By Neale Monks
Mollies, Platies, and Swordtails are the most popular and widely kept of
livebearing fishes in the aquarium hobby. They are generally hardy, colorful,
and easy to maintain. Best of all, they readily breed in the home aquarium with
little effort on the part of the aquarist. But like all other aquatic organisms,
they respond positively to good care and if maintained improperly will get sick
Taxonomy and distribution
Mollies, Platies, and Swordtails all belong to a single family of freshwater
fish known as the Poeciliidae. All share a similar body shape with a
distinctly upturned mouth, and in most cases the males are substantially smaller
than the females. Males also tend to be more brightly colored, and also have a
modified anal fin known as a gonopodium, used to fertilize the female.
Guppies and Mollies belong mostly to the genus Poecilia
while the Swordtails and Platies belong to the genus Xiphophorus. Species
within each genus sometimes hybridize, and Guppy-Molly crosses and
Platy-Swordtail crosses are not uncommon.
relatively recently mollies were placed in their own genus, Mollienesia,
from which comes the "Molly" name. Older books may also place the guppies in the
genus Lebistes, and while this name is occasionally used in the modern
literature, it is generally considered to be an obsolete name. Another old name
that may be encountered is Platypoecilus, a genus to which the platies
were assigned (and from which they derive their common name, Platy).
range of these livebearers is centered on Mexico and Central America but most
have been introduced to various tropical and subtropical locations outside their
natural range. Guppies, in particular,have been used as anti-malaria fish, being
placed in waters such as ponds and canals to eat the mosquito larvae found
there. Whether or not these introductions had much effect on malaria is
debatable, but feral populations of Guppies can now be found in places as
diverse as Albania and Zambia. In some cases, the introduced livebearers have
had a negative effect on native fish faunas.
reticulata Peters 1860 is the common Guppy. Originally found across northern
South America from Venezuela to Brazil, as well as the some of the Caribbean
islands including Barbados and Trinidad. Now widely distributed elsewhere.
Females to about 2", males smaller and less robust, but more colorful. Lots of
variation in the wild, but these distinct "races" are rarely traded. "Feeder"
Guppies are closest to the wild Guppies in size and color. Numerous "fancy"
varieties; these may be more brightly colored than wild Guppies but are often
smaller and generally much less hardy. Remarkably adaptable in terms of water
chemistry, but ideally pH 7.0-8.0, hardness 10 dH or more. Does well in brackish
water and, if acclimated properly, can be kept in saltwater conditions as well.
Temperature 18-28 C.
wingei Poeser, Kempkes, & Isbreucker 2005 is known as the "Endler Guppy" to
aquarium hobbyists but the "Campoma Guppy" to scientists, this latter name a
reference to the Campoma region of Venezuela where these Guppies are naturally
found. Similar to the common guppy in appearance but a little smaller and though
very variable its colors are often even more vivid than those of common Guppies.
In general terms, its maintenance is identical to that of the common Guppy,
except that warm water conditions are preferred, ideally around 26-28 C. Common
and Endler Guppies hybridize readily, and most of the Endler Guppies available
in pet stores are in fact hybrids of the two species. While perfectly nice fish
in themselves, aquarists after pure-bred Endler Guppies will do better by
obtaining them from aquarium clubs, auctions, etc.
Micropoecilia picta (Regan 1913) is known as the "Swamp Guppy" in the trade
and is referred to as Poecilia picta in many older aquarium books. It is
a small species, reaching about an inch or so in length and rather resembles a
wild-type Guppy at first glance. It is a bit more streamlined than the average
Guppy though, and its tail is not so large. Coloration is very variable, and a
number of aquarium strains have been developed. Typically the fish is
silvery-green with patches of yellow, blue, and black. Males are smaller but
more colorful than the females. A brackish water species, the Swamp Guppy does
not do well kept in a freshwater tank; pH 7.5-8.0, hardness 20 dH or more,
specific gravity 1.003-1.005. Temperature 26-28 C.
