Least Killifish: Complete Care Guide

The Least Killifish is a small species, great for nano-tanks. They’ve also got some quirks to their care that you should learn before you add them to your tank.

Ready to dive in? I’ll show you everything you need to know about Least Killifish Care.

Least Killfish


Least Killifish Quick Care Sheet

  • Common Name: Least Killifish
  • Alternate Common Name(s): Dwarf Top Minnow, Lesser Killifish, Mosquito Fish
  • Latin Name: Heterandria formosa
  • Care Level: Beginner
  • Tank Size: 5+ Gallons
  • Size: Males ⅘”, Females 1 ⅖”
  • Diet: Omnivorous
  • Behavior: Peaceful Schooling Fish
  • Lifespan: 4-5 years
  • Reproduction Type: Livebearing 
  • Water Temperature: 64° -78°F (18°-26°C)
  • pH: 6.5-7.5
  • Water Hardness: 5–25 dKH

Introducing the Least Killifish

Let’s get one thing straight right out of the gate: the Least Killifish is not a killifish species. It’s also not a top minnow despite its alternative common names. Instead, it’s part of the Poeciliidae family, which includes such perennial favorites as the Fancy Guppy, Mollies, and Swordtails.

It is, in fact, the world’s smallest known livebearing fish. The males reach only ⅘” in length, while the females come in a bit bigger at 1 ⅖” in length and a bit more girth. Unlike a lot of livebearers, Heterandria formosa lacks bright coloration.

The species is usually an olive color up top, with a horizontal stripe. The belly underneath the stripe is white or greyish. They also generally have a dot on their dorsal fins, and the females have a dot on their anal fins as well.

Least Killifish don’t have a specialized diet. Fish flakes work fine, without the need for frozen or live foods. Culturing Daphnia can help reduce the workload on the aquarist by keeping a constant supply of live prey in the tank, but it’s not necessary.

The Least Killifish is among the smallest fish in the world. They are the smallest fish present in North America.

Despite being a bit small and drab, they’re a favorite for some aquarists. An incomplete list of reasons why:

  • Native Fish- Some folks really like to keep fish from closer to home. The Least Killifish is an American fish and one of the few that’s readily available in the aquarium trade.
  • Planted Tanks- For those who want fish but don’t want them to steal the show from their aquascaping.
  • Unique Breeding- The Least Killifish is a bit different from other livebearers. We’ll discuss this in the breeding section below.
  • Brackish Friendly- These fish can survive in brackish conditions, making them a good choice for smaller brackish tanks.
  • Beginners- Least Killifish are very hardy, and their parameter requirements make them a forgiving fish to keep.

Not a Pet-Let’s Talk About Colonies

Least Killifish aren’t really “pets” in the usual sense.

Or in the sense that fish like Bettas, Goldfish, and Oscars are.

As far as they’re concerned, the two of you are in different worlds. They may or may not even recognize that you’re the one who brings them food.

So, you’ll need to change the way you think about fish tanks to have long term success.

A Least Killifish tank is best thought of as a colony rather than a pet enclosure. Generations of fish will live and die in the aquarium without any specific input from you. When you make decisions about the tank you’ll need to keep in mind the entire colony although individual fish may need medical treatment on occasion.

In my opinion, this makes them great beginner fish for those interested in aquaria. The level of interest between someone who keeps a goldfish bowl and someone who begins creating purposefully designed aquarium systems is just a matter of depth of interest.

These fish are hardy and breed easily in most water parameters. You can start with just a few of them and watch as they breed to match their environment and the level of food that’s found in it.

If you remember that your goal is to keep a colony alive, rather than individual fish, you’ll have a much easier time. Your colony will include both plants and invertebrates in addition to these fish, and reaching the perfect equilibrium is the goal for most of us.

If you’re looking for a livebearer that’s closer to a pet for youngsters, you’ll find that Fancy Guppies or Mollies are just as easy to keep and display a lot more individual traits.

Origins of the Least Killifish

The Least Killifish is a North American native fish. They’re found along the Gulf and Atlantic coasts and farther inland. Their use as mosquito fish has caused them to spread a bit out of their natural territory, but they’re not considered invasive.

