Least Killifish: Caring For The World’s Smallest Livebearer

Reading Time: 7 minutes
least killifish

Least killifish aren’t killifish at all. They’re also called lesser killifish or dwarf top-minnows, though they’re not top-minnow, either. So… what are they?

These guys are the smallest livebearer in the world, and one of the smallest fish species in the world as well. They’re a fun, unique, and hardy little US native. They make a fantastic fish for beginners, beginner breeders, summer tubs, nano aquariums, and small-species community tanks – you really can’t go wrong with these guys!

I honestly can’t think of another fish that you have quite as many opportunities with as you will these guys.

With that said, they do have some care requirements that need to be met – albeit some easy ones.

Disclosure: we’re reader-supported! So if you buy a product I recommend, I might make some coffee money at no cost to you.

Table of contents

least killifish care


Heterandria formosa Classification

IUCN Status: Least Concern – last assessed 2/14/2012

Class: Actinopterygii are ray-finned fishes, which is a subcategory of boney fishes. These fish are characterized by the bony structures that support their webbed fins. This group makes up nearly half of all living vertebrates.

Order: Cyprinodontiformes is an order of ray-finned fishes that consists of mostly small, freshwater fish, and includes killifish and livebearers.

Family: Poeciliidae, even though the whole family is known as “live bearers,” there are some fish in the family that are eggs scatterers who utilize external fertilization.

Genus: Heterandria is a genus of livebearers that mainly come from Guatemala and Mexico. They look similar to top minnows at first glace, hence why most are called “killifish,” even though they’re not.

Species: Heterandria formosa

What does Heterandria formosa mean?

Heterandria means “different male.” This is likely because every fish in the genus has some clear sexual dysmorphism.

Formosa means “finely formed” or “beautiful.” Given their small size, it’s likely that “finely formed” means both in stature and appearance.

Though the males are much smaller at 3/4,” compared to the 1.5″ females.

Find Other Fish

Looking for something specific? You can discover other fish with similar characteristics! They open in a new tab so you can keep reading too!

Distribution & Natural Habitat

Midget livebearers are found predominantly in freshwater, but can occasionally be found in brackish conditions. Though I don’t recommend them in your tank. They span six states in the southern US from the southern parts of North Carolina down to Florida and west from North Carolina to Louisiana.

They can usually be found in stagnant ponds or sluggish waterways, within heavy vegetation or marginal plants.

heterandria formosa wild habitat
Cape Fear River, NC

If you live where they can be collected, it might be tempting to go collect them yourself, but you should look into local permits and regulations to make sure you can. And, even if you can, you’re going to want to make sure you have a well-stocked medicine cabinet to treat any parasites, as they’ll likely be your biggest issue with wild-caught livebearers.

Aquarium Care

Difficulty: Beginner
Size: 1.4″ (3.5 cm) max
Lifespan: 2 – 3 years
Tank Size: 5 gallons (20 liters)
Diet: Omnivore – unfussy
Temperature: 68 – 80 F (20 – 27 C)

pH6.5 – 8.0
Hardness: 5 – 20 dKH
Temperament: Usually peaceful, but not shy
Breeding: Easy
Swimming: Everywhere they can
Availability: Uncommon, but easy to find online

These guys do well in any condition above but, like any livebearer, they do best in warm slightly hard water. They can be tough to find, but most online stores that deal with native species would be a good place to start if you want to buy some.

Tank Specs

A 5-gallon tank works well if you want to keep a handful of dwarf killis in a species only tank. If you want to add some tankmates or keep a larger colony, a 10-gallon would be a better starting place. Unless you have a ton of them, I wouldn’t put them in anything much larger than that because they may have a hard time finding food without you polluting the tank.

They do well at room temperature, so you don’t need a heater, and they don’t seem to be jumpers, so you probably don’t need a lid, either. If you’re interested in getting into summer tubbing, these guys would make a wonderful candidate!


