This is a club you can’t afford not to join: the nearly untouchable Killifish clan. They have connections around the globe and a presence on every continent.
Even in places like Australia where they are not naturally found, specimens can be found spying from their tanks of captivity waiting for the next jailbreak.
There are at least 1270 known species around the world run by the powerful Aplocheilidae, Cyprinodontidae, Fundulidae, Profundulidae, and Valenciidae families.
Their robust nature shows a vitality even where other fishes dare to swim. Far from sinister in any way, this brightly colored, freshwater fish makes an excellent addition to most aquariums if you recreate their habitat correctly.
So how exactly do we hook you up to join this family without finding yourself swimming beneath these fishes?
Species Overview
Killifish are a huge group of small, ray-finned, freshwater fish belonging to the order Cyprinodontiformes.
Although typically found in shallow seasonal ponds and slow-to-turbid rivers and streams, they have also been found surviving in very inhospitable habitats with temperature swings, harsh pH, high salinity.
Through miraculous evolutionary tenacity, some killifish have developed the ability to survive in places with heavy metals and dioxins or to temporarily pause their development when conditions are unfavorable through suspended animation.
| Category | Rating |
| Life Span | 3 Months – 5 Years |
| Families | Cyprinodontiformes |
| Color Variety | Various vivid colors |
| Care Level | Beginner to Intermediate |
| Compatability | Peaceful with Most Types of Fish |
| Food and Diet | Carnivore |
| Temperament | Peaceful but territorial |
| Tank Size (Min.) | 10 Gallons for each specimen |
| Tank Setup | Dim lighting, minimal decor |
| Size | 2-9 inches |
Rather than any killer instinct, the name of these wonder-fish derives from the Dutch word kil, meaning a small stream where these fish were traditionally found by Europeans.
They are social fish and cohabitate easily with most other non-predators of a similar size. Just understand that if food becomes scarce, a fully-grown 6-inch killifish won’t think twice about whacking very small neighbors as potential prey.
Overall though they are easy to keep and generally have a good temperament. The most difficult thing to do when choosing a killifish is to decide on which species to buy from the many, many varieties, colors, patterns, and lifespans that are on offer.
Although coloration is subjective and nearly limitless, there is one key differentiation among individual family members, longevity.
Those coming from cold environments tend to live longer than their tropical cousins. So do you want your pet for a single season, one that lives about a year (an annual), or last several breeding seasons?
If you are looking for a fast-breeding killifish with highly adaptive environmental responses for genetic research, the African turquoise killifish (Nothobranchius furzeri) would be a perfect match. They live only a few months and are prone to very high rates of mutation.
Conversely, If you are looking for a brilliantly colored, prolific breeder for your home aquarium, the Steel-Blue Killifish might be more your calling. They survive for up to three years and are amongst the most visually striking of their species.
Either way, there is a killifish for every owner in this club that you cannot afford not to join.
Lifespan
There are two main divisions among the members of the Order Cyprinodontiformes, longevity and the nature of their reproduction.
Some will develop from eggs while others are born live, killifish always develop eggs that separate them from most other members of the Order.
Amongst killifish, there are also divisions about their longevity, those that live short lives tied to seasonal weather changes (drought) and others that live longer durations in colder water that is more permanent.
The average lifespan in nature is under one year, typically tied to cyclical weather conditions. In captivity, in perpetual wetness, and with utmost care this can be extended to about 5 years, but their natural lifespan is mostly limited to seasonality.
The most common strategy for increased longevity involves regulating their food intake to ensure both physical wellness and bio-fauna health into maturity. Getting your fish from a reputable breeder will ensure that your killifish are of the highest pedigree, are brilliantly colored, and live as long as possible with optimum care.
Appearance
Most killifish can be described as medium-sized, highly hydrodynamic, vividly-colored fish. They come in every color imaginable and are often prized for their opalescent shimmer. Many individual species are named after these bold colorations directly.
These you will see identified in pet stores under all spelling variants and many common, informal names being used: kelli fish; kilifish, killi fish, killie fish, killiefish, killies, killifish, killy fish or even toothcarp and pupfish.
Rest assured each animal can be more precisely identified. Always ask by binomial name for a specific member of this very large family to avoid misunderstandings; bring pictures if it helps.
The one’s you’ll often find labeled correctly are the:
Aphyosemion cognatum, Redspot Panchax
Epiplatys annulatus, Banded Panchax, (Common name: Clown Killifish)
Aplocheilus lineatus, Striped Panchax,(Common name: Golden Wonder Killifish)
and Fundulopanchax gardneri, or Blue Lyretail
In these cases, their purity, pedigree, and brilliant colorations garner a premium in this flashy, fishy, family.
Be sure to also check out our care guides for some of these specific species when planning your own killer mob. And just for accuracy, only Australian animals like kangaroos and human crime syndicates form mobs; fish come in shoals, armies, troops, gleans, swarms and schools depending on the type.
Size
The males are larger and more colorful, often pea-cocking like similarly sexually dimorphic fish (and birds) for a mate. Most are only a few inches long but even this can show variations by individual micro-environment.
In areas devoid of predators with slow-moving water they tend to spawn larger and fewer specimens; less favorable conditions produced more numerous and smaller offspring.
Killifish Care
Your choice of killifish species will determine the level of care required. Although they adapt easily to live in varied conditions, their longevity, health, and especially their breeding patterns will be affected by their ecosystem.
Some types only breed under optimal conditions (drying-out ponds before the rainy season) while others do so prolifically without aid. Some will also benefit from vitamin and mineral supplements to promote good health including skin condition and vibrancy.
Tank Size
Killifish love the company of others and can often be found schooling with others and sometimes isolating themselves with their own kind.
