Koi Tricolour Swordtail (Xiphophorus hellerii)

What the Koi Tricolour Swordtail is and what you receive

The Koi Tricolour Swordtail is a bred colour form of Xiphophorus hellerii, the swordtail of southern Mexico and Central America. Seriously Fish calls the species one of the world's most recognisable aquarium fishes, and Wikipedia lists it among the most popular tropical fish in the hobby, valued for its hardiness and for how well it fits a community tank. Underneath the colour this is the same animal as the olive-green wild fish: the same eventual size, the same liking for hard, alkaline water and the same busy, slightly pushy character. Breeders have changed the paintwork, not the fish.

The name is borrowed from koi carp. In koi terms, kohaku is a white fish with red markings, sanke is that same red and white with small black markings added, and sanshoku simply means "three colours". A swordtail sold as koi tricolour belongs to that family of names: the line is bred for a broken mix of red, white and black rather than one flat body colour. Patterned fish are individuals, though, so read the name as a description of the line, not a promise of where each patch will sit on the fish you receive.

The offer on this page

Each order unit on this listing is one fish of the size you select: 3–4 cm, 4–5 cm or 6–7 cm. That is the size it leaves us at, not the size it will finish at. FishBase records the species at up to 14 cm total length for males and 16 cm for females, so a fish of this size still has most of its growing ahead of it. Plan the aquarium for the adult, not for the youngster in the bag.

The listing does not say whether you will get a male or a female, and we don't promise either. That matters more with swordtails than with many community fish, because the balance of males and females decides how peaceful the tank stays. If you want a group, the sensible route is to buy several, give them time to mature, and adjust the mix once you can see who is who. The section on temperament below explains why.

Why the name now has two i's

The species is very often written Xiphophorus helleri, with a single "i", and Seriously Fish points out that the slip turns up in scientific papers as well as hobby books. FishBase, Seriously Fish and Wikipedia all give hellerii. Johann Jakob Heckel described the fish in 1848 and named it after Karl Bartholomaeus Heller, an Austrian botanist who came across it while exploring Mexico between 1845 and 1848, and the shorter spelling is treated as an error. Same fish, corrected name. The genus name comes from Greek and means "sword bearer", which tells you exactly what to look for on an adult male.

Telling males from females as it grows

Swordtails are among the easier livebearers to sex once they are adult, and the clue is in the common name. Mature males carry the sword: the lower lobe of the tail fin grows out into a long, straight extension. In wild-type fish Wikipedia describes it as yellow with a black lower edge. Males also have a gonopodium, the anal fin modified into an organ used to fertilise the female internally, which is the standard livebearer arrangement. Females are the larger sex and, as Wikipedia puts it, lack the sword.

There is one complication with bred forms. Seriously Fish notes that some ornamental strains show fin extensions in both sexes, so on fancy lines a long tail on its own is not proof of a male. The gonopodium is the reliable check. Compare the anal fin, on the underside just behind the vent: on a male it has become the gonopodium, on a female it has not.

How big, and how quickly

Total length is measured to the tip of the tail fin; standard length stops at the base of the tail. So the Seriously Fish figure, if anything, describes a slightly longer fish overall than the FishBase male record. Both are maximum records. Not every swordtail reaches them, and you should not treat them as a growth target, but they are the right numbers to plan a tank around. We use 14 cm in the care details on this page.

FishBase gives eight to twelve months for sexual maturity. Depending on its age when it reaches you, a 4–5 cm fish may or may not show its adult features clearly yet, so give it time before deciding what you have. FishBase also notes that sex reversal has been recorded in the species under certain conditions, which is worth knowing if an older fish ever seems to change.

Why the sword matters to a swordtail

The sword is not just ornament. Seriously Fish describes female swordtails showing preferences for a long list of male traits: sword length (long, short or none at all), colour pattern, body size, vertical barring on the body, chemical cues and particular courtship behaviours. Wikipedia reports that the length of a male's sword has been found to significantly affect his chances of mating.

