Not a killifish, and four of our six sources say so without being asked
The least killifish is a livebearer in the guppy family, and the name it is sold under belongs to a different group of fish entirely. The record on this site for a fish that is a killifish sits at pH 4.0 to 7.0; this one sits at 7.0 to 8.0. They touch at one point and share nothing else.
Wikipedia puts it plainly: despite the common name least killifish, it belongs to the family Poeciliidae and not to one of the killifish families. Aquarium Co-Op names the relatives, and they are the fish anybody would recognize: a midget livebearer from the same family Poeciliidae as guppies, platies, and mollies. Aquarium Store Depot and Aquariadise say the same in their own words.
The names it also carries are mosquito fish, dwarf topminnow, dwarf livebearer and midget livebearer, and three of those four describe it better than the one on the tank. Seriously Fish dates the binomial to Girard, 1859.
The second thing worth knowing is what it does instead of having a breeding season. A female carries up to six broods of embryos at different stages of development at the same time and drops one to eight fry every few days. Station 03 has that, and it is the fact the rest of the record depends on.
Why this record was written now
- The gap
- Three thousand six hundred searches a month, no AI overview on the result page, and the best ranked competitor at position seven. It is the smallest fish we have written and it arrives the week after the second largest.
- What it changes
- The third surface record here and the only one that also uses the middle, the first record to carry superfetation, and a male that ties the smallest fish on this site.
- What it does not
- No growth rate, because no source publishes one, and only one source publishes a lifespan at all.
The water, and a band three degrees narrower than the fish it describes
| Source | Temperature | pH | Hardness | Tank | Adult size |
|---|---|---|---|---|---|
| Seriously Fish | 68 to 79 °F | 7.0 to 8.0 | 5 to 20 dGH | 12.5 L for a pair | 0.8 in max |
| Aquarium Breeder | 68 to 78 °F | 6.5 to 8.0 | 5 to 20 dGH | 10 gal | 0.6 to 0.8 male, 1 to 1.5 female |
| Aquarium Co-Op | 68 to 79 °F | above 7.0 | harder, with minerals | none | 1 in |
| Aquarium Store Depot | 65 to 80 °F | 7.0 to 8.0 | none | 5 gal | 0.8 to 1.4 in |
| Aquariadise | 75 to 79 °F | 7.0 and over | none | 5 gal | 1.2 in |
| Wikipedia | none | none | none | none | 0.8 male, 1.2 female |
| This record | 75 to 78 °F | 7.0 to 8.0 | 5 to 20 dGH | 5 gal | 0.6 to 1.5 in |
Read the temperature column and the row this record prints is set by one source. Four of the five bands start at 65 or 68; Aquariadise starts at 75, and the span all five contain is three degrees. That is what the site rule produces, and it is worth saying what it costs: the agreement is warmer and narrower than the animal.
Wikipedia puts the native range in the southeastern United States from North Carolina south to Georgia and Florida, west to Louisiana and recently eastern Texas. Aquarium Store Depot says outright that it tolerates a wide range of temperatures including unheated tanks. A fish that overwinters in North Carolina is not a fish with a three degree requirement, so the row is the consensus and the note is the tolerance, and a keeper in a cool room should read the 65 as well as the 75.
The pH column is the opposite: four sources put the floor at 7.0 and only Aquarium Breeder goes below neutral at 6.5. That is a real constraint and it decides the company this fish keeps. Our platy record runs 7.0 to 8.0 and our ghost shrimp record the same, which is what the family predicts, since Aquarium Co-Op names guppies, platies and mollies as the relatives and our guppy record runs 7.0 to 8.5.
The hardness column carries something this site rarely sees. Two sources publish general hardness and publish the identical figure, 5 to 20 dGH. For contrast, our common pleco record has one page giving the same numbers as dGH in one place and dKH in another, and our rabbit snail record has sources measuring in units that cannot be compared at all.
Six broods at once, and a fish that may not see a second summer
Most livebearers carry one brood and release it. This fish does something else, and Seriously Fish gives it its name: superfoetation, the formation or development of a second foetus when one is already present. Fry are dropped continually rather than in defined broods.
Aquarium Breeder puts numbers on it: females hold up to 6 broods of embryos at different stages of development and produce small clutches of one to eight fry at intervals of a few days to two weeks. Wikipedia gives the total: as many as 40 fry released over a 10 to 14 day period, occasionally longer. Aquarium Co-Op describes the mechanism from the mother's side, the unusual ability to carry multiple fetuses conceived at different times, so she gives birth intermittently over the course of many days instead of all at once.