Given their modest size but a propensity to gather in often shrinking waters they are more tolerant to crowding than others. So, give each one 10 gallons to roam about and they will make themselves at home with their neighbors.
Water Parameters
Normally killifish make their homes in a wide variety of slow-moving streams, rivers, estuaries, swamps, waterholes, and creeks throughout the world.
While they are highly adaptable and tolerant of extremes as with many domesticated pets, you are trying your best to mimic their ideal, natural habitat.
Regardless of the variety you choose, they are equally resilient to a wide range of water pH balances, temperatures, and degrees of salinity; you always want to aim for optimum conditions. A tank near 70F, with neutral to slightly acidic (pH 6.8), salt-free water to make these river fish feel at home.
What To Put In Their Tank?
Again we are trying to mimic the ideal conditions of their natural habitats. Since they like slow or non-moving water, there is no need to mimic a current.
Killifish also spend most of their day below the surface of shallow pools away from predators. You can stick with natural materials for your substrate and aim for minimal decor with a few good hiding spots in this simulated home environment.
This will ensure that there are defined boundaries allowing females and immature males to pass through or hide temporarily during their transit of defined male territories.
This normal territorialism becomes most acute during periods of breeding. Ensuring a good ratio of males to females (1:3) not only ensures the survival of your killifish but also helps to ensure good genetic diversity.
Once your visitors have arrived, also be certain to affix a good lid. Killifish are prolific jumpers and liable to make a run from the law if given the chance.
In nature, this would help them escape vanishing ponds for better ones nearby. So to avoid your killifish from going on the run, be sure to keep the lid securely fashioned.
Common Diseases
Like similar small freshwater species, killifish have excellent immune systems and do not suffer from many endemic illnesses. The most problematic illnesses they suffer come from fungal and viral sources.
While having a good diet and a clean, healthy tank are your best bets for prevention, if they do get sick, most health issues can be controlled through prudent quarantine of sick animals and timely use of therapeutic medications.
Like in humans tetracycline is often used to treat killifish bacterial infections; when using this therapy always follow the manufacturer or veterinarian’s instructions on dosage and possible complications for other tank mates.
While clean, optimum conditions are ideal for killifish, they continue to surprise science with their tenacity. They have been found in bodies of water near industrial sites with many thousands of times the lethal dose of heavy metals and PCBs mulling about.
What Do Killifish eat?
While their general care and upkeep are flexible and forgiving, their choices in dining range from opportunistic to plain old picky.
They are natural hunters preferring to take down and enjoy live foods whenever possible, only resorting to carrion or vegetal grazing in times of short food supply. In the aquarium, this equates to regular live feeding of aquatic worms and crustaceans supplemented by flake food.
A few members of the order have adapted to more omnivorous diets again demonstrating their amazing, swift adaptations at survival. Others have found niches like eating mosquitoes in malarial swamps.
A correlation has even been made in these areas where the prevalence of malaria (transmitted by mosquitoes) is directly proportionate to the number of killifish living in these pools.
Behavior & Temperament
Killifish don’t mix well with large or aggressive fish and they may see very small neighbors as a possible food source. Overall they are hardy and non-aggressive, but like many fish become stressed in the absence of food or a shortage of plausible mates.
Most of their time they leisurely cluster below the surface in small ad-hoc groups. Protection through numbers is their racket and it has served their cause very well to the power of several billion individuals worldwide.
Killifish Tank Mates
Temperamentally, Killifish are fine to be with most community aquarium fish. However, given their specific dietary needs and enjoyment of cooler temperatures, it will somewhat limit who will also want to call this environment home.
Beginners hobbyists should consider keeping a species-specific killifish tank until you’ve sufficiently researched potential tank mates who like living in similar conditions.
The most commonly kept killifish companions are:
- Danios
- Neon Tetras
- New World, Dwarf Cichlids
- Rasboras
- Swordtails
Be sure to also consider adding bottom-feeders like corydoras and plecos as companions. These won’t compete for the same food sources, can clean up the carnivorous mess left behind by your killifish, and still thrive in their modestly low oxygen environment.
Please also always research the precise species of killifish you intend to buy before mixing them with other fish. Although we have detailed primers on the most popular ones there are still over 1270 individual species to choose from.
Although many of their characteristics can be generalized to accurately describe them all some can prove more ornery or docile than others.
Breeding
If your male killifish goes from handsome one week to an absolute knockout the next, be wary: he’s hormonal and ready to breed.
At this point, it is best to separate a female and her potential mate from the rest of the tank to lay their eggs. Too many others will see the eggs or juveniles as a food source and they require some extra nurturing to hatch into fry.
Like all other cases in keeping wild animals, we do our best to simulate their natural environment and optimum breeding conditions.
Normally killifish lay eggs in sandy, calm shallows; so if you can replicate those conditions in your holding tank, it will prove a fruitful breeding ground. Once the eggs are fertilized and safely tucked away, remove both parents from the holding tank; now comes the only tricky part.
You have to wait out a simulated dry season for them to hatch. This involves keeping the fertilized eggs and sand warm and moist for around 3 months. After this dry season is over you can return the eggs to the water in your holding tank where they will develop into fry.
While the fry are small they will survive by eating their egg sacs. Later on, as they develop into fingerlings they will be able to eat larger and hunt down more complex items. Fingerlings are adept at taking down small aquatic arthropods and love newly hatched brine shrimp.
Once they have evolved into juveniles they will be big enough to compete against others, feed themselves, and re-join the greater community.
Final Thoughts
If you create a space similar to their own biome your killifish community will flourish.
These beautiful, adaptable time-twisting fish will not only add beauty to your own home but perhaps one day give humans the keys to controlling longevity.
Although far from being Time Lords or holding the chalice of immortality we can learn from their genetic adaptations to identify the aging clock in other species including humans. Now that’s a club I’d like to join!
