Swordtail

Quick summary

Swordtails are a hardy, easy-to-breed livebearer well suited to beginners, thriving in a peaceful community tank of 115 litres or more with hard, alkaline water. They need at least one male to two or three females to reduce aggression, and a secure lid, but otherwise ask little beyond stable water quality and a varied diet.

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In this guide:

• Introduction
• Quick Stats
• Appearance
• Varieties and Morphs
• Natural Habitat
• Tank Size and Setup
• Water Parameters
• Filtration and Equipment
• Diet and Feeding
• Behaviour and Temperament
• Tank Mates
• Breeding
• Common Diseases
• FAQs

Introduction

The swordtail is one of the most recognisable and enduringly popular freshwater aquarium fish in the hobby. Named for the distinctive elongated lower caudal fin ray found on males, Xiphophorus hellerii has been a staple of community tanks since the early twentieth century. Its hardiness, ease of breeding, and the dazzling array of colour morphs developed through decades of selective breeding make it an excellent choice for beginners and experienced aquarists alike.

Originally collected from the rivers and streams of Central America, the swordtail belongs to the family Poeciliidae, the same group that includes guppies, mollies, and platies. Like its relatives, it is a livebearer, meaning females give birth to free-swimming fry rather than laying eggs. This reproductive strategy, combined with the species’ adaptability and peaceful disposition, has cemented its place as one of the most widely kept tropical fish in the world.

Wild swordtails are typically olive-green with a red or brown lateral stripe, but the captive-bred varieties available today span an extraordinary spectrum of colours and fin shapes. Whether you are drawn to the classic red swordtail or a more exotic pineapple or koi variety, there is a form to suit virtually every taste. This care guide covers everything you need to know to keep swordtails healthy and thriving in your home aquarium.

Appearance

The swordtail is a moderately sized, streamlined livebearer with a torpedo-shaped body that speaks to its active, fast-swimming nature. Males are the more visually striking sex, possessing the eponymous “sword”, a long, pointed extension of the lower rays of the caudal fin that can add several centimetres to overall length. This sword is typically edged in black and can be vividly coloured depending on the morph. Females lack the sword entirely and tend to be slightly larger and deeper-bodied, particularly when gravid.

In wild-type specimens, the base colour is an olive-green to yellowish-green, with a prominent red or reddish-brown lateral stripe running from behind the eye to the base of the tail. The belly is typically pale, and the dorsal fin may carry subtle spotting. However, the wild form is relatively uncommon in the aquarium trade today, having been largely supplanted by the vast range of selectively bred colour morphs.

Males also possess a modified anal fin called a gonopodium, a narrow, rod-like structure used to deliver sperm during mating. This is one of the most reliable ways to sex swordtails, as the gonopodium develops as males mature. Females retain a standard fan-shaped anal fin. It is worth noting that swordtails can occasionally undergo late sex reversal, with older females developing male characteristics including the sword and gonopodium, though such individuals are generally infertile.

Varieties and Morphs

Few freshwater species can rival the swordtail for the sheer number of captive-bred colour morphs and fin varieties available. Decades of selective breeding, and some hybridisation with the closely related platy, Xiphophorus maculatus, have produced an impressive catalogue of forms. Below are some of the most popular and widely available varieties.

The Red Swordtail is arguably the most iconic morph and the one most people picture when they hear the name. It features a solid, vivid red body with a matching red sword edged in black. This variety has been established in the hobby for many decades and remains one of the most commonly sold forms.

The Red Wag Swordtail shares the red body of the standard red morph but adds jet-black fins, including the dorsal, caudal, and pelvic fins. The contrast between the bright red body and the dark fins gives this variety a particularly striking appearance.

The Pineapple Swordtail displays a warm yellow to orange body with red or reddish-orange suffusion, particularly along the dorsal surface and into the fins. The overall effect is reminiscent of the fruit from which it takes its name, and it is a popular choice for planted community tanks.

The Neon Swordtail features a pale, almost luminous green or blue body with an iridescent sheen that catches the light beautifully. This variety is sometimes marketed under slightly different trade names depending on the exact shade and intensity of the iridescence.