Where wild swordtails live

The wild species belongs to Mexico and northern Central America. Seriously Fish gives the native range as southeastern Mexico, central Guatemala, southern Belize and northwestern Honduras, running south from the Río Nautla in Veracruz to the Río Usumacinta drainage in Guatemala. FishBase and Wikipedia summarise it as Veracruz to northwestern Honduras. The type locality, where the specimens used for the original description were collected, is Orizaba in Veracruz state.

It is a fish of moving water, though not only of moving water. Seriously Fish describes populations from sea level to around 1,500 m, in fast-flowing rocky streams, springs, ditches, ponds and rivers, in water anywhere from clear to turbid and usually less than 1.5 m deep. Adults favour the stronger current while juveniles keep to the quieter margins. FishBase records adults mainly in rapidly flowing streams and rivers with heavy vegetation, and Wikipedia adds warm springs and canals to the list.

The sources do not agree on the plants. FishBase and Wikipedia stress heavily vegetated habitat, while Seriously Fish says many of these waters hold little aquatic vegetation. We take the practical middle line in the aquarium: plenty of cover, with open water left clear for swimming. FishBase also lists the species from brackish as well as fresh water, another sign of how adaptable it is.

What that means for your tank

• A fish from flowing streams appreciates some water movement, so set the filter to push a gentle current along the tank.
• Adults use open water; young fish in the wild keep to calmer edges, so a planted, quieter end of the tank suits a 4–5 cm newcomer.
• The fish copes with water from clear to cloudy in the wild, but that is not a reason to tolerate poor water quality at home.

Never release swordtails

This adaptability has a downside. Seriously Fish reports feral populations on every continent except Antarctica, FishBase reports feral populations from Natal and eastern Transvaal in South Africa and from Lake Otjikoto in Namibia, notes adverse ecological effects reported after introductions, and classes the species as a potential pest. Wikipedia attributes the damage to its ability to reproduce rapidly in high numbers. The IUCN lists the species as Least Concern. Unwanted swordtails or their young should go to another aquarist or a shop, never into a pond, stream or ditch.

Tank size and layout for a fish that reaches 14 cm

It is easy to buy a 4–5 cm swordtail for a tank that suits a 4–5 cm fish. The trouble starts later, when a full-grown male is patrolling a short tank and has nowhere to go but round in circles. Seriously Fish recommends a tank base of at least 120 × 30 cm, which it puts at roughly 108 litres. FishBase's aquarium note gives 80 cm as the minimum length and pairs it with a warning that males are aggressive towards each other. We use 108 litres as the minimum in the care details on this page.

Footprint matters more than volume here. Swordtails swim lengthways, and males chase each other. A long, fairly shallow tank gives them a straight run and gives a pursued fish distance to escape. A tall cube or column of the same volume gives them neither, so if you are choosing between two tanks of similar capacity, take the longer one.

Furnishing it

For the wild form, Seriously Fish suggests a heavily planted set-up with a dark substrate, adding floating plants and driftwood roots or branches to diffuse the light and give a natural look. The same scheme works for a koi pattern. A dark gravel or sand base makes the red and white stand out far better than a pale one, and floating plants take the glare off the surface.

Plan the layout around how the group behaves. Plant the back and both ends densely, leave a clear swimming lane along the front, and use wood or tall stems to break up the line of sight between one end and the other. When males settle their pecking order, a lower-ranking fish can then drop out of view rather than being pinned in the open.

Filter, flow and heater

Seriously Fish says the filtration does not need to be especially powerful, but the species does appreciate a degree of water movement. For a tank of the size it recommends, it suggests a filter rated at roughly 430–540 litres per hour and a 75–125 watt heater. Aim the filter outlet along the length of the tank so the current runs the full distance, and keep one planted corner calm for resting fish and any youngsters that appear.

Whatever you choose, have the tank set up, heated and running with a matured filter before you order. An immature filter is one of the most common reasons new fish struggle in their first weeks, and swordtails are no exception.