There’s not a whole lot of ways this can go wrong. Like most livebearers, you’ll want more females than males if you want to get a good breeding colony going, but they don’t seem to be as obnoxious as guppies, so you don’t need to watch your male to female ratio as closely. Males can get a little feisty with each other when competing for a female, but they’re not large or aggressive enough to do any damage.

Still, a 1:1 ratio is probably the most I would do, though a 3:1 ratio (like with guppies) would certainly provide the least stressful conditions for everyone.


Of course, lots of plants are welcome! But, other than that, there’s not a ton I would suggest. If you want to get spider wood, which usually ups your pH, just make sure you keep an eye on it while it leaches tannins so the pH doesn’t go nuts. You could also use crushed coral, limestone, or Texas holey rock to keep your pH up. Dragon stone is another great rock option, but it doesn’t typically affect pH.

dragon stone
Dragon stone

River stones or sand are also goods options for substrate. I like HTH pool filter sand, which is linked just so you can see what it looks like and if it’s near you. I wouldn’t buy it online if you don’t have to, it’s always cheaper in store. If you plan on trying to grow difficult plants, I suggest one of the planting substrates below instead.

Best Plants For Least Killifish

There aren’t any plants that mosquito livebearers wouldn’t do well with, but the best plants you can get them – if you want a breeding colony – are going to be bushy and fast-growing. These will be the most helpful to fry, both as a place to hide as well as a source of infusoria.

But bushy plants will also help the adults feel secure and be able to get away from each other if they get stressed out.

Water Wisteria

Water wisteria is a fast-growing plant good for keeping ammonia, nitrates, and nitrites under control. It’s great for providing shelter for scared fish and fry, keeping cyanobacteria (aka blue-green algae) at bay, and has anti-microbial properties.

Difficulty: Bulletproof
Growth: Slow
Temperature:  64 – 86F (18 – 30C)

pH: 5.0 – 8.0
Hardness:  2 – 25 dKH
Placement: Anywhere, basically

Bacopa sp.


Bacopa comes in a variety of species and variants – some more demanding than others – but the least demanding is Bacopa carolinia. It is banned in a few states because of its invasive species status, but it makes a wonderful addition to most aquariums if you can get it.

Difficulty: Easy
Growth: Moderate
Temperature: 59 – 86F (15 – 30C)

pH: 5.0 – 8.0
Hardness: 3 – 12 dKH
Placement: Planted in substrate

Cabomba sp.


There are several species of cabomba, the green is the easiest. It’s difficult to get in certain states because in some places it’s considered invasive. Somewhat ironically, we seem to struggle growing it in aquariums. If you can get your lighting high enough, it’s worth it.

Difficulty: Moderate
Growth: Moderate
Temperature:  72 – 82 F (22 – 27 C)

pH: 6.5 – 7.5
Hardness: 3 – 12 dKH
Placement: Floating or planted

Guppy Grass

guppy grass

Guppy grass is a great floating plant that adds depth, structure, and cover to any tank. It’s a super easy plant to grow and takes up tons of ammonia, nitrates, and nitrites, but – beware – it grows fast and can easily overrun your tank. Overall, it’s an amazing plant to have if water quality is your top concern.

Difficulty: Easy
Growth: Rapid
Temperature: 50 – 86F (10 – 30C)

pH: 6.0 – 7.0
Hardness: 2 – 25 dKH
Placement: Floating, planted, weighted

Guppy Grass

guppy grass

Guppy grass is a great floating plant that adds depth, structure, and cover to any tank. It’s a super easy plant to grow and takes up tons of ammonia, nitrates, and nitrites, but – beware – it grows fast and can easily overrun your tank. Overall, it’s an amazing plant to have if water quality is your top concern.

Difficulty: Easy
Growth: Rapid
Temperature: 50 – 86F (10 – 30C)

pH: 6.0 – 7.0
Hardness: 2 – 25 dKH
Placement: Floating, planted, weighted

Java Moss

Java moss

Java moss is an almost bulletproof plant that requires almost no care. It doesn’t grow nearly as slowly as it’s java fern cousin, and can create lush moss beds that blow and billow in the current. They’re a great option for grazers, fry, or those of you with the blackest of thumbs.