latipinna (Lesueur 1821) is known as the "Sailfin Molly" on account of the
very large dorsal fin sported by male fish. Native to the Southern United
States, primarily in brackish water, sometimes in the sea. They use the fin for
threat displays between one another and to impress females. Body greenish with
numerous blue, white, and yellow spots. Various artificial varieties: black,
chocolate, orange, etc. Mollies are distinctly herbivorous and require plenty of
green foods in their diet. Algae or algae-based flake foods ("livebearer flake")
should be used instead of ordinary meat-based flake foods. Will also peck at
algae in the aquarium as well as soft vegetables such as blanched lettuce and
thinly-sliced cucumber. Enjoys insect larvae, daphnia, etc. Water conditions: pH
7.5-8.0, hardness 20 dH, temperature 20-28C. Does best in brackish water, and
can be acclimated to marine conditions as well.
mexicana Steindachner 1863 is one of the short-fin Mollies that may turn up
for sale, though it isn't all that common in the hobby. Native to Mexico and
parts of Central America, this species has a relatively small dorsal fin
compared with the Sailfin Molly, but otherwise similar in size and shape. Around
3-4", females larger than males. Requires similar care as Poecilia latipinna.
Water conditions: pH 7.5-8.0, hardness 20-30 dH, temperature 23-28C. Does well
in brackish water.
salvatoris Regan 1907 is known as the "Liberty Molly". Naturally found only
in El Salvador. Silvery green body, enlivened with red, white, and blue dorsal
and tail fins (from whence its common name, in the US at least). Apparently
confined to freshwater habitats in the wild, but otherwise requires similar care
to the other mollies. Water conditions: pH 7.5-8.0, hardness 20 dH, temperature
sphenops Valenciennes 1846 is another of the short-fin Mollies and most
likely the chief ancestor of the Balloon Molly and Black Molly. Natural range
runs from Mexico to Colombia; widely introduced elsewhere. Smaller (2-3") but
otherwise similar to Poecilia mexicana. Water conditions: pH 7.5-8.0,
hardness 20 dH, temperature 25-28C. Does very well in brackish and marine
aquaria (often used to mature marine aquaria).
velifera (Regan 1914) is the "Giant Sailfin Molly". A spectacular but rather
uncommonly-traded species. Endemic to Mexico. Maximum length of males is 6",
females 7". A large, deep aquarium is essential because of the size of these
fish. Maintenance is otherwise similar to that of the other mollies but brackish
water conditions seem to be more important; pH 8.0, hardness 20-30 dH,
temperature 25-28 C, SG 1.005-1.025. Does very well in marine aquaria, and said
to grow larger under such conditions.
Xiphophorus maculatus (Gunther 1866), the Platy of the hobby but "Southern
Platyfish" to the scientists. Native to Mexico and parts of Central America.
Relatively small but very stocky. Maximum size about 2", often smaller. Males
and females similar in length but the female is much more robust. Wild fish
greenish with a black patch at the caudal peduncle. Numerous artificial
varieties. Quite hardy and easy to keep. Herbivorous in the wild, and
algae-based flake foods should be used as a staple. Also enjoys small
invertebrates such as daphnia and bloodworms. Very peaceful. Adaptable, but does
best around pH 7.0-8.0, dH 10-20, temperature 25-28 C.
Xiphophorus variatus (Meek 1904), the "Variatus Platy" of the hobby but
"Variable Platyfish" to ichthyology. Endemic to Mexico, but like many other
livebearers, widely introduced elsewhere. Larger than the common Platy (around
2.5"). Its common name refers to its variable coloration in the wild, but
typically with a series of longitudinal stripes along the flanks. Many
artificial varieties, likely including crosses with the common Platy. Robust and
easy to keep, this fish is notable for being tolerant of subtropical water
conditions much cooler than those favored by most other livebearers in the
hobby. Care is otherwise similar to that of the common Platy. Water conditions:
pH 7.0-8.0, dH 10-25, temperature 15-25 C.
Xiphophorus xiphidium (Gordon, 1932), the Swordtail Platy. Endemic to
Mexico. Small (around 1.5") and not as brightly colored as the other Platies,
but notable for the short "swordtail" borne by the males. Not commonly traded,
but easily obtained at fish auctions, etc. Care is similar to the common Platy;
water conditions: pH 7.5-8.0, dH 5-20, temperature 22-26 C.