The Least Killifish comes from a blackwater environment. Blackwater runs through swampy and marshy environments and the water suffers from a lack of clarity. Blackwater usually has a lower pH than normal rivers.

More importantly for the home aquarist in this case, is the fact that the Least Killifish’s normal environment is weedy with almost overwhelming amounts of plants. The areas they’re found in could be described as choked by vegetation.

These fish are great for aquatic studies. They’re much hardier than most fish out there and can tolerate an incredible range of water conditions.

In the wild, the Least Killifish is primarily a micro predator. They will eat algae and other plants on occasion but it’s not their preferred food. At their size microorganisms like Daphnia are a great meal for them and it’s not a bad idea to keep a culture going.

On the other hand, being the smallest fish in their natural environment means they’re snapped up by every fish or bird that finds them.

Killifish naturally spend a lot of their time going through plants. It provides natural cover for them and allows them to feel secure.

The takeaway for aquarists is this: Least Killifish are hardy and their natural environment is quite easy to replicate at home.

Ideal Tank Setup

Least Killifish don’t require a lot of specialized equipment, but plant choice is the key to keeping a good colony going.

Least Killifish colonies are sometimes created in a self-sustaining, no equipment fashion. While these are beginner fish, self-contained systems are not something for a beginner to tackle.

An ideal Least Killifish tank will have the following qualities:

  • Slow Flow- These fish don’t care much about ultra-clean water, but they can be thrown around easily by currents. Using in-tank or sponge filters is ideal in this case and lets you get clean enough water without disturbing the fish.
  • Heavy Vegetation- The key to a thriving colony of Heterandria formosa is keeping the tank heavily planted. We’re talking heavily planted, or creating plant-choked areas in a larger tank.
  • Cycled Tank- You can do a fish-in cycle with these fish, but it’s a lot of extra work. Just get your tank cycled and use the week or two required to establish the plants in your tank.

There’s not a whole lot to the care of these fish other than being able to keep plants growing. Keep reading for some ideas on what to do next.

Recommended Equipment

You’ll just want the following as you get the tank started.

  • 5+ Gallon Aquarium
  • Nutritious Substrate
  • Aquarium Light
  • Hardscape Elements
  • In-Tank or Sponge Filter
  • Heater
  • Water Conditioner
  • Aquarium Test Kit

You can actually skip the substrate and hardscape in this case, but make sure you’ve got a bit of a green thumb for floating plants.

I also strongly recommend getting aquarium fertilizer and a CO2 booster. The SeaChem lineup (commonly found in all pet stores) is more than adequate in this case.

Lighting should be around 1W or 50 Lumens per gallon. Try to stay close to that range. If you go much higher then you’ll have a tank that requires constant trimming.

Plant Ideas

There are a couple of ways to go when it comes to these fish, and I’m going to cover both.

A standard planted tank allowed to overgrow is my preferred method. It does require you to have some experience with these tanks. Purposeful overgrowth can be hard to manage.

That said if you’re just learning about planted aquaria these fish are perfect. You can make mistakes that would outright kill some of the more vulnerable nano-species and Least Killifish won’t be phased.

The following are good choices for beginners, but all will need to be rooted:

  • Cryptocoryne Wendtii- A good, basic plant that spreads by runners. When properly fed, Cryptocoryne species create a dense, jungle-like effect. Just be sure to trim dying leaves and be prepared for the initial “Crypt Melt.”
  • Vallisneria sp.- Any Val will work out great and help create a thickly planted environment without much mess. Let them grow along the top of the water and they can work as the sole plant in the tank.
  • Echinodorus amazonicusThe famed Amazon Sword is a great centerpiece for a small tank. When provided with CO2 and enough nutrients they create a thick layer of leaves from their initial basal rosette and they’re forgiving for newbies.

I don’t recommend Java Fern or Anubias sp. in this case. While both are great beginner plants, their slow rate of growth means they’ll serve a tertiary, at best, function in a tank designed for Least Killifish.

The other option is to create a densely packed, bare-bottom tank. This is commonly done for breeding colonies of these fish, and they really don’t suffer for it.