Difficulty: Easy
Growth: Moderate
Temperature: 59 – 86F (15 – 30C)

pH: 5.0 – 8.0
Hardness: 3 – 12 dKH
Placement: Floating or attached

Lighting & Filtration

Like most fish, least killifish do best with subdued lighting, though they’re not as opposed to it as most other species seem to be. You can subdue the lighting with floating plants since they don’t get as spooked by it as some fish do.

They also do best with slow-moving water, so a slowly bubbling sponge filter would be the best solution. Particularly in such small tanks.

Water Care

Dwarf livebearers are hardy and can handle a good deal of mistakes, but that doesn’t mean you should try to neglect them. Obviously, the more you change their water, the better, but you can probably get away with twice a month if your tank is well planted, well cycled, and lightly stocked.

killifish tank

And, again, even though they can be found in brackish water, you shouldn’t put them in a brackish tank unless you have a good reason to do so. They don’t usually have issues with diseases, but if they’re wild-caught, a good deal of yuck could easily pop up. I have almost all the meds you should need to get yourself through quarantining wild-caught or even medicating tank bred least killis.

Feeding Least Killifish

Least killis aren’t picky fish by any means, but they are small fish and omnivores at that, so you should try to mix up their diet with tiny food offerings. If you want to get them into tip-top condition, live food is the best option. Not only will live food make them and their fry healthier, but it’ll also make them more colorful.

dwarf livebearer care

They’ll gladly gobble baby brine shrimp (both babies and adults!), mosquito larvae, bloodworms, small daphnia, wingless fruit flies, small grindal worms, and other small creatures they can fit in their mouth. If you’re not into feeding live food, frozen is the next best option. They’ll take a similar range of food, but you can also include things like mysis shrimp to their diet.

Though not my favorite recommendation, you can also feed them dried foods. As always, I recommend avoiding flakes because they’re just not healthy enough to be worth your hard-earned cash. However, golden pearls, decap brine shrimp eggs, baby brine, bloodworms, tubifex worms (crumbled), freeze-dried blackworms (also crumbled), and some plant-based foods are all great options for these guys.

Anything under 800 microns should be good for your adults, and babies should be able to eat anything under 200 microns even immediately after they’re born. If you don’t know where to start, I have a complete shopping list below for you that should help you get started.

Common Least Killifish Diseases

Dwarf livebearers don’t seem to be particularly prone to illnesses, but there are a few that do pop up with relative frequency. Of course, like any living creature, it can get a host of illnesses, parasites, or infections – these are just the most common ones to look out for that seem to present the biggest issues:

Skin & Gill Flukes

skin and gill fluke

Skin and gill flukes are worm-like parasites that embed into your fish. Since they’re so small, they’re almost invisible to the naked eye and incredibly difficult to diagnose. 

Before you freak out, flukes are present in almost every tank and are harmless under normal conditions.


  • Excess mucus on skin
  • Redness in gills and on skin
  • Itchiness
  • Scratching
  • Labored breathing (if in gills)


  • Generally, stress
  • Previous illness
  • Overcrowding
  • Wrong water parameters



Tapeworms are small rice-like worms that can drop out of your fish’s anus with or without passing poop with it. Due to the small size of the segments these worms break into, it can be incredibly hard to diagnose in fish.


  • Sunken stomach
  • Inability to grow
  • Generally not thriving


  • Infected by another fish

Bacterial Infection

bacterial infection

Bacterial infection is a broad term, the bacteria family can cause a wide range of symptoms and come from varying causes. Generally, you can treat them with a broad spectrum antibacterial regardless of the particular bacteria at hand. It’s diagnosing that’s usually the hard part.

Making matters even more difficult, fish can have an internal or external bacterial infection.


  • Red streaks
  • Red ulcers
  • Fuzzy growths
  • Pop eye
  • Bloating


  •  Poor water quality
  • Food that’s gone bad
  • Keeping fish in inappropriate water parameters
  • Stress

Fungal Infection

fungal infection

Fungal infections are a tricky bunch – not only do they have a huge family that presents a wide variety of species and symptoms – but some bacterial infections look strikingly similar to a fungus. 

