Your Comprehensive Guide to Killifish Care and Species Diversity

Killifish

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 are 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.

Natural Environment

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 to 75 F (22 to 24 C) and low lighting. Fish coming from this type 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 (2 or 3 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.

Annual Fish

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 a very short life span. 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 life span of this fish is short, and the fact that a dry season never comes in the aquarium does not change that.

Killifish in Aquariums

The killifish are 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 bloodworms. 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 6 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, but if it is rainwater from within a city or industrial area, 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.

Housing

Because they’re small, killifish can do well in a smaller tank, provided that you don’t overcrowd it with too many fish—always consider the number of killifish you plan on keeping, as well as the number of other species you plan on housing with them. Generally, an aquarium that is at least 20 gallons is best. If you want to keep just a few killifish, such as three of them, you can house them comfortably in a 10-gallon tank.

Low, subdued lighting is best for killifish, so choose plants that don’t need a lot of light when decorating your tank. Keep the water temperature anywhere from 72 to 75 F, using a heater if necessary. And for substrates, choose options that are dark, such as sand or gravel that won’t harden the water.

Basic Breeding

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 lay their eggs on.
• For the bottom-spawners: The peat should be about 1 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.

Chocolate Lyretail (Aphyosemion australe)

This popular killifish comes from West Africa, where they live in small streams and ponds in the rainforest. They grow to 2 1/2 inches. They are very easy to sex, with 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.

Blue Lyretail (Fundulopanchax gardneri)

This killifish also comes from West Africa and grows to about 3 inches. They exist 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. This is another egg-hanger, with eggs taking up to 21 days to hatch, but sometimes more depending upon the temperature.

Striped Panchax (Aplocheilus lineatus)

This hardy robust "panchax" comes from India, where they grow to about 4 inches. They 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. This is a typical egg-hanger and the eggs take about two to three weeks to hatch depending upon the temperature. They will eat dried foods. An aquarium-bred variant of this species with a more yellowish coloration is known as golden wonder killifish.

Bivittatum (Aphyosemion bivittatum)

This killifish also comes from West Africa and grows to about 2 inches. They have 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 eggs on the roots of floating plants in the wild, but accepts a floating nylon mop in the aquarium on which to lay eggs. Eggs hatch in 14 to 21 days. Like all "killies" (egg-laying tooth carps), this is not a good community tank fish.

Blue Gularis (Aphyosemion sjoestedti)

This fish also comes from West Africa, and 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 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.

Dwarf Argentine Pearl Fish (Cynolebias nigripinnis)

This egg-burying (bottom-spawning) killifish comes from Argentina and grows up to 1 1/2 inches. They live in ephemeral ponds that evaporate in the dry season, and bury eggs in the mud at the bottom of the pond. In the aquarium, these fish need 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.

American Flagfish (Jordanella floridae)

This fish comes from Florida and Central America, where they live in ponds and swamps. The male grows to nearly 3 inches in length, and the female is slightly smaller. They can be aggressive 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.