killifish is a small fish even in the wild where they are predators, mainly feeding on insect larvae. This is much the same habit as their cousins the guppies and mollies in their constant search for mosquito larvae. They prefer live foods in the aquarium but can live on dry flake food supplemented with frozen brine shrimp and blood worms. They are not really a great community aquarium fish because of their dietary needs and preference for cooler water temperatures, but species are temperamentally compatible to be with most community aquarium fish.

    The small fish of the genus Aphyosemion, Nothobranchius, and Cynolebias can be kept in small plastic containers (12 inches by six inches), much like you would keep a betta. But unlike a betta, special attention must be paid to the water conditions (remember that the betta is a labyrinth fish and is not so concerned with water oxygen). These small fragile fish prefer soft, acidic water and temperatures slightly lower than most fish. If soft tap water is not available, rainwater may do the trick, unless it is rainwater from within a city or industrial area, then try to get reverse osmosis water from a local tropical fish store for best results. Remember that even collecting water from a cement roof or cistern will make the water alkaline.

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    Basic Breeding

    Male and female killifish swimming in an aquarium

    The Spruce / Thomas Reich

    Breeding these fish is a challenging but attainable project. For the top-spawners, a 1/2-inch layer of previously boiled peat moss on the bottom of the breeding tank makes the water acidic and also provides a dark bottom, preferred by these fish. The peat needs to be boiled for five minutes and then squeezed dry to extract all the excess acidity in the peat. Provide floating aquarium plants for them to to lay their eggs on.

    For the bottom-spawners, the peat should be about one-inch thick to allow enough depth for egg-laying. Remember that these species must be given the illusion that they are burying their eggs deep enough to last through the coming drought.

    In spawning killifish, it is better to put one male with three females because the males are hard drivers. The males are usually easily distinguished because in many species they have lyre-shaped fins and are much more colorful than the females.

    The eggs of the egg-hangers take about three weeks to hatch, whereas the bottom-spawner eggs need to have water removed from the aquarium and kept in just moist peat for about three months (depending upon the species) before water is added back into the tank.

    It is possible to experience the miracle of the killifish lifecycle by actually buying eggs from breeders online. These eggs arrive in moist peat moss and have been already properly aged. You add the proper water as instructed and within hours you have hatched fry. This is a cheaper and easier way to acquire a collection of killifish than buying adult fish, and since they have such a short life span, you get to experience them for a longer period of time.

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    Chocolate Lyretail (Aphyosemion australe)

    Two Chocolate Lyretail (Aphyosemion austrail) killifhs

    The Spruce / Thomas Reich

    This popular killifish comes from West Africa where it lives in small streams and ponds in the rainforest. It grows to 2 1/2 inches. They are very easy to sex, the male having the characteristic, lyre-shaped caudal fin. They need soft, slightly acidic water and are egg-hangers. They will lay a few eggs every day among the strands of a nylon mop suspended from a cork.

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    Blue Lyretail (Fundulopanchax gardneri)

    Brightly colored Gardneri (Aphyosemion gardneri) in aquarium

    The Spruce / Thomas Reich

    This killifish also comes from West Africa and grows to about 3 inches. It exists in two color forms: the yellow where the edges of the tail, anal, and dorsal fins in the male are yellow, and the blue form where the yellow is replaced by a pale blue. The female is very similar to all Aphyosemion females. It is another egg-hanger with eggs taking up to 21 days to hatch, sometimes more depending upon the temperature.

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    Striped Panchax (Aplocheilus lineatus)

    Two yellow Limeatus (Aplocheius lineatus) in water.

    The Spruce / Thomas Reich

    This hardy robust ‘panchax’ comes from India where it grows to about four inches. It can live well in the community aquarium but may eat smaller fish and guppy fry. The male is brighter colored than the female, which has a more rounded tail. It is a typical egg-hanger and the eggs take about two to three weeks to hatch depending upon the temperature. It will eat dried foods. An aquarium-bred variant of this species with a more yellowish coloration is known as golden wonder killifish.

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    Bivittatum (Aphyosemion bivittatum)

    Shimmery Bivittatum (Aphyosemion bivittatum) in a tank.

    The Spruce / Thomas Reich 

    This killifish also comes from West Africa and grows to about 2 inches. It has has an elongated, brightly colored body with two dark bands made up of red spots running along each side, with the colors varying between populations. Individual males are commonly found with golden-brown dorsal surfaces, bluish sides, and golden ventral surfaces. This fish is a typical egg-hanger (surface-spawner) laying its eggs on the roots of floating plants in the wild, but accepts a floating nylon mop in the aquarium on which to lay its eggs. Eggs hatch in 14 to 21 days. Like all "killies" (egg-laying tooth carps), it is not a good community tank fish.

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    Blue Gularis (Aphyosemion sjoestedti)

    Red-striped Blue Gularis (Aphyosemion sjoestedti) swimming in a tank.

    The Spruce / Thomas Reich

    This fish also comes from West Africa. It is a larger fish than most Aphyosemion species, growing to 4 1/2 inches. The male is easily recognized by his three-pointed tail. This fish lays its eggs on the bottom among peat fiber. The eggs should be collected from there and put in a sealed plastic box in just-moist peat for about six weeks. Adding soft water will then hatch the fry.

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    Dwarf Argentine Pearl Fish (Cynolebias nigripinnis)

    Two Dwarf Argentine Pearl Fish (Cynolebias nigripinnis) swimming in a tank.

    The Spruce / Thomas Reich

    This egg-burying (bottom-spawning) killifish comes from Argentina. It grows up to 1 1/2 inches. It lives in ephemeral ponds that evaporate in the dry season, and it buries its eggs in the mud at the bottom of the pond. In the aquarium, it needs layering of peat or mud substrate, a temperature of about 70 F, and soft water. The eggs need incubating in just-moist peat for 16 weeks.

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    American Flagfish (Jordanella floridae)

    Two American Flagfish (Jordanella floridae) swimming in a tank.

    The Spruce / Thomas Reich

    This fish comes from Florida and Central America, where it lives in ponds and swamps. The male grows to nearly 3 inches in length, and the female is slightly smaller. It can be an aggressive fish during spawning season. The female is usually plumper than the male and is duller than the colorful male, with a dark mark on the dorsal fin. The male’s coloration resembles an American flag in a really good example of this fish, thus the common name. The fish will lay eggs on clumps of peat fiber in the tank bottom. Eggs hatch in seven to 10 days.