Why a fish would do this is answered by the lifespan row, and only one source publishes one. Aquarium Breeder gives up to 1.5 years in captivity under optimal conditions and lower temperatures, and in the wild a maximum longevity between 120 and 150 days. Four to five months. An animal that may not see a second summer does not wait for a breeding season.
The practical consequence is the same on every source that mentions it, and two do. Aquarium Store Depot: population control is the main concern; they breed constantly and a small colony will multiply quickly. Seriously Fish: it is nigh on impossible to prevent it breeding if both sexes are present.
So the plan for the fry is part of the purchase rather than a problem for later, which is why it is in the kit list below. Aquarium Co-Op notes the one mercy: the newborns are large enough to immediately eat live baby brine shrimp and crushed flakes, and they are ignored by the adults.
The full record
| Field | Recorded | Where it comes from |
|---|---|---|
| Minimum tank | 5 gal | Four sources and three different framings, because each answers for a different number of fish. Aquarium Store Depot gives 5 gallons, 19 liters, for a group of six or more. Aquariadise gives tanks as small as 5 gallons, 20 liters, for a harem of one male and two females. Seriously Fish goes smaller and states the dimensions rather than the volume: 12 by 8 by 8 inches, 30 by 20 by 20 cm, 12.5 liters, for a pair, which is about three and a quarter gallons. Aquarium Breeder asks for 10 gallons, about 40 liters. The row prints the 5 gallons two sources give for a group, and the spread is a factor of three rather than the factor of seventeen on our common pleco record, because on a fish this size the disagreement is about how many you keep and not about how big it gets. It is the same volume as our clown killifish and chili rasbora records, and our nerite snail record is still the smallest on the site at 2 to 3 gallons. |
| Temperature | 75 to 78 °F | Five bands, and the span they all contain is three degrees. Seriously Fish gives 20 to 26 C, 68 to 79 F. Aquarium Co-Op gives 68 to 79 F, 20 to 26 C. Aquarium Breeder gives 68 to 78 F, 20 to 26 C. Aquarium Store Depot goes widest at 65 to 80 F, 18 to 27 C. Aquariadise goes narrowest at 75 to 79 F, and it is Aquariadise that sets the floor this row prints. The agreement is warmer and narrower than the animal. Wikipedia puts the native range in the southeastern United States from North Carolina south to Georgia and Florida, west to Louisiana and recently eastern Texas, and Aquarium Store Depot says outright that it tolerates a wide range of temperatures including unheated tanks. So the row is what the sources agree on rather than what the fish will take, and a keeper in a cool room should read the 65 as well as the 75. The band still contains the 75 to 76 °F the rest of this site shares, which not every record manages. |
| pH | 7.0 to 8.0 | Three sources publish the same range and two publish a floor without a ceiling. Seriously Fish 7.0 to 8.0. Aquarium Store Depot 7.0 to 8.0. Aquarium Co-Op asks for pH levels above 7.0 and harder water with minerals. Aquariadise gives 7.0 and over. Aquarium Breeder is the only source that goes below neutral, at 6.5 to 8.0, and the row prints the 7.0 the other four agree on. This puts the fish in the hard water group on this site with our platy record at 7.0 to 8.0 and our ghost shrimp record at the same, which is the company the family would predict: Aquarium Co-Op names guppies, platies and mollies as its relatives, and our guppy record runs 7.0 to 8.5. |
| Hardness | 89 to 357 ppm (5 to 20 dGH) | Two sources publish general hardness and they publish the identical figure: Seriously Fish 5 to 20 dGH and Aquarium Breeder GH 5 to 20. Converted, that is 89 to 357 ppm, and it is the row. Aquarium Co-Op asks for harder water with minerals without a number, and the other three publish no hardness at all. Two sources agreeing exactly is rare on this site, and it is worth noting where it does not happen: our common pleco record has one page publishing the same numbers as dGH in one place and dKH in another, and our rabbit snail record has sources measuring in units that cannot be compared at all. |
| Adult size | 0.6 to 0.8 in male, 1 to 1.5 in female | The dimorphism is the point, and one source contradicts the rest. Aquarium Breeder gives males up to 0.6 to 0.8 inches, 1.5 to 2 cm, and females 1 to 1.5 inches, 3 to 3.5 cm. Wikipedia gives males about 2 cm, 0.8 inches, and females about 3 cm, 1.2. Aquarium Store Depot gives 0.8 to 1.4 inches, 2 to 3.5 cm, females larger. Aquariadise gives 1.2 inches and Aquarium Co-Op gives one inch, 2 to 3 cm. Seriously Fish publishes a maximum standard length of 20 mm, 0.8 inches, for the species, which is below the female figure the other four give, and it notes in the same entry that males are much smaller than females. The row prints both sexes because no single figure describes this animal. On this site the male at 0.6 inches ties our ember tetra record for the smallest fish, and the female at up to 1.5 is more than twice him; our nerite snail record is smaller still at 0.5. |