Water conditions: hard, alkaline and steady

This is a hard-water fish. FishBase gives a temperature range of 22–28 °C, a pH of 7.0–8.0 and hardness of 9–19 dGH. Seriously Fish agrees on pH, gives a wider hardness band of 10–25 dGH, and lists a temperature range of 16–28 °C. The figures in the care details on this page follow FishBase.

The 16 °C lower figure from Seriously Fish describes what the species can tolerate across its wide altitude range. It is not a setting for a tropical aquarium. In a heated tank we would set the thermostat somewhere in the middle of the 22–28 °C band and leave it there, rather than run near either edge where a heater fault or a hot summer spell would push the fish outside its range.

Matching your tap water

Swordtails suit hard, alkaline tap water well. If your water already tests at a pH of 7 or above with moderate to high hardness, you may barely need to change anything apart from removing chlorine and chloramine with a dechlorinator. If your water is very soft or acidic, talk to us before ordering. Raising hardness and pH is possible, but it should be done steadily in an established tank weeks before fish arrive, not on the day the parcel lands.

FishBase lists the species from brackish as well as fresh water. That shows its tolerance; it is not an instruction to add salt. A freshwater community tank at the right hardness is all a koi swordtail needs.

Keeping it steady

Swordtails are hardy, but hardy fish still dislike sudden swings. A simple routine works well:

• Change part of the water every week, using dechlorinated water at the same temperature as the tank.
• Test ammonia and nitrite in the first weeks of any new set-up and after adding fish; both should read zero.
• Keep nitrate down with regular changes rather than occasional large ones.
• Check pH and hardness every few weeks, so any drift shows up before the fish feel it.
• Clean the filter media in old tank water, never under the tap, so the bacteria that process waste survive.

Seriously Fish mentions that introduced swordtail populations show tolerance to polluted conditions. Please don't read that as permission to let things slide. A fish that survives poor water is not the same as one that thrives, and you chose this one for its colour.

Feeding a growing swordtail

Swordtails are easy to feed. In the wild, FishBase lists worms, crustaceans, insects and plant matter in the diet, and Seriously Fish describes a generalised omnivore taking aquatic and land-dwelling invertebrates, organic detritus, algae and other plant material. In the aquarium, Seriously Fish says the species is unfussy and will accept just about anything offered, recommending good-quality dried foods alongside freeze-dried bloodworm and freeze-dried Artemia (brine shrimp).

Unfussy does not mean any old food. A fish that arrives at 4–5 cm and may finish at 14–16 cm does a lot of growing, and the diet over those months shows in its size, shape and colour. We would build the menu like this:

• Staple: a good flake or small pellet sized for the fish's mouth, fed every day.
• Plant content: a food with algae or spirulina in it several times a week, since plant material and algae are part of the wild diet.
• Meaty extras: freeze-dried bloodworm or brine shrimp, as Seriously Fish suggests, or the frozen equivalents, two or three times a week.
• Variety: rotate between two or three staples rather than relying on a single tub.

How much and how often

Feed small amounts once or twice a day, giving only what the fish clear in a couple of minutes. A swordtail that rushes to the glass when you walk past is not necessarily hungry; most aquarium fish soon link people with food. Uneaten food sinks into the plants and breaks down, and overfeeding is one of the most common causes of poor water in a tank of otherwise healthy fish.

A young fish benefits from two smaller meals rather than one large one. Once the group is adult, one or two meals a day with an occasional lighter day suits them. If you go away for a long weekend, healthy adult swordtails in an established, planted tank are better left unfed than fed by a well-meaning neighbour with the whole tub.

Feeding in a mixed tank

Swordtails are unfussy, active feeders. If you keep slower or bottom-dwelling fish with them, put sinking food in at the far end at the same time as the swordtails are busy at the surface. Watch the first few feeds after arrival: a fish that takes food within a day or two is usually settling well.

Temperament, group balance and tank mates

With other species, a swordtail is usually a good community citizen: Seriously Fish describes it as suitable for community set-ups and social within a group, and Wikipedia credits its popularity partly to its suitability for community tanks. The friction is almost all between males. FishBase states plainly that males are aggressive towards each other, and Seriously Fish explains that in confined spaces groups of males form dominance hierarchies and spend a good part of their time defending their position.