If you’re not sure if you’re dealing with a fungus or a bacterial infection, I find it helpful to treat with Ich X and Erythromycin (provided it’s 100% erythromycin) at the same time to be sure I’m treating for both.


  • Cottony growths on body, fins, eyes, and/or gills


  • Prior untreated injury
  • Stress
  • Water quality-related issues
  • Prior untreated infection (bacterial, parasitic, etc.)

Columnaris (Cotton Mouth Disease)

columnaris disease

Occasionally called false neon tetra disease or cottonmouth, this is caused by a gram-negative bacterium. it can also, quite understandable, be confused with fungal infections. 


  • Discolored scales
  • Scales appear to be popping off (not “pineconing”)
  • Grey spots
  • Lesions on the back
  • Legions around the mouth
  • May result in fuzzy patches due to secondary infections


  •  Flavbacterium columnare (bacteria)

15 Least Killifish Tank Mates

Least killifish are naturally found alongside elassoma species (pygmy sunfish) as well as bluefin killifish, swamp darters, and ghost shrimp, so those make good choices for US native tanks or some biotope setups. I would, personally, avoid shrimp because smaller shrimp can easily be eaten and ghost shrimp can (and do!) eat smaller fish. But most people don’t have issues with them and shrimp.

If a native tank isn’t your cup of tea, no worries, you can look for small, peaceful fish that aren’t too active or boisterous.

Though dwarf killifish aren’t picky eaters, they’re not particularly aggressive eaters either, so they can be easily outcompeted for food or spooked by over-active tankmates. They also do best in well-planted tanks at room temp with sluggish, somewhat hard water, so keep that in mind as you’re looking at roommates. Even still, there’s no shortage of options.

Pygmy Sunfish (Elassoma sp.)

pygmy sunfish

There are currently seven recognized species of elassoma, though the easiest to find are E. evergladi and E. gilberti. All species stay under 1.5″ (4 cm) and need live food and dense vegetation to thrive in the aquarium. Though each species will have their own care requirements you should research, the general requirements for all the species are below.

pH: 6.5 – 8.0 (sp. dependent)
dKH: 5 – 20 (sp. dependent)
Temp: 70 – 80 F (22 – 26 C)

Size: 1.5″ (4 cm) max
Temperament: Peaceful and shy
Swimming: Bottom & in plants

Swamp Darter (Etheostoma fusiforme)

swamp darter

Swamp darters are US native species, and one of the few darter species that doesn’t like fast-flowing water. This makes it one of the easier darters to care for, though they typically only take live food. Their range of temperature, pH, and dKH makes them impressively adaptable in the wild, but be sure to check what your darter’s natural pH was before introducing it to your tank so you don’t shock them.

pH: 4.0 – 8.0
dKH: 5 – 20
Temp: 50 – 80 F (10 – 26 C)

Size: 1.5″ (4 cm)
Temperament: Peaceful and shy
Swimming: Bottom & structure surfaces, will dig in sand

Bluefin Killi (Lucania goodei)

bluefin killi

Bluefins do best with little to no water movement and plenty of plants, but they can handle a wide variety of temps and pH. They’re hardy, adaptable, eat easily, and do well kept in pairs or by themselves. If they don’t have enough plants, they can be a bit shy and need a lid because they can (and do!) jump pretty well.

pH: 6.5 – 8.0
dKH: 5 – 10
Temp: 50 – 80 F (10 – 26 C)

Size: 1.1″ (3 cm)
Temperament: Peaceful, can be shy
Swimming: Everywhere

Emerald Dwarf Rasboras (Microrasbora erythromicron)

emerald dwarf rasbora

A beautiful shoaling species that needs to be kept in groups of six or more. Even still, they can be a little timid without the right plant coverage. Once settled, they’re an active, gregarious, and beautiful addition to the right aquarium!

pH: 7.0 – 8.0
dKH: 12 – 20
Temp: 68 – 76F (20 – 24C)