Xiphophorus alvarezi Rosen 1960 is the "Chiapas Swordtail", and one of many
"oddball" Swordtails that turn up in tropical fish shops occasionally. Native to
Mexico and Guatemala. Compared with the common Swordtail, this species has three
prominent horizontal bands running along the flanks. Grows to around 2.5-3" plus
the sword. Maintenance similar to the common Swordtail, but less adaptable and
demands closer attention to water quality and diet. Males rather quarrelsome.
Water conditions: pH 7.5-8.0, dH 5-20, temperature 24-28 C.
Xiphophorus hellerii Heckel 1848 is the common Swordtail or "Green
Swordtail". Native to Mexico and parts of Central America. Larger (typically
around 3") and more streamlined in shape than the Platy. Males immediately
recognizable by the long "sword" jutting from the lower lobe of the tail fin; in
some cases this sword may be almost as long as the body of fish itself. Many
geographical varieties, some relatively small (around 2.5") others much larger
(up to 4.5") and remarkable for their differences in coloration as well. Body
may be green, silvery, or orange; one or more horizontal red bands may be
present on the flanks; some varieties have black spots on the body that others
lack. These varieties are often named after their place of origin, e.g.,
Yucatan, Rio Belize, Rio Atoyac, etc. There are also numerous artificial forms.
Wild-caught fish are often sensitive to poor water quality and show a preference
for small live foods such as Daphnia and mosquito larvae; captive-bred fish
generally hardy and thrive on flake foods. Male swordtails are aggressive
towards one another and may also chase other fish they perceive as rivals, such
as Platies. Because of their relatively large size and very active natures,
these fish are best suited to large aquaria with plenty of swimming space. Water
conditions: pH 7.5-8.0, dH 5-20, temperature 24-28 C.
belizianus Kner 1860, the "Pike Topminnow" or "Pike Livebearer". Native to
Mexico and Central America, but now found in parts of the United States and
deemed by many to be a serious threat to native fish faunas. Big (up to 8", more
typically 4" for males, 6" for females) and highly predatory. Does not gorge
like many other predatory fish, but eats a few small prey every day. Difficult
to care for. Females much larger than the males, and as apt to eat potential
mates as breed with them; fry highly cannibalistic. Prefers live foods;
invertebrates when young, almost entirely fish when mature. Wild-caught fish
very difficult to wean onto dead foods, tank-bred fish a bit easier. Best kept
in brackish water; pH 8.0, hardness 20-30 dH, temperature 25-28 C, SG
genus, hybrids will occur readily. That is, all
Poecilia will hybridize with each other, as will all Xiphophorus.
So unless you want to produce hybrids, Guppies and Mollies should all be kept
apart, one species to a tank, and likewise swordtails and Platies should be kept
apart, one species to a tank as well. Hybrids across genera don't seem to
happen; so guppies, Poecilia reticulata, and Platies,
Xiphophorus maculatus could be mixed for example without any problems.
all Mollies and Platies are already hybrids of a number of species, so questions
of "genetic purity" don't really mean much with these fish. Swordtails and
common Guppies may be "pure" species at a crude level, but the geographical
variations typical of these species have long since been lost. On the other
hand, commercial stocks of Endler Guppies are almost certainly hybrids of the
"true" Endler Guppy Poecilia wingei with the common Guppy Poecilia
Water Chemistry and Quality
livebearers prefer hard water with a pH around 7.5-8.0. When kept in soft or
acidic waters these fish tend to be more prone to diseases, particularly finrot
and fungus. Mollies are also prone to a condition known as the "shimmies", a
neurological condition where the fish seems to tread water and loses the ability
to swim properly; eventually it dies. In soft water areas, Malawi or Tanganyikan
salt mix can be used to raise the pH and hardness. Alternatively a calcareous
substrate such as coral sand can be added to the aquarium. Salt-tolerant
livebearers can be kept successfully in brackish water tanks but it is important
to use marine salt mix not aquarium "tonic" salt for this. Only marine salt mix
contains the minerals that will raise the pH and hardness of the water.
is very important over the long term. Mollies, in particular are very sensitive
to high levels of nitrate (above 20 mg/l) when kept in freshwater conditions. By
contrast, in brackish and salt water mollies are extremely robust, and are
routinely used to cycle marine aquaria. Although often recommended as suitable
fish for maturing aquaria, fancy Guppies are actually not all that hardy and
should only be kept in mature aquaria.
platies are more herbivorous than Guppies and Swordtails. When feeding Mollies
and Platies, providing them with green foods is essential to long-term health.
