Killifish

Quick Take

• Killifish can't survive on fish flakes alone, and skipping what they actually need could cost you the fish entirely. Captive feeding requirements →
• Some killifish live less than a year by design, and their survival strategy is stranger than you'd expect. Annual killifish lifespan explained →
• Killifish are rarely sold by a name you'd recognize, which is something that trips up even experienced aquarium hobbyists before they buy. Over 1,270 species classified →
• A secure tank lid is not optional with killifish, and the reason why tends to surprise most first-time owners. Why secure lids matter →

Killifish are vibrant freshwater fish widely distributed across the world. Antarctica and Australia are the only two places where they don’t occur. Because of their strikingly beautiful colors, they are very popular among fish enthusiasts who keep home aquariums.

These fish are often sold under their scientific names, making it hard for novice fish enthusiasts to find or know what they are buying. Therefore, purchasing and housing killifish requires considerable research.

Three Amazing Killifish Facts

• Killifish are primarily small fish, generally measuring between 2 and 3.5 inches long. However, larger species, like the Blue Gularis, can grow up to 6 inches.
• Their diet mainly consists of insects floating on the surface, worms, insect larvae, algae, and plants, but some species are predatory and hunt smaller fish.
• Distinguishing between killifish genders is easy, as males are much brighter than females and come in various colors, like blue, yellow, red, and green.

Classification and Scientific Name

Killifish are ray-finned fishes that are classified under the order Cyprinodontiformes. The order contains multiple families and over 1,270 species. These fish are generally small with bright colors. The major families include Nothobranchiidae, Rivulidae, and Cyprinodontidae. Killifish are classified according to their habitat, and range from annual (living in temporary pools) to non-annual species.

There are 1270 species of killifish, which all vary in color, size, and shape, but what they all have in common is their:

• Striking patterns
• Bright coloring
• Flatheads
• Mouths are located at the tip of their face or below it
• Long, pointed, and curved teeth

These are five of the most popular killifish to keep in an aquarium.

Red-Striped Killifish (Aphyosemion striatum)

The red-striped killifish is the perfect species for novice fish owners because they have peaceful temperaments and stunning neon colors. They have narrow blue and orange bodies covered with red dots and asymmetrical stripes.

Red-striped killifish are native to Equatorial Guinea and Gabon and inhabit freshwater swamps and shallow, slow-moving rivers in rainforests.

Chocolate Lyretail (Aphyosemion australe)

Aquarists are drawn to the chocolate lyretail because its bright orange coloring adds character to any aquarium. In addition, they are very adaptable and can acclimate to many ecosystems.

These fish have orange and light brown bodies littered with bright red spots. Male lyretails are brighter than females and are distinguished by their lyre-shaped tails and spiked fins. They are native to West Africa and occur in freshwater habitats like ponds or small rivers.

Two-Stripe Lyretail (Aphyosemion bivittatum)

The brightly colored two-stripe lyretail has an elongated body with two luminescent red lines running down each side. Males have colorful and distinct dorsal and anal fins ranging from orange to blue and green.

This tiny fish inhabits the rivers of Nigeria and Cameroon. However, it is easy to replicate their habitat in captivity by providing various floating plants and bogwood. The water in the aquarium must be warm, with slight acidity.

Two-stripe lyretails are predators and prey on shrimp and other small creatures in their habitat, but in captivity, their diet consists of the following:

• Mosquito larvae
• Bloodworms
• Flakes

American Flagfish (Jordanella floridae)

The American flagfish is breathtakingly beautiful and admired by fish enthusiasts around the world for its vivid colors and adaptability.

They were actually named after the American flag because of their red and blue markings. In addition, both genders have a characteristic black dot on their side.

These stunning fish are native to Florida, where they occur in ponds, slow-running streams, and swamps. In captivity, they thrive in larger tanks with plenty of hardy plants and several hiding spots.

Blue Lyretail (Fundulopanchax gardneri)

The blue lyretail is a breathtaking fish, with its light blue body scattered with vivid red spots and patches of bright yellow on its fins.

They have a distinct lyre shape, and the male’s coloring is more prominent than the female’s. In addition, these fish have large mouths and will eat anything that can fit into them.

Blue lyretails occur in lakes and swamps in African countries like Nigeria and Cameroon. They thrive in warm, soft waters but are highly adaptable and acclimate well to freshwater tanks. However, males can become aggressive and will fight if there are too many blue lyretail males in the tank. The best ratio is 1 or 2 males to 4 or 5 females.

These tiny fish are omnivores, and their diet contains a lot of variety. In captivity, owners will feed them pellets, flakes, algae, and live foods.

Appearance

Due to the large number of killifish species, they exhibit significant variations in appearance. However, all these species share one trait in common: their vibrant coloring and eye-catching patterns.

In addition, males are much more striking than females, which are usually brown and dull. Furthermore, the top of their heads is flat, and their mouths are filled with long, pointed, and curved teeth located at the tip of their faces or below.

