What to Know About Killifish

Medically Reviewed by Vanesa Farmer, DVM on December 02, 2022
4 min read

Killifish sound much more vicious than they are. These tiny fish adapt and spread far in the wild but make beautiful fish for display in your home. 

Killifish is a group of around 1,270 species of small fish. Killifish have a wide variety of appearances, habitats, and traits because of the number of species. 

Killifish habitats. Killifish can seemingly live almost anywhere. But no species have been found in Northern Europe, Australia, or Antarctica.

Popular species of pet killifish come from South America and Africa. More species are popping up around the U.S., which has recently become the topic of some research.

Annual killifish. Some areas of South America and Africa have dry and wet seasons. Killifish in these areas can be annual killifish because they only live the length of the wet seasons.

In the wild, annual killifish lay and bury their eggs in the mud at the bottom of ponds. Those ponds dry up during the dry seasons, and the killifish die.

When the mud at the bottom dries, the encased eggs are in a state of suspended development. It's as if the eggs are frozen in time.

During the wet seasons, the eggs develop again and hatch when the rain fills the pond. The pond fills with killifish, which lay eggs, and the cycle continues. 

Killifish and science. Scientists have an interest in killifish for their ability to stop growing. This ability is called diapause.

When the annual killifish eggs are in diapause, they can completely stop their cells from growing. They can also thrive in environments deprived of oxygen. 

The annual killifish's diapause trait isn't fully understood. Getting to the bottom of killifish diapause may reveal ways humans can slow or stop cancer growth and prevent or treat heart attacks or strokes.

Another point of interest in killifish is their ability to adapt and spread. They've spread far across the U.S., adapting to toxic waters that most fish can't survive in.

This extraordinary adaptation, while rescuing populations of killifish from being wiped out, has also made some species invasive.

Every species of killifish is different. The traits highlighted below apply to the majority of species and the species often kept as pets. 

Killifish size. Adult killifish can range from 20 millimeters (about 0.78 inches) to 152 mm (about 6 inches) long, depending on the species. Most are on the small to medium size, around 50 to 76 millimeters (2 to 3 inches).

Killifish colors. Killifish in the wild tend to be neutrally colored, with stripes or spots. They typically have olive, yellow-brown, and silvery-blue colors, with darker brown or black markings. 

Popular genera of killifish often kept as pets have a variety of vibrant, colorful species. For example, the Aphyosemion genus can be a chocolatey brown, gold, or vibrant orange.

Pet killifish still have spots and markings like their wild cousins, but the colors are more diverse and vibrant. As species breed, more color patterns appear.

Killifish lifespan. Typical captive killifish live two to three years. Annual killifish only live for one wet season (about 6 to 36 weeks) in the wild. Even in captivity, annual killifish only live for about a year.

Popular killifish species. The most common genera kept as pets include Aphyosemion, Fundulopanchax, Epiplatys, and Nothobranchius. But, finding killifish in your local pet store may be difficult.

Common species of killifish that you may find in a pet store are:

  • Aphyosemion australe
  • Fundulopanchax gardneri
  • Fundulopanchax sjoestedti

If you're looking for an annual killifish to display or breed, some species are more beginner-friendly. For example, the Nematolebias whitei is a species of South American annuals that's great for beginner killifish owners.

Since killifish are harder to buy, you may need to find a reputable seller to get the species you want. If your local store only has a particular species, you'll need to research that species before bringing it home.

The care setup and routine are different depending on whether you're keeping killifish for display purposes or breeding. If you want to keep annual killifish, breeding is necessary unless you intend to buy new annual killifish every year. 

Aquarium and aquarium maintenance. Depending on the fish's size, you can keep a killifish in a 2.5-gallon aquarium. Larger killifish species need a larger aquarium, typically a gallon or more per inch of the fish.

If you intend to keep multiple killifish in the same aquarium, make sure they're female. Male killifish breed with females, and multiple males in an aquarium will fight.

Filtration. Along with the aquarium tank and stand, you'll need a biological filter to remove your killifish's waste products. You'll also need a mechanical filter like gravel to remove particulates.

Water quality. Each species has specific water pH and hardness needs, so you'll need meters and kits to measure your tank's water quality. Talk to your killifish seller about the water the fish are living in to provide your fish with the same water quality. 

Depending on your area, you may need a heater for your killifish tank. Most killifish prefer temperatures between 72°F and 75°F.

Aquascaping. Killifish thrive in low-light aquariums, so plenty of cover, plants, and hides will be appealing and comfortable for the fish. If you use real plants, they should thrive in similar low-light conditions.

Plants can complicate egg collection if you're breeding killifish. But you can still put plants and decor in the tanks used for rearing newly hatched killifish. 

Killifish diet. Killifish eat small crustaceans, flying and aquatic insects, and some seeds in the wild. Captive killifish eat live or frozen brine shrimp, daphnia (a small crustacean) mosquito larvae, white worms, and some dry foods.

Killifish are beautiful, vibrant fish to keep on display. They're lower-maintenance than other fish, so they can be great starter fish for new breeders. 
With over a thousand species, it's hard to make sweeping judgments about all killifish. Talk to local sellers and fish specialists to learn more about specific breeds.