The following are good choices if you’re going to rely on just floating plants for cover. They all fit well into regular planted tanks as well.

  • Java Moss- Java Moss is green, lush, and usually remains alive in an aquarium unless it’s sterilized. You can use fishing string or twine to tie it to a rock or piece of driftwood and keep it loosely in place, or just set it free to roam with the current.
  • Ceratophyllum demersum– Commonly called Hornwort or Coontail. This dense plant has tons of needles and becomes nearly unstoppable once established in a tank.
  • Elodea sp.Anacharis is available in most aquarium stores, and is often sold as bunched stems to be planted. Elodea is a floating plant, not a rooting one. Just throw it in there for the best results, when planted it will rot under the substrate before coming loose.

Pro Tip: You can manipulate both Anacharis and Hornwort to create denser growth by selective trimming. The basic idea is to trim these plants right above the growth nodes on the stems. This causes the plant to branch and you can create an exponential amount of stems by doing this.

For brackish tanks, your plant choice will be limited and it’s hard to find answers online. I can only say from experience that Java moss and Anacharis are your best bets for plants in a brackish tank. Drip acclimate over a day or so for the best results.

For these fish do not use silk or plastic plants. The low flow required by their small size limits your maximum filtration flow. You want real plants to act as an additional nutrient sink to keep the water column clean.

Tank Cycling

Cycling your tank is something that every aquarist should be familiar with. A fishless cycle is always the best choice. If you’re not familiar with cycling, here’s a quick breakdown:

  1. Set up the aquarium without fish.
  2. Place a small amount of food in the tank each day.
  3. Test water daily for ammonia. Wait until you have detectable levels.
  4. Test water daily for nitrites once ammonia has been detected. Continue testing until the nitrite levels are at 0ppm.
  5. Test for nitrates daily until at an acceptable level (<30ppm for Least Killifish)

Then you can begin to add your fish. Since these fish require heavy vegetation you should begin to establish the plants in the tank as well.

For those in the back, I do not recommend or advocate cycling a tank with fish in it. It’s ethically murky and can result in fish deaths. It also takes longer and requires more work.

Facing reality requires us to acknowledge there is a subset of fishkeepers who will do it regardless of any advice against it.

You’ll proceed the same way, with your fish in the tank, but there are a few key differences:

  • Test twice daily. In a nano-tank water parameters can swing wildly in just a couple of hours. You owe it to the fish to test at least twice a day and prevent any unnecessary problems.
  • Water must go through at least a 25% change every day until cycled. No exceptions.
  • Once ammonia and nitrite begin to show on the test kit you need to up that water change to 50%.
  • Due to all the water changes, fish-in cycles usually take 3-7 days longer than a fishless cycle.

Least Killifish, realistically, are unlikely to die during a fish-in cycle. Following the above guidelines will help keep them healthy, and prevent any unnecessary consequences in the near future.

Least Killifish Behavior

Least Killifish are rather shy but can become rather lively once they’ve figured out they’re safe in their enclosure.

They’re small schooling fish and it’s best to keep them in groups of at least 6. Like other livebearers, keep a ratio of one male to two females for the best results. For a 5-gallon tank, 6 is more than enough.

Their behavior is pretty simple for the most part. They swim in thick vegetation, school a little bit, and eat when they can. They’re quite peaceful for the most part and relaxing to watch.

Detecting Illness

Since Least Killifish don’t display much out-of-the-ordinary behavior, behavioral differences can mean they’re getting ill. Keep an eye out for the following:

  • Lack of appetite
  • “Flashing” or scraping the body against aquarium decor
  • Buoyancy
  • Tilted swimming

Basically, anything out of the ordinary is a good cause to suspect disease. From there you’ll have to try to make a diagnosis and treat the fish in the appropriate manner.

Inbreeding Depression

Inbreeding depression seems to occur within the Least Killifish at an increased rate compared to most livebearers. The concept is pretty simple: Least Killifish tend to breed less when they’re inbred.

This is the case in a lot of fish, which is why outbreeding is so important.

