Least Killifish: Caring For The World’s Smallest Livebearer

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.

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Table of contents

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.

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

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!

Stocking

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.

Decor

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.

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

Bacopa sp.

Cabomba sp.

Guppy Grass

Guppy Grass

Java Moss

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.

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.

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:

Columnaris (Cotton Mouth Disease)

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 Corydoras (Corydoras Pygmaeus)

Scarlet Badis (Dario dario)

Norman’s Lampeye Killifish (Poropanchax normani)

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.

Spawning

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.

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.

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.

Further Reading & Resources

NANFA.org – North American Native Fishes