Size: .75″ (1 cm)
Temperament: Shy shoaling fish
Swimming: Mid to top

Rosy Loach (Petruichthys sp. ‘rosy’)

Rosy loaches are best kept in groups of at least 6 and they do best in planted setups. They’re often wild-caught, so most of them are in poor condition when you buy them. They also need to be added to a mature aquarium since they don’t do well with environmental swings.

pH: 6.5 – 8.5
dKH: 5 – 15
Temp: 68 – 79 F (20 – 26 C)

Size: 1.2″ (3 cm) max
Temperament: Peaceful and active
Swimming: Mid to bottom

Celestial Pearl Danio (Danio margaritatus)

Celestial Pearl Danio

CPDs or Galaxy Rasbora (although they’re not a rasbora or a danio,) are small shoaling species. When stocking, buy as many as possible,  8 being the minimum I’d personally suggest. There have been reports that they’re hard to transition onto prepared foods, but this may be wild-caught specimens. There are numerous reports that they’ll take finely crushed flakes and micropellets.

pH: 6.5 – 7.5
dKH: 5 – 15
Temp: 69 – 79 F (20 – 26 C)

Size: under 1″ (2.5 cm) 
Temperament: Peaceful, can be shy
Swimming: Mid-water shoaling

White Clouds (Tanichthys albonubes)

These fish are best kept in groups of eight or more, though 10 is better. There’s little information of just how far spread these fish are, but they’ve been observed slow-moving white and blackwater streams in and around China. 

pH: 6.0 – 7.0
dKH: 5 – 20
Temp: 60 – 72F (15 – 22C)

Size: 1.5 – 2″ (3 – 5 cm)
Temperament: Peaceful shoaling fish
Swimming: Mid to top

Cherry Barb (Puntius titteya)

cherry barb

Cherry barbs are small and peaceful. They’re undemanding and pack a colorful punch when cared for correctly, making them an ideal community inhabitant. They’re shoalers, so they need to be kept in groups of 6 or more to bring out their best behavior.

pH: 6.0 – 8.0
dKH: 2 – 20
Temp: 68 – 81 F (20 – 26 C)

Size: 1.5 – 2″ (4 – 5 cm)
Temperament: Peaceful and active
Swimming: Top to midwater shoaling

Nerite Snails (Neritina natalensis)

nerite snail

The most common complaint about snails is that they can reproduce like crazy, this is especially true for tanks with tons of leftovers! Nerite snails, however, can’t reproduce in freshwater so this isn’t a concern for the average aquarium. A simple remedy to keeping shrimp in soft water or water with little calcium is to add Tums to the tank for them to munch on to get their calcium fill.

pH: 7.0 – 8.9
dKH: 6 – 12
Temp: 70 – 80 F (21 – 27 C)

Size: 1″ (2 cm) although somewhat species dependent
Temperament: Peaceful
Swimming: Everywhere there’s food

Otocinculus (Otocinclus sp.)


Otocinclus, like most peaceful fish, enjoy the company of their own kind – four or more is a good start. They enjoy cleaning algae and debris off glass, decor, and plants – but will always clean plants first if they have the choice. It’s important to add these guys to a well-established tank not only because it needs to have enough food for them to munch, but also because they’re highly sensitive fish.

pH: 6.0 – 7.5
dKH: 6 – 15
Temp: 72 – 82F (22 – 28C)

Size: 1 – 2″ (3 – 5 cm)
Temperament: Peaceful, social fish, usually shy
Swimming: On surfaces

Pygmy Corydoras (Corydoras Pygmaeus)

The pygmy cory (corydoras pygmaeus) is the smallest of all the corydoras species and – possibly – the smallest catfish in the world. It’s a peaceful shoaler that appreciates sandbeds and at least six of their own kind.

pH: 6.2 – 7.4
dKH: 2 – 15
Temp: 60– 78F (15 – 25C)

Size: 1.3″ (3.5 cm) max
Temperament: Peaceful shoaling fish
Swimming: Everywhere

Scarlet Badis (Dario dario)