In practice that means a top male who chases, displays and pushes rivals into the plants, and lower-ranking males who hide, hold their fins tight and can lose condition if they have nowhere to go. Females also get a lot of attention, which is why the usual advice is to outnumber the males. Wikipedia recommends one male to three or four females.

Building a balanced group from unsexed fish

Because this listing does not specify a sex, you build the balance over time:

1. Buy a small group rather than a single fish, so the odds give you more than one of each.
2. Let them grow on. As fish mature, the sword and gonopodium show you which are males.
3. If you end up with more males than the tank can absorb, rehome the extras to another aquarist or a shop.
4. In a tank at or near the 120 cm minimum, one male with several females is the calmest arrangement.

A larger, well-planted tank gives you more room to experiment, but watch the lowest-ranking fish. One that hides constantly and stops coming up to feed needs moving.

Choosing companions

Good companions are peaceful fish of a broadly similar size that want the same water: hard and alkaline at 22–28 °C, pH 7.0–8.0. Avoid anything large or predatory enough to swallow a 4–5 cm fish, and avoid species that need soft, acidic water, because one side of the tank would always be compromised. Very timid, slow fish can be overwhelmed at feeding time by a swordtail's speed.

Platies need a note of their own. Seriously Fish records hybridisation between swordtails and both Xiphophorus maculatus (the platy) and X. variatus (the variatus platy), and Wikipedia states that most aquarium swordtails are hybrids with southern platyfish. If you keep both, with both sexes of each, expect cross-bred young.

Other swordtail colour forms are the same species, with the same size, water and temperament, so the male-balance rule applies across them all. If you like the koi look, the Koi Sanke swordtail is named after the three-colour sanke koi, and the Koi Kohaku Swordtail after the red-and-white kohaku. For the species' wild colouring, compare the Green Swordtail.

Breeding and what to do with the young

Wikipedia describes swordtails as some of the easiest fish for an amateur aquarist to breed, and in a mixed group you should expect young whether you planned them or not. Like the rest of the family Poeciliidae, they are livebearers. The male uses his gonopodium to fertilise the female internally, and the young develop inside her and are born alive.

FishBase gives the numbers: a gestation of 24 to 30 days, and broods of 20 to 200 young. A pregnant female develops a dark gravid spot near the anal fin, according to Wikipedia. Seriously Fish adds a point that surprises many new keepers: females can store viable sperm for months. A female bought from a mixed group may give birth after you have her, and again later, even with no male in your tank.

Giving the young a chance

In the wild, Seriously Fish notes that juvenile swordtails keep to quiet marginal areas while the adults hold the current. That is a good model for the aquarium. If you want some young to survive in the main tank, give them shallow, densely planted, low-flow areas. Fine-leaved plants and floating plants with trailing roots give newborns somewhere to stay out of the way.

If you want to raise a larger number, a separate small tank with a gentle sponge filter is the reliable route. Move the heavily gravid female across before she gives birth and return her once she has finished. Feed the young finely crushed flake or a fry food, little and often, and keep their water very clean.

Plan for the numbers

Up to 200 young per brood, with a gestation of under a month, is a lot of fish. Before you let a group breed freely, decide where the youngsters will go, because they will reach a size that needs a big tank. Never solve a surplus by releasing fish outdoors: FishBase classes the species as a potential pest outside its range.

Two more things to remember if you are breeding on purpose. Pattern varies between individual koi-type fish, so don't expect every youngster to look like its parents. And if platies share the tank, Seriously Fish's record of hybridisation means some youngsters may be crosses.

Health: what to watch and how to prevent problems

Swordtails earn their reputation for hardiness, and most problems we hear about come back to the same few causes: an immature filter, overcrowding, overfeeding, too many males in too little space, or sudden changes in temperature or water chemistry. Get those right and a koi swordtail is a straightforward fish to keep.