Most of the killifish family members are slender and pike-shaped, making them great swimmers. Others have cylindrical bodies with short, round fins. However, some species have long, wide fins.

While their body shape varies significantly, they all have a dorsal fin located towards the back of their bodies. In addition, it is common for the males of this family to have more prominent dorsal and anal fins.

Killifish are generally 2 to 3.5 inches long, but there are smaller species, like the Dwarf Medaka and the Hummingbird Lampeye, which are less than an inch long. In addition, some species get pretty big, like the Orestias, which can reach lengths of 10.5 inches.

Behavior

Most killifish species are territorial, while some species will shoal. However, certain populations can be dense, and they can shift territories relatively quickly, especially when inhabiting shallow waters.

Furthermore, several species form passive social groups in small streams, dominated by alpha males that fiercely defend their territory while still allowing females and juvenile males to pass through.

In Captivity

Killifish are highly sought after for their peaceful nature and ability to adapt to most aquarium communities. However, as mentioned above, male killifish can be pretty aggressive towards other males. So, novice owners should keep just two killifish in a species-only tank until they become more knowledgeable.

However, more experienced owners can keep a group of killies in a bigger tank as long as there are enough places for them to hide. Alternatively, aquarists can keep one male with several females, but should avoid adding any similar-looking male species. In addition, female killifish do well in a group without any males present.

These fish adjust well to communities of small, peaceful fish that require the same water conditions, like rummy nose tetras and neon tetras.

All owners should know that killifish are excellent jumpers, and their tanks should always have secure lids or hoods. But keep in mind that most killies are tiny and can jump through small openings.

Habitat

These luminous fish inhabit tropical and subtropical waters all over the world except for Australia and Antarctica. Killifish primarily live in swamps, creeks, pools, and shallow streams. However, some species inhabit brackish estuaries.

Other species prefer colder temperatures and live in water bodies surrounded by overhanging trees and shrubs, which help cool the water and create subdued light. However, certain species flourish in desert pools with water temperatures exceeding 90°F, like the pupfish that occur in the southwestern regions of the USA.

Aquarium Habitat

Smaller killifish are perfect for desktop or nano aquariums because of their tiny size. A 5 to 10-gallon tank is recommended for 3 killifish. However, if owners want a community-type setup, they need a 20-gallon tank at a minimum, especially if they want more than one male.

These fish need subdued lighting, and non-breeding aquariums should contain peat moss or driftwood, which softens the water and lowers the pH levels. In addition, the tank needs light-tolerant plants like Java fern, Java moss, and Cryptocoryne. Lastly, these sneaky fish are incredible jumpers, so a secure lid is essential.

Diet

These tiny fish are primarily carnivores, and their diets vary depending on their location and environment. However, they generally eat insect larvae, crustaceans, and worms. Other species of killifish are omnivores, and their diet consists of algae.

Diet in Captivity

When in captivity, killifish need live food to survive. They cannot live on flakes alone, so if owners cannot keep live prey in their homes, this is not the fish for them.

When aquarists obtain killifish for breeding purposes, they need to ensure these fish have a varied and well-balanced diet. Therefore, expert breeders will typically make their own food to guarantee they meet the exact nutritional requirements.

The best diet for killies includes:

• Brine shrimp is always a winner with killifish and a good source of nutrition, and owners can purchase frozen brine shrimp.
• Daphnia is often used as live food, and owners can obtain it from ponds; however, too much can act as a laxative, so feed daphnia in moderation.
• Mosquito larvae can also be collected from ponds, but they are only available seasonally.
• Fruit flies
• Paste foods
• Dry foods
• Beefheart

Killifish benefit from a variety of the foods listed above; feeding too much of the same food is never a good idea.

If you want to breed these fish, it’s also essential to know what the fry eat, as the fry’s diet differs from the adults’. The best food to feed fry is:

• Newly-hatched brine shrimp
• Vinegar eels
• Microworms
• Infusoria
• Grindal worms

Predators and Threats

The killifish falls prey to birds like terns and gulls, wading birds like egrets and herons, and predatory fishes like bluefish and striped bass. In addition, fishermen use these fish as bait.

Unfortunately, wild killifish populations are declining due to human interaction, climate change, and habitat loss. For example, industrial developments like forestry activities and road building can negatively affect their numbers.

Reproduction, Babies, and Lifespan

Killifish fall into three classes: annuals, semi-annuals, and non-annuals. Annuals lay their eggs in mud and live in swamps and ponds. In addition, they are fast growers and breed before the water dries. However, they have a short lifespan, living only 6 to 9 months.

Semi-annuals inhabit both wet and dry environments. Species in this classification leave their eggs to hatch unattended. They live longer than the annuals, usually around 2 to 3 years in cooler conditions. If they live in warmer climates, they only live for approximately 12 months.

Non-annuals live the longest, up to 5 years, depending on the species, and inhabit permanent water bodies.

Population

As there are many species of killifish, it’s impossible to know their exact population size. In addition, their conservation status varies depending on the species. Some types of killifish have abundant numbers, but others are endangered or even extinct.