| Diet | Omnivore, and it browses algae | Seriously Fish is the one that describes it: unfussy and omnivorous, it will accept most foods offered, is particularly fond of small live or frozen varieties such as brine shrimp or Daphnia, and will browse on algae. Aquarium Breeder lists it as an omnivore. Aquarium Co-Op does not describe the adult diet but gives the newborn one, which is the more useful fact on a fish that breeds continually: the young are large enough to immediately eat live baby brine shrimp and crushed flakes. Aquarium Store Depot, Aquariadise and Wikipedia publish nothing on the diet at all. Nothing here is a specialist requirement, which is unusual on a fish this small: our otocinclus record needs algae it cannot always find and our pea puffer record refuses flake entirely. |
| Water column | Top to middle | One source states a layer and one describes the habitat. Aquarium Breeder gives the position as top to middle. Seriously Fish describes the wild water rather than the layer: shallow, heavily vegetated marginal areas of still or sluggish bodies of water. The other four publish nothing on it, which is why this row rests on a single source even though two of the names the fish is sold under, dwarf topminnow and top minnow, point the same way. It joins our clown killifish and betta records as the third surface fish on this site, and it is the only one of the three that also uses the middle. |
| Lifespan | Up to 1.5 years, on one source | Only Aquarium Breeder publishes a lifespan, and it publishes two. In captivity, under optimal conditions and lower temperatures, up to 1.5 years. In the wild, a maximum longevity of 120 to 150 days, which is four to five months. The other five sources give no figure at all. A single source on a row is thin and the record says so rather than dressing it up, but the wild figure is worth printing because it explains the reproduction row: an animal that may not see a second summer does not wait for a breeding season, and station 05 has what it does instead. For scale on this site, our platy and cherry shrimp records floor at one year and our clown loach record reaches thirty. |
| Growth rate | Not recorded yet | Not stated by Seriously Fish, Wikipedia, Aquarium Co-Op, Aquarium Store Depot or Aquariadise. The Aquarium Co-Op page is a livebearer roundup and the sizes printed near this entry belong to platies and goodeids rather than to this fish, which is the trap a roundup source sets on a record like this one. Aquarium Breeder is the sixth source here and its page will not serve a scripted request. |
The same nine fields for every animal on this site are in the A to Z index.
Where our sources disagree
Two disagreements on this record and they are different in kind. The first is a name four sources call wrong. The second is a ratio two sources invert.
Aquarium Breeder, on how many of each sex
3-4 males per 1 female
Aquariadise, on the same question
one male and two females
Seriously Fish, on whether the question arises
Very easy. It’s nigh on impossible to prevent it breeding if both sexes are present
What we record: we print both and take neither, because the two are not a rounding apart: they are inverted. Three to four males per female and one male to two females are opposite prescriptions, and the reasons usually given for each point in opposite directions too, one to spread male attention and one to limit it.
What both sources agree on is the floor, six fish, which is the figure Aquarium Store Depot gives as well. So the row carries the group and not the ratio, and a keeper who buys six unsexed fish has satisfied every source we hold.
Seriously Fish makes the argument almost academic: it is nigh on impossible to prevent it breeding if both sexes are present, so whatever ratio you start with is not the ratio you will have.
Two records, one word, and one of the two is actually a killifish
| Field | Least killifish | Clown killifish |
|---|---|---|
| Is it a killifish | No, a livebearer | Yes |
| Family | Poeciliidae, on four sources | not stated on our record |
| Scientific name | Heterandria formosa | Epiplatys annulatus |
| pH | 7.0 to 8.0 | 4.0 to 7.0 |
| Temperature | 75 to 78 °F | 68 to 79 °F |
| Adult size | 0.6 to 1.5 in | 1.4 in |
| How it reproduces | Live fry, continually | Eggs |
Put the two records beside each other and the pH row is the one to look at. Our clown killifish record runs 4.0 to 7.0 and this one runs 7.0 to 8.0. They meet at a single value and nowhere else. One is an egg-laying fish published for water more acidic than rain; the other is a livebearer that four sources say wants minerals in the water.