The Marigold Swordtail is a bright golden-yellow to deep orange morph that lacks the red tones of the pineapple. Clean, bold colouration makes this a visually appealing variety, especially in groups.

The Koi Swordtail, also sometimes sold as the Kohaku Swordtail, features a white or pale cream body overlaid with patches of red and occasionally black, mimicking the markings of koi carp. Each individual tends to have a unique pattern, adding interest to a group.

The Black Swordtail has a deep, velvety black body that can appear almost solid in well-bred specimens. Some individuals may show slight iridescence under certain lighting. This morph can be slightly more delicate than others and may be more susceptible to melanoma, a condition linked to the melanophore genetics involved in producing black colouration.

The Tuxedo Swordtail has a bicoloured pattern, typically featuring a lighter front half, often red, orange, or green, and a dark black rear half and tail region. The tuxedo pattern is well established across several Xiphophorus species and hybrids.

The Hi-Fin or Lyretail Swordtail is a fin-shape variant rather than a colour morph. These fish have dramatically elongated dorsal fins that flow like a banner, and in lyretail forms, both the upper and lower caudal rays are extended, creating a forked, lyre-like tail. Hi-fin swordtails are available in various colour forms and add an elegant dimension to the species.

The Berlin Swordtail, also referred to as the Montezuma-cross Swordtail in some circles, is a variety developed in Europe that typically features a robust body, often in red or orange, with a particularly long and well-developed sword. It is prized by breeders for its close resemblance to wild-type proportions combined with enhanced colouration.

Natural Habitat

Xiphophorus hellerii is native to the Atlantic slope of Central America, with its natural range stretching from the Río Nautla drainage in the Mexican state of Veracruz southward through Guatemala, Belize, and into north-western Honduras. The species has also been introduced, either deliberately or accidentally, to numerous countries outside its native range, including parts of Africa, Asia, and even southern Florida in the United States.

In the wild, swordtails inhabit a diverse range of freshwater environments. They are most commonly found in fast-flowing streams and rivers with rocky or gravelly substrates, but they also occur in quieter pools, springs, canals, and even warm sulphur springs. Vegetation is typically present in their habitats, ranging from submerged aquatic plants and algae-covered rocks to overhanging riparian growth that provides shade and cover. Water clarity varies from crystal-clear spring-fed systems to slightly turbid lowland streams.

The water in these habitats tends to be moderately hard to hard and slightly alkaline, reflecting the limestone geology prevalent across much of their range. Temperatures vary with altitude and season but generally fall between 22 and 28 °C. Current strength ranges from gentle to moderate, and swordtails are strong swimmers well adapted to life in flowing water. Understanding these natural conditions helps inform the ideal aquarium setup and explains why swordtails thrive in harder, more alkaline water that might not suit soft-water species.

Tank Size and Setup

Despite being frequently sold as a small community fish, the swordtail is an active and moderately large livebearer that benefits from a spacious aquarium. A minimum tank size of 115 litres is recommended for a small group, and longer tanks are strongly preferred over tall ones, as swordtails are horizontal swimmers that make good use of lateral space. A standard 100 × 40 × 40 cm tank or equivalent is an ideal starting point. If you plan to keep multiple males, or a larger group, consider upgrading to 150 litres or more to reduce territorial tension.

Swordtails appreciate a well-structured environment that offers both open swimming space and areas of cover. A moderate to dense planting scheme works well, with hardy species such as Vallisneria, Java fern, Anubias, and Hygrophila providing shelter without overwhelming the swimming area. Floating plants like Amazon frogbit or water lettuce can help diffuse lighting and provide additional refuge for fry. Driftwood and smooth rocks can be used to create visual barriers that break up sightlines and reduce aggression between males.

The substrate is not critical for swordtails, and both fine gravel and sand work well. A darker substrate can help bring out the colours of many morphs. Ensure that the tank has a secure, tight-fitting lid, as swordtails are surprisingly capable jumpers, particularly when startled or when males are chasing one another during displays of dominance.

Water Parameters

Swordtails are adaptable fish, but they do have clear preferences when it comes to water chemistry. They are a hard-water species and perform best in moderately hard to hard, alkaline conditions. Keeping them in very soft, acidic water can lead to chronic stress, reduced immune function, and a shorter lifespan. If your tap water is naturally soft, consider using a mineral supplement or blending with a remineraliser designed for livebearers.