Without enough green matter in their diet, these fish are prone to constipation
and do not develop their best colors (this is especially true with wild-caught
fish). Flake foods designed for livebearers are the ideal, containing large
amounts of algae as well as protein. Also consider leaving some of the algae in
the tank for your livebearers to nibble on. All livebearers enjoy small
invertebrates such as mosquito larvae and these make an excellent treat,
particularly for pregnant females.
livebearers are not really schooling fish in the same way as tetras or barbs.
Males are often aggressive towards one another. This is especially true with
Swordtails which will fight and chase one another. Male Mollies and Guppies are
somewhat less aggressive, and Platies least of all. Females of all types get
livebearers with other community species is usually not a problem provided water
chemistry is taken into consideration. However, fin-nippers such as Serpae
Tetras and Tiger Barbs will certainly pester male Guppies. Their small size also
makes Guppies at risk of being eaten by other fish, including Angelfish. Male
Swordtails and Mollies can sometimes be a bit boisterous with more placid fish,
so tankmates for these should be chosen with care. Platies are usually the
safest species for the community tank, being neither too small nor too
often dismissed as being simply a case of "just add water", breeding livebearers
is a little more complex than this. In the wild, newborn livebearers swim
towards shallow water where they hide among the vegetation. The parents have no
protective instinct at all, and if they come across a newborn fish, they'll try
and eat it. So, in the average community tank, few if any fry survive to
There are two
ways to solve this problem. One is to to place some floating plants in the
aquarium to give newborn fish some cover, and then to check once or twice a day
for fry and remove them to another aquarium whenever they are found. A rearing
tank need not be large: a 10 gallon tank will be ample, since all you need to do
is get them to a size large enough that they won't be eaten by anything in the
approach is to place a pregnant female in a breeding trap. These are floating
boxes that contain compartments into which the fry drop after being born. The
idea is that because the female cannot get to the fry, and the fry cannot get
loose in the aquarium, it is easier to keep the newborn fish safe. However, most
traps are far too small for anything other than guppies or perhaps small
platies. Mollies and Swordtails certainly cannot be kept in breeding traps, and
often miscarry when placed in them (assuming they simply don't jump out).
livebearer fry is not difficult. They will take algae and finely powdered flake
food immediately, and should be fed 4-6 times per day. Some floating plants
should be provided to offer shade and at least weekly 25% water changes are
important to maximize growth rate. Guppies will reach a sellable size in about
2-3 months, Mollies and swordtails may take a little longer. Sailfin Mollies,
especially depend upon being kept in a large tank with good water quality if the
males are to develop their dorsal fins properly.
are generally trouble-free as far as healthcare goes. While they do get things
like Whitespot and Fin Rot just as readily as any other fish, they react
positively to copper-based treatments making curing any such diseases easy.
Mollies are the exception here: in freshwater tanks they are very prone to a
variety of sicknesses, possibly related to their intolerance of high levels of
nitrate. When kept in brackish or saltwater conditions they are altogether more
Endnote: perfect fish?
livebearers the perfect aquarium fishes? Many aquarists think so, and they are
consistently among the most widely sold and frequently bred freshwater fishes in
the trade. But familiarity often breeds complacency, and far too often these
fish have been kept in under-filtered and overstocked aquaria with the wrong
water chemistry. It is a testament to their fundamental hardiness that
livebearers usually come through such abuse, but these are such nice fish that
they deserve better. Whether you're looking for a good beginner's fish,
something with bright colors, or an introduction to fish breeding, livebearers
have lots to offer.
Livebearing Fishes (Fishkeeper's Guides), Interpet Publishing, 1999, ISBN
Livebearing Fishes: A Guide To Their Aquarium Care, Biology and Classification,
Blandford, 1995, ISBN 0-7137-2592-3
(editor): Brackish Water Fishes, TFH 2006, ISBN 0-7938-0564-3
Schäfer F &
M. Kemkes: All Livebearers and Halfbeaks, Aqualog 1998, ISBN 3-931702-77-4