Guppies, mollies, platies, and swordtails 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 and die.
Taxonomy and distribution
The guppies, 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.
Until 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).
Their natural 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.
Poecilia 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.
Poecilia 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 getting to 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.
Poecilia 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.
Poecilia 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. 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.
Poecilia 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 25-28C.
Poecilia 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).
Poecilia 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.
Belonesox 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 1.005-1.025.
Within each 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.
In reality, 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 reticulata.
Water chemistry and quality
All these 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.
Water quality 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.
Mollies and 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.
These 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 along well.
Mixing 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 quarrelsome.
Although 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 maturity.
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 main aquarium.
The other 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).
Raising 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.
Livebearers are generally trouble-free as far as healthcare goes. While they do get things like whitespot and finrot 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 robust.
Endnote: perfect fish?
Are 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.
Scott, P: Livebearing Fishes (Fishkeeper's Guides), Interpet Publishing, 1999, ISBN 1-9023-8962-X
Dawes, J: Livebearing Fishes: A Guide To Their Aquarium Care, Biology and Classification, Blandford, 1995, ISBN 0-7137-2592-3
Monks N. (editor): Brackish Water Fishes, TFH 2006, ISBN 0-7938-0564-3
SchÃ¤fer F & M. Kemkes: All Livebearers and Halfbeaks, Aqualog 1998, ISBN 3-931702-77-4