Stability is just as important as hitting specific numbers. Avoid sudden swings in temperature or pH, and always ensure that the tank is fully cycled before introducing swordtails. Regular water changes of 20–30% weekly will help maintain low nitrate levels and replenish essential minerals. A small amount of aquarium salt, around one to two teaspoons per 20 litres, can be beneficial but is not strictly necessary if water hardness is adequate.

Filtration and Equipment

Swordtails are not especially demanding when it comes to filtration, but they do come from flowing-water habitats and appreciate good water movement. A hang-on-back filter or internal power filter rated for the tank’s volume is generally sufficient. Canister filters are an excellent choice for larger setups, offering superior mechanical and biological filtration with adjustable flow rates. Aim for a turnover rate of approximately six to eight times the tank volume per hour, which provides adequate current without creating an excessively turbulent environment.

A reliable heater is essential for maintaining a stable temperature within the recommended range. Adjustable submersible heaters are the most practical option, and in tanks over 150 litres, using two smaller heaters rather than one large one provides redundancy and more even heat distribution. A thermometer, ideally a digital model, should be used to monitor temperature consistently.

Standard aquarium lighting is perfectly adequate for swordtails. They are not particularly light-sensitive, and the choice of lighting can be driven primarily by the needs of any live plants in the tank. A timer set to provide eight to ten hours of light per day establishes a natural photoperiod and helps regulate the fish’s behaviour. An air stone or additional powerhead is not usually necessary if the filter provides sufficient surface agitation for gas exchange, but it can be beneficial in warmer weather when dissolved oxygen levels drop.

Diet and Feeding

Swordtails are omnivores with a slight herbivorous tendency, and they are refreshingly unfussy eaters. In the wild, their diet consists of algae, plant matter, small invertebrates, insect larvae, and detritus. Replicating this varied diet in captivity is straightforward and will keep your fish in excellent condition.

A high-quality flake or micro-pellet food designed for tropical fish or livebearers should form the staple of their diet. Look for products with a good proportion of vegetable matter, such as spirulina or kelp, alongside animal proteins. Supplement this base diet with regular offerings of blanched vegetables, courgette, spinach, and shelled peas are all readily accepted, as well as occasional treats of frozen or live foods such as bloodworm, daphnia, and brine shrimp. These protein-rich foods are particularly beneficial for conditioning breeding adults and supporting the growth of fry.

Feed swordtails two to three times daily, offering only as much food as they can consume within two to three minutes per feeding. Overfeeding is a common mistake that leads to poor water quality, obesity, and associated health problems. If you notice leftover food settling on the substrate, reduce the portion size. Swordtails will also graze on soft algae growth within the tank, which provides additional nutrition and enrichment.

Behaviour and Temperament

Swordtails are generally peaceful, active fish that spend much of their time swimming in the middle and upper levels of the aquarium. They are social animals and do best when kept in groups, though the sex ratio within that group requires careful consideration. Males can be assertive and will compete with one another for dominance and mating opportunities. This competition rarely results in serious injury but can involve persistent chasing, fin flaring, and lateral displays that may stress subordinate males if there is insufficient space or cover.

The recommended approach is to keep swordtails in a ratio of one male to two or three females. This distributes the male’s mating attention across multiple females and prevents any single individual from bearing the brunt of his persistence. If you wish to keep multiple males, a larger tank with plenty of visual barriers is essential, and keeping at least three males can sometimes help diffuse aggression so that no single subordinate is singled out.

Swordtails are not aggressive towards other species and generally make excellent community tank inhabitants. They may occasionally chase or nip at fish with long, flowing fins, particularly if those fish are slow-moving, but this behaviour is relatively uncommon and usually limited to individual troublemakers rather than being characteristic of the species as a whole. They are curious and will investigate all areas of the tank, including the surface, where they may learn to recognise their keeper at feeding time.

Tank Mates

Swordtails are well suited to a peaceful community aquarium and coexist happily with a wide range of similarly sized, non-aggressive species. The best companions share their preference for moderately hard, alkaline water and occupy different areas or niches within the tank. Avoid housing swordtails with very small species that might be viewed as food, with fin-nipping species that could damage the male’s sword, or with large, aggressive fish that could prey on or bully them.