Signs worth acting on

• Clamped fins and hiding: often a lower-ranking male under pressure, or a fish stressed by water quality. Test the water, then look at the group balance.
• Rapid breathing at the surface: check ammonia, nitrite and temperature first, and make sure the surface is moving enough to exchange gas.
• Loss of appetite: a swordtail that ignores food for more than a day or two is unusual and worth watching closely.
• Frayed or split fins: look for a bully in the tank or a sharp ornament before assuming disease.
• White spots, flashing or rubbing against décor: common signs of skin parasites; isolate and treat promptly with a product suited to the fish in the tank.
• Fading colour: a stressed fish often loses intensity. On a patterned fish this can be your first clue that something has changed.

Prevention that works

Quarantine is the most useful habit you can build. A spare tank with a mature sponge filter lets you watch new fish for a few weeks before they join an established group, and gives you somewhere to separate a bullied male or a female about to give birth. It doubles as a hospital tank when you need one.

Beyond that, keep the routine boring: weekly water changes, steady temperature, moderate feeding and a stocking level the filter can handle. Remember that the fish you buy at 4–5 cm will be several times heavier when adult, and that a mixed group adds young to the bioload every month or so. A tank that is comfortably stocked on day one can be overstocked within a year if nobody is counting.

Finally, give the males space. Seriously Fish's description of males spending much of their time maintaining a hierarchy is a description of chronic stress for whoever is at the bottom. A longer tank, more cover and fewer males are the best medicine for most swordtail health problems.

Arrival day and the first week

Before you order, please read our delivery and live arrival information. The aquarium must be ready when the parcel arrives: filter running and matured, water tested, and the heater holding the temperature you intend to keep. Someone must be at the delivery address to receive the parcel.

How the fish travels

Your swordtail travels in an oxygen-filled bag inside insulated packaging, with heat packs when conditions call for them, using a tracked courier licensed to carry live animals. We email you the confirmed delivery date and tracking details so you know when to be in.

Film the unboxing

Start a continuous video before you open the parcel, while it is still sealed, and keep filming in the same take as you open it and show the fish in its still-sealed bag. If a fish arrives dead, dying or clearly compromised, email hello@tropicalfishco.co.uk on the day of delivery with your order number and that video. Photos on their own do not meet the policy. The resolution is a replacement with your next order or a refund for the affected fish, subject to the guarantee terms.

Acclimatising your swordtail

1. Turn the tank lights off or down.
2. Float the sealed bag in the aquarium for 20–30 minutes so the water inside reaches tank temperature. This matches temperature only, not pH or hardness, so if your water is very different from typical hard, alkaline swordtail water, ask us for advice before the fish arrive.
3. Open the bag and net the fish across into the tank.
4. Throw the bag water away. Never pour it into your aquarium.

The first week

Keep the lights low for the rest of the first day and leave the fish alone to find its way around. Offer a small feed the next day. Over the first week, watch three things: breathing, appetite and behaviour. A 4–5 cm swordtail that is exploring, swimming in open water and taking food within a couple of days is settling in well. If it hides constantly, breathes hard or refuses food, test the water first and then look at what the other fish in the tank are doing.

If you are adding several swordtails to a tank that already holds adults of the same species, rearrange a few plants or pieces of wood before releasing the newcomers. Breaking up established territories gives everyone something new to explore and takes the pressure off the new arrivals.

Further reading

These are the sources behind the facts on this page. They are worth reading if you want to go deeper into the species.

• FishBase: Xiphophorus hellerii covers maximum lengths for males and females, water ranges, distribution, wild diet, gestation and brood size, and the aquarium note on male aggression.
• Seriously Fish: Xiphophorus hellerii covers habitat, recommended tank size, water conditions, diet in the aquarium, male hierarchies, female mate choice, sperm storage and hybridisation with platies.
• Wikipedia: Green swordtail covers the naming history and spelling, sexual differences, recommended sex ratio, the gravid spot and the species' spread outside its range.
• Wikipedia: Koi explains the kohaku, sanke and sanshoku colour terms that ornamental swordtail names borrow.