The difference is of little interest to most casual aquarists. There are plenty of colonies that have been going on for years and years without outbreeding. The most significant problem that arises is the fish lose their female bias when it comes to sexual characteristics. 

Inbred populations will tend to even out the numbers of each sex, potentially creating an imbalance from the preferred male-to-female ratio.

That said, if you want to keep your fish in ideal health you’re best off throwing some fresh ones from a supplier in the batch every few generations. Starting with a colony of 25+ in a larger tank can sidestep this problem for the most part. It appears the problems begin when it’s direct siblings breeding and they can withstand a shallow genetic pool.

Identifying Sex

Least Killifish are sexually dimorphic. The size difference is the most obvious indication but there is one other major sign.

The anal fins of the fish vary based on sex.

Males have extended anal fins used during reproduction. They’re easily identified, especially if there’s a female nearby. In any case, they’ll generally look oversized.

Females, on the other hand, have normal-sized anal fins with a black spot.

Suitable Tankmates

Finding tankmates for nano-fish is always a bit rough. Their small size naturally limits the fish that you can keep them with, but if you keep the following in mind you can make the right call:

Any fish placed with Least Killifish should be less than 2 ½” long and have a peaceful temperament. As long as you have a sufficient density of vegetation you can keep them with active fish without spooking them.

Some good examples are the following:

Bottom dwellers can fudge the size requirements a little bit, but only if they’re not predatory fish. Catfish (excluding the above), in particular, can quickly make your Least Killifish population disappear.


Every good aquatic system has some invertebrates included.

In this case, you’re good to go with the standard mollusks and micro shrimp that can be found in most pet stores.

I recommend some of the following:

  • Ramshorn Snails- Breed easily and come in a few different colors. Great detritus cleaners and harmless overall. They breed less prolifically than some other small freshwater snails.
  • Pond Snails- Breed very rapidly and will consume any algae or detritus in their path. Pond snails are a good choice if you’re not a frequent water tester, a population explosion means an increase in nitrates for instance.
  • Malaysian Trumpet Snails- Good for tanks with a substrate. They stay under the soil while the light is on. Their movements aerate the roots of plants and they’re decent cleaners.
  • Apple or Mystery Snails If smaller snails aren’t your bag, these larger snails are often available in pet stores. One per 5-10 gallons is usually a good population density, but you’ll have to manually remove the younger snails when they get larger.
  • Cherry Shrimp- Neocaridina davidi. They come in a ton of colors nowadays and they’re easy to keep alive. They clean well and breed prolifically which makes them a good long-term choice for the colony. They’re durable and an initial seeding of 5-6 per 5 gallons will result in a continuous shrimp colony.
  • Red Crystal Shrimp- If you can maintain good enough water quality for these fish, you can mix them with Least Killifish. RCS’s are much harder to keep than Least Killifish so the colony should be built around them instead of the fish.

Least Killifish are small and not particularly aggressive with invertebrates despite being a predator by nature. They will eat the smallest shrimplets and snails, but they’re not able to bother anyone else.

All of these species will be able to reach an equilibrium with Least Killifish, creating a mostly self-sustaining micro-ecosystem.

Pro Tip: If you keep snails with exponential growth potential, then you may want to have a way to control them. The Assassin Snail, specifically Clea helena is widely available. I use them as temporary snail population control in the event of a population boom. They can be kept in a bare-bottom 2.5-gallon tank when not needed, all you need to do is drop in a couple of snails once a week.

Brackish Companions

Brackish tanks require different fauna for the most part. 

Unfortunately, most of the common brackish fish are right out. They’re far too big and most are very predatory.

There are a couple of creatures that are of interest to someone keeping a brackish Least Killifish tank:

  • Fiddler Crabs- Fiddler crabs are harmless to fish and easy to care for. They’ll need a small area above the water level to crawl onto but require little more than a bit of fish flake apart from that.
  • Bumblebee Goby- One of the most popular brackish fish. Bumblebee Gobies are tiny and lively. They’re easy to care for as well, which makes them good company for your Least Killifish.
  • Olive Nerite Snails- Hail from brackish waters and do a good job cleaning up detritus. Other Nerite species may or may not survive in brackish water for an extended period of time.