Good tank mates

• Platy (Xiphophorus maculatus), a close relative with identical water requirements and a similarly peaceful disposition, making them natural companions.
• Molly (Poecilia sphenops), another hardy livebearer that thrives in the same hard, alkaline conditions and adds variety to a community setup.
• Bristlenose Pleco (Ancistrus sp.), a peaceful bottom-dweller that occupies a different niche entirely and helps manage algae growth in the tank.
• Corydoras Catfish (Corydoras spp.), Sociable, bottom-feeding catfish that pose no threat and add activity to the lower levels of the aquarium.
• Cherry Barb (Puntius titteya), a gentle, colourful barb that is too peaceful to cause problems and appreciates similar temperatures.
• Harlequin Rasbora (Trigonostigma heteromorpha), a calm, schooling species that stays in the mid-water column and adds a flash of warm colour.
• Honey Gourami (Trichogaster chuna), a peaceful, slow-moving gourami that occupies the upper levels without competing aggressively for space.
• Kuhli Loach (Pangio kuhlii), a shy, nocturnal bottom-dweller that stays out of the swordtail’s way entirely.
• Rainbowfish (Melanotaenia spp.), Active, colourful mid-water swimmers that enjoy similar water hardness and complement swordtails well in larger tanks.

Fish to avoid

• Tiger Barb (Puntigrus tetrazona), a notorious fin-nipper that is highly likely to target the male swordtail’s elongated caudal extension.
• Oscar (Astronotus ocellatus), a large, predatory cichlid that would readily consume swordtails and requires an entirely different setup.
• Convict Cichlid (Amatitlania nigrofasciata), a territorial and aggressive cichlid, especially when breeding, that would harass and stress swordtails.
• Red-Tail Shark (Epalzeorhynchos bicolor), a territorial species that may aggressively chase swordtails, particularly in smaller tanks.
• Betta (Betta splendens), Male bettas may be antagonised by the sword’s resemblance to flowing fins, and the swordtail’s active nature can stress a slower betta.
• African Cichlids (various Mbuna species), highly aggressive and territorial species that will bully and injure swordtails despite sharing a preference for hard water.
• Green Terror (Andinoacara rivulatus), a large, aggressive South American cichlid that poses a serious predation risk to swordtails.
• Chinese Algae Eater (Gyrinocheilus aymonieri), Adults become aggressive and may latch onto the sides of swordtails, damaging their skin and slime coat.

Breeding

Breeding swordtails is one of the simplest reproductive endeavours in the freshwater hobby, and it will often occur without any deliberate effort on the keeper’s part. As livebearers, swordtails practise internal fertilisation. The male uses his gonopodium to transfer sperm to the female, and she carries the developing embryos internally for a gestation period of approximately 28 days, though this can vary between 24 and 35 days depending on temperature and the individual.

A gravid female will become noticeably plumper as her pregnancy progresses, and a dark gravid spot, a darkened area near the base of the anal fin where the developing fry can be seen through the body wall, will become increasingly prominent. When she is close to giving birth, she may become reclusive, seeking out dense vegetation or sheltered corners of the tank. A single brood can range from 20 to over 80 fry, depending on the size and age of the female.

Fry are born fully formed and free-swimming, capable of feeding almost immediately. However, swordtails offer no parental care, and both adults and other tank inhabitants will readily consume newborn fry. To maximise survival, provide dense planting, particularly floating plants and fine-leaved species like Java moss, where fry can hide. Alternatively, move the gravid female to a separate breeding tank shortly before she is due to give birth, and return her to the main tank once the fry have been released.

Swordtail fry can be fed on crushed flake food, powdered fry food, or freshly hatched brine shrimp from day one. Growth is relatively rapid under good conditions, and fry will begin to show colour and sex characteristics within two to three months. It is worth noting that females can store sperm from a single mating and produce several successive broods without further contact with a male, which means a female purchased from a shop may continue to produce fry for months after being brought home.

One fascinating aspect of swordtail biology is the phenomenon of late-onset sex change. In rare cases, a mature female that has previously given birth may begin to develop male characteristics, including the sword and gonopodium. While this has been well documented, such sex-reversed individuals are typically unable to father offspring.