A shopper who reads killifish as a category and buys for it will get the water wrong by the width of the scale. That is not a hypothetical: it is what the shared word invites, and it is the reason this record spends a station on a name.
What the two do share is the surface. Our clown killifish record and our betta record are the two surface fish on this site, and this one joins them, though Aquarium Breeder places it top to middle rather than at the film. It is the only one of the three that uses the middle as well.
And both are small enough to be eaten. The clown killifish record is 1.4 inches and this one is 0.6 to 1.5, which puts both in the group of fish that a community tank sized by temperature alone will quietly remove. Station 06 has who does the removing.
This is what a species name is for, and why this record leads with one. Heterandria formosa tells you the family, the family tells you it bears live young and wants minerals, and both of those decide the tank. The trade name tells you none of it and points at the wrong half of the pH scale. Every one of our six sources agrees on Heterandria formosa; four of them go out of their way to say the common name is misleading; and the shop label is the one piece of information a buyer actually receives.
The same species is sold as a nano fish, a pond fish and a feeder in different places, which is a fair description of how wide its tolerances are. What does not change with the label is the size: a fish of 0.6 to 1.5 inches is a nano fish in any tank, and the 5 gallon figure two sources give is a figure about the species rather than about the fashion it is sold under.
Peaceful and not a community fish, which is one sentence rather than two
Seriously Fish writes the whole problem in a clause: this is not a community fish in the general sense, although it is very peaceful, because the males are so small that other fish may eat them. A male here is six tenths of an inch.
Three records on this site are refused on that sentence and their own diet rows. Our oscar record states a predatory omnivore that eats fish, at 10 to 12 inches, and overlaps this one across 75 to 78 °F. Our redtail catfish record states predator, at 47 to 55 inches, and overlaps at 75 to 76, which is the whole band this site shares. Our betta record runs 72 to 86 and contains this record's band entirely.
The useful half of that sentence is the other half, and Seriously Fish names the company it would accept: pencil fish, dwarf Corydoras, Otocinclus and freshwater shrimp. Four of those five have records here, and on this site freshwater shrimp means our cherry shrimp, amano shrimp and ghost shrimp records.
The water works for some of those and not all of it. Our ghost shrimp record runs 7.0 to 8.0 on pH, the same as this one, and 72 to 80 on temperature. Our otocinclus record runs 6.0 to 7.5, so the two overlap only at 7.0 to 7.5, and our corydoras record runs 6.5 to 7.8. The recommendation and the chemistry do not entirely agree, and the pairings grid on our tank mates page prints each overlap rather than a verdict.
What no source disputes is the temperament. Every one of the six that mentions behavior calls it peaceful, and the group figure they agree on is six.
Least killifish questions, answered from the record
Is the least killifish actually a killifish?
No, and four of our six sources say so. Wikipedia: despite the common name least killifish, it belongs to the family Poeciliidae and not to one of the killifish families. Aquarium Co-Op calls it a midget livebearer from the same family as guppies, platies and mollies. It is a livebearer, it gives birth to free-swimming young, and the true killifish on this site is our clown killifish record, which lays eggs.
How big does a least killifish get?
Males 0.6 to 0.8 inches and females 1 to 1.5 inches, on Aquarium Breeder, and the other sources sit inside that: Wikipedia gives 0.8 and 1.2, Aquarium Store Depot 0.8 to 1.4, Aquariadise 1.2, Aquarium Co-Op one inch. Seriously Fish is the outlier with a maximum standard length of 20 mm, 0.8 inches, for the species, which is below the female figure the others give. On this site the male ties our ember tetra record for the smallest fish.
What size tank does a least killifish need?
5 gallons for a group, on Aquarium Store Depot and Aquariadise. Aquarium Breeder asks for 10. Seriously Fish goes smaller and gives dimensions instead of a volume, 12 by 8 by 8 inches for a pair, which is about three and a quarter gallons. The spread is a factor of three and it is about how many fish you keep, not how big they get.
What temperature does a least killifish need?
75 to 78 °F is the span all five published bands contain, and it is narrower than the fish. Four sources start at 65 or 68 and Aquariadise starts at 75. Aquarium Store Depot says it tolerates a wide range including unheated tanks, and Wikipedia puts the native range as far north as North Carolina, so read the 65 as well as the 75.