Dario dario

Scarlet badis are tiny micro predators that are easily intimidated by large, boisterous species, so they rarely make good tank mates. In a few cases, however, it can be done well with care and planning.

pH: 6.0 – 8.0
dKH: 1 – 15
Temp: 65 – 79 F (18 – 26C)

Size: .75″ (2 cm) max
Temperament: Timid
Swimming: Bottom of tank

Norman’s Lampeye Killifish (Poropanchax normani)

Lampeye killifish

Norami killis, or Norman’s lampeye killifish, are non anual killifish (meaning they won’t die in a year) from Central and West Africa. It’s best to get these guys in schools of nine or more to see them at their best. Their blue “eye” nearly glows in the dark and is spectacular to see in person!

pH: 6.0 – 7.0
dKH: 4 – 15
Temp: 73° – 78° F (23° – 26° C)

Size: 1.6″ (4 cm)
Temperament: Peaceful, generally active
Swimming: Mid to top

Japanese Rice Fish (Oryzias latipes)

A bit challenging to find, but insanely easy to keep, Japanese rice fish can handle a wide variety of temperatures and make a fantastic beginner fish! It’s best to keep them in shoals of 8 or more.

pH: 6.5 – 8.0
dKH: 9 – 19
Temp:  64 – 75 F (17 – 23 C)

Size: 1.6″ (4 cm)
Temperament: Peaceful and active
Swimming: Mid to top

Asian Stone Cat (Hara jerdoni)

Asian stone catfish

Asian stone cats are tiny, adorable catfish that need to be kept in groups of six or more. They need plenty of hiding places, a good deal of dissolved oxygen, and a low flow.

pH: 5.6 – 7.6
dKH: 8 – 15
Temp: 64 – 75 F (18 – 24 C)

Size: 1.2″ (3 cm)
Temperament: Peaceful, can be shy
Swimming: Bottom

Breeding Least Killifish

Breeding dwarf livebearers isn’t hard and since the fry come out so large, getting the fry to live isn’t terribly difficult, either. They breed best in a colony situation, which you can keep heavily planted or use the rockpile and an airlift connected to a second tank just for the fry. Unfortunately, removing the females before they drop isn’t a great option since they drop fry almost daily.

Telling males and females apart is a pretty-straightforward process as well, in part due to their massive size difference, but also because the huge gonopodium.

male dwarf livebearer
Male (usually 3/4″) with gonopodium
female dwarf livebeaerer
Female (at 1.5″) with no gonopodium


Spawning is also a straightforward affair that you’ll likely never see because it’s lightning-fast. The male will extend his gonopodium to deposit sperm, much like all livebearers. There isn’t much information as to if least killifish store sperm like other livebearers do, but I would be surprised to find out that they don’t.

Fantastic video from Cory at AquariumCoOp.com

Pregnancy & Fry Care

A female can be pregnant with as many as 15 or more fry at a time, but most of them will be at different stages of development, with only one or two out of the 15+ close to being born at any time. This is known as superfoetation, a process where there are multiple fetuses at different stages of development in the same womb. The gestation for each “batch” of fry is about four weeks.

Because of this process, the fry are born quite large and usually only one or two at a time. For larger females, they can drop fry daily or every other day. But most of the time, you’ll see one or two appearing every 3 – 4 days per female.

The parents don’t usually predate on their fry, but the growing fry are more than happy to predate on their younger siblings. This can make raising multiple generations a real challenge if you don’t have a well-planted tank. Dwarf livebearer fry are born so large that they can accept baby brine shrimp immediately after being born. Which is still pretty small, really.

Baby least killi scale
Baby least killi next to a penny

Variants Of Least Killifish

I only know of one color variant for least killifish, which is even more difficult to find than the normal colored least killi.

White least killi & white guppy

Gold Least Killifish

Gold least killifish are incredibly hard to find and will likely cost you more than a pretty penny for a small breeding colony. They’re completely devoid of any black markings and look close to white guppies. Some strains are more snow-white while others are more goldenrod yellow.

Further Reading & Resources

NANFA.org – North American Native Fishes

2 Responses

Leave a Reply