Do not keep Least Killifish with Red Claw Crabs, Vampire Crabs, or any other freshwater crab. Fiddlers are comparatively harmless compared to the wide world of brackish crustaceans. They’re dedicated bottom feeders and I’ve never seen one fight back against a nosy fish. Let alone try to go after one.

The other species can and will kill smaller fish whenever they can. They’re also more dangerous to handle for the inexperienced. While they’re not a serious risk, I can confirm that a full-grown Red Claw Crab can do enough damage to require a pair of stitches.

The majority of brackish fish are unsuitable as companions for Least Killifish. While the reasons for most are obvious, don’t think you can get away with keeping a dwarf or pygmy puffer with Least Killifish. Puffers, even the smallest, are far too predatory to mix well.

Breeding Least Killifish

Breeding Least Killifish is usually as easy as keeping them in a tank that suits them. Breeding may become suppressed if there’s not enough food available or the water is too dirty. Some also report that they have trouble breeding when the temperature is low.

The most interesting aspect of Least Killifish reproduction is the way they release the fry.

Fish like Guppies and Mollies dump a ton of fry at once. 40-50 fry swimming around isn’t unusual after a single female has given birth.

Least Killifish undergo a process called superfetation. The core idea is that there are a ton of babies in there but they’re all developing at different stages. When a female least Killifish initially becomes pregnant the eggs hatch and fish begin to grow.

From there, the female will slowly release the fry as they mature. The cycle continues indefinitely. In most well-kept Least Killifish tanks there are one or two fry dropped each day.

Remember what I said above about colonies?

The coolest thing about Least Killifish is they offer the ability to build an active, thriving colony of fish to a complete beginner.

Least Killifish fry are quite large when they’re born. Unlike a lot of fish, they’re able to accept powdered flake or baby brine shrimp as soon as they’re born as well.

Oh, and their parents don’t eat the fry in most cases. I’ve heard of more cases of the big fry eating the smaller ones than parents chasing down their offspring. 

If you’ve ever kept livebearers you know how big of a headache keeping the parents from eating the fry are. Guppies, for instance, are known and prolific cannibals.

As such, you don’t necessarily need to set up a separate nursery tank for the fish. Instead, focusing on making sure there’s enough vegetation to provide coverage for the babies as they grow out.

What to Do If Your Least Killifish Aren’t Breeding

Least Killifish reproduction doesn’t require much on your part in most cases. Throw them in, let them acclimate to the tank, and enjoy the constant stream of fish fry as you watch them grow into adults.

In some cases, however, people have found that they don’t seem to breed no matter what. In some cases, this may just be a newbie error like having a “harmless” tank companion that’s eating them.

On the other hand, you may also have water parameters in the wrong place. Dirty water is one cause, but if you’re planning to breed fish it should have been ruled out long before you made it here.

The two biggest parameter adjustments that matter for breeding are temperature and pH. Try to keep the pH of the tank at 8.0 or lower, and raise the temperature to 82°-84°F (27°-28°C).

A Note About Selective Breeding and Least Killifish

On occasion, aquarists like to create their own “lines” of fish. Livebearers often make this easier since they breed so prolifically and easily.

The process is simple: you pick the male and female that have the traits you’ve selected for. You isolate them together, get the fry, and continue to select the traits you’re looking for. A bolder black stripe, for instance, or trying to expand hints of color that showed up in one of your fish.

Here’s the issue: Least Killifish females can store sperm for up to 3 months.

So, you may end up with babies that don’t actually share traits with the fish you thought was the father.

For that reason, you should put more weight on the desired traits in the mother fish than on the father. It also helps to keep detailed logs.

Remember these fish seem to suffer from inbreeding depression at a higher rate than most. Keep a few mated pairs in separate tanks that are selected for the same properties and you’ll be able to avoid having a shallow gene pool.

Not a Killifish, and Certainly Not the Least

The Least Killifish is a unique creature, and it’s really not the least of anything. Instead, these unique North American livebearers are the perfect introduction to the world of aquarium keeping beyond just keeping pets. 

If you’re ready for the education, they’ll be waiting!