Can you keep least killifish in an unheated tank?
One source says so directly. Aquarium Store Depot: it is native to the southeastern United States, tolerates a wide range of temperatures including unheated tanks. Its published band there is 65 to 80 °F. No other source we hold addresses it, so the record prints the consensus band of 75 to 78 and names the tolerance rather than the other way round.
How do least killifish breed?
By superfetation, which Seriously Fish defines as the formation or development of a second foetus when one is already present. Aquarium Breeder: up to six broods of embryos at different stages at once, and clutches of one to eight fry every few days to two weeks. Wikipedia: as many as 40 fry over a 10 to 14 day period. Aquarium Co-Op: the mother gives birth intermittently over many days instead of all at once.
How many least killifish should I keep together?
Six is the figure two sources give and the one this record carries. The sex ratio is where they disagree, and they disagree in opposite directions: Aquarium Breeder says 3 to 4 males per female, Aquariadise says one male and two females. Buying six unsexed fish satisfies every source we hold, and Seriously Fish makes the point moot anyway: it is nigh on impossible to prevent it breeding if both sexes are present.
How long do least killifish live?
Up to 1.5 years in a tank, on one source. Aquarium Breeder is the only one of the six to publish a figure, and it publishes two: up to 1.5 years in captivity under optimal conditions and lower temperatures, against a wild maximum longevity of 120 to 150 days. Four to five months in the wild is the figure that explains the breeding.
What do least killifish eat?
Seriously Fish: unfussy and omnivorous, it will accept most foods offered, with a fondness for small live or frozen foods like brine shrimp or Daphnia, and it will browse on algae. Aquarium Breeder lists omnivore. Aquarium Co-Op gives the newborn diet, which matters more here: the fry can immediately eat live baby brine shrimp and crushed flakes.
What can live with a least killifish?
Seriously Fish calls it not a community fish in the general sense, although it is very peaceful, because the males are small enough to be eaten. The mates it names are pencil fish, dwarf Corydoras, Otocinclus and freshwater shrimp, and four of those have records here. This record refuses our oscar, redtail catfish and betta records on the same sentence.
What is a gold least killifish?
None of our six sources mentions a color form, gold or otherwise. The keyword exists and the record cannot answer it, so the row stays empty rather than being filled from a shop listing. If a source we trust publishes one, it will be added and named here.
Is a least killifish the smallest fish on this site?
The male ties it. At 0.6 inches he matches our ember tetra record, and our chili rasbora record is 0.7 to 0.8. The female is the interesting figure, at up to 1.5 inches, more than twice the male. The smallest animal on this site is not a fish at all: our nerite snail record starts at 0.5.
What a least killifish needs that a guppy does not
Two of these are plants and one of them is a decision you make before you buy.
- A heavily planted tank, and slow waterSeriously Fish asks for a gently filtered, heavily planted setup with the flow kept to a minimum, which is the wild habitat it describes: shallow, heavily vegetated marginal water
- Somewhere for the fry to gotwo sources say population control is the main concern and one says it is nigh on impossible to prevent breeding, so the plan for the fry is part of the purchase rather than a later problem
- Minerals in the watertwo sources publish 5 to 20 dGH and a third asks for harder water with minerals, so this is not a fish for the soft acid water most of our plant records prefer
Every item our records ask for is gathered on the tank setup topic.
Nothing here is a paid link. It is the shortfall between what this record asks for and what a boxed kit contains.
How this record was compiled
- By
- Verified
- Sources
Seriously Fish, Heterandria formosa, Wikipedia, least killifish, Aquarium Co-Op, top 10 livebearers, Aquarium Store Depot, smallest aquarium fish, Aquariadise, nano fish for small tanks and Aquarium Breeder, least killifish
- Conflicts
Printed rather than resolved: two sources publish inverted sex ratios, one publishes a maximum length below the female size four others give, and the temperature the sources share is narrower than the range the fish is sold as tolerating.
- Blanks
Growth rate stay empty until a source we trust states them
- Not recorded
A growth rate, because no source we hold publishes one; a lifespan from more than one source, because only Aquarium Breeder gives a figure at all; what the gold strain sold as gold least killifish is, because none of our six sources mentions a color form; and whether the fish tolerates the brackish water Aquarium Co-Op says it is found in, because saying where an animal occurs is not the same as saying what to keep it in