Common Diseases

Swordtails are generally robust fish, but like all aquarium species, they are susceptible to a number of common ailments, particularly when kept in suboptimal conditions. Maintaining excellent water quality, providing a balanced diet, and minimising stress are the best preventive measures against disease.

Ich, or white spot disease, caused by the protozoan parasite Ichthyophthirius multifiliis, is one of the most frequently encountered illnesses. It presents as small, white, salt-grain-like spots across the body and fins, often accompanied by flashing (rubbing against objects) and clamped fins. Treatment involves raising the temperature gradually to 28–30 °C and administering a proprietary ich medication. Swordtails are generally tolerant of the mild salt treatments sometimes used alongside medication for this condition.

Fin rot is a bacterial infection that causes progressive deterioration of the fins, which may appear ragged, discoloured, or frayed at the edges. It is almost always secondary to poor water quality or physical damage from fin-nipping tank mates. Addressing the underlying cause and treating with an antibacterial medication will usually resolve the condition.

Velvet disease, caused by the dinoflagellate parasite Piscinoodinium, can affect swordtails and presents as a fine, gold or rust-coloured dusting on the skin. It is often harder to spot than ich in its early stages. Treatment typically involves copper-based medications and darkening the tank to inhibit the parasite’s photosynthetic ability.

Some colour morphs of swordtails, particularly those with heavy black pigmentation such as the black swordtail, can be prone to melanoma. This is a genetic condition linked to the melanophore (pigment cell) genes responsible for their dark colouration, and it can result in abnormal dark growths on the body. There is no treatment for melanoma, and affected fish may live normal lifespans or may deteriorate depending on the severity. Sourcing stock from reputable breeders who select against this trait can reduce the risk.

Internal parasites, including intestinal worms, can occasionally affect swordtails, particularly wild-caught or poorly maintained specimens. Symptoms include weight loss despite a healthy appetite, white, stringy faeces, and lethargy. Antiparasitic medications containing praziquantel or levamisole are effective treatments.

FAQs

How many swordtails should I keep together?

A group of at least four to six swordtails is ideal, maintained at a ratio of one male to every two or three females. This ratio minimises harassment of individual females and reduces male-on-male aggression. Keeping a single male with several females in a well-planted tank of 115 litres or more is the most straightforward approach for beginners.

Can swordtails live in an unheated tank?

Swordtails prefer tropical temperatures between 22 and 28 °C and are not well suited to unheated aquariums in most British homes, where room temperatures typically fall below this range, especially in winter. A heater is strongly recommended to maintain a stable, appropriate temperature year-round.

Why is my female swordtail developing a sword?

Swordtails are known to undergo late-onset sex reversal, whereby an older female may begin to develop male characteristics including the elongated caudal sword and even a gonopodium. This is a natural, if uncommon, phenomenon. While the fish may take on a fully male appearance, sex-reversed individuals are generally unable to reproduce as males.

Do swordtails eat their own fry?

Yes, swordtails will readily consume their own fry. They exhibit no parental care, and newborn fry are small enough to be seen as food by adults. To improve fry survival rates, provide abundant dense planting or floating cover, or separate the fry into a dedicated rearing tank shortly after birth.

Can swordtails crossbreed with platies?

Swordtails and platies both belong to the genus Xiphophorus and can interbreed, producing fertile hybrid offspring. In fact, many of the colour morphs available in the hobby today are the result of historical hybridisation between the two species. If you wish to maintain pure lines, it is best to keep them in separate tanks.

Related Guides

• Platy Care Guide, a closely related livebearer with similar care requirements, ideal for those who enjoy keeping Xiphophorus species.
• Molly Care Guide, another popular livebearer that thrives in hard, alkaline water and makes an excellent swordtail companion.
• Guppy Care Guide, the most widely kept livebearer in the hobby, sharing many husbandry principles with swordtails.
• Endler’s Livebearer Care Guide, a smaller Poeciliid with vibrant colouration and easy breeding habits, offering a useful comparison to swordtail keeping.
• Livebearer Breeding Guide, a comprehensive overview of breeding techniques and fry care applicable to swordtails and their relatives.