Koi Shiro Utsuri Swordtail (Xiphophorus hellerii)

Meet the Koi Shiro Utsuri swordtail

The Koi Shiro Utsuri swordtail is a colour line of Xiphophorus hellerii, the green swordtail, a livebearing fish in the family Poeciliidae. It is bred for its pattern rather than collected from a river, but under the colour it is exactly the same animal, and every care figure on this page comes from the species itself.

The name is borrowed from Japanese koi keeping. Shiro means white, and utsuri comes from a word meaning "to print", because the black on these koi looks like ink pressed into the skin. A Shiro Utsuri koi is a black fish with white markings. Swordtail lines sold under koi names take their labels from the koi patterns they are bred to echo, so in plain shop terms this is the black-and-white member of the koi swordtail group, where the Koi Kohaku swordtail is the red-and-white one and the Koi Sanke swordtail adds black to red and white.

Patterned fish are individuals, and this listing has no gallery photos. We are not going to describe a particular fish or promise how much black or white yours will carry. What we can do is tell you the features every swordtail shares, so you know what you are looking at when it lands in your tank:

• The sword. A mature male grows a long extension of the lower lobe of his tail fin. That is where the common name comes from; females do not normally have one.
• The gonopodium. In males the anal fin is reshaped into a narrow, pointed rod used to fertilise the female internally. Seriously Fish describes it as prominent, and it is the clearest way to sex a swordtail once it starts to mature.
• Size and build. Females are the larger sex. FishBase gives maximum total lengths of 14 cm for males and 16 cm for females.
• Finnage. Some ornamental swordtails carry an extended upper tail lobe or longer fins as well. This line's name refers to colour, not fin shape, so it is sold for its pattern.

You will often see the species written helleri, with a single i, including on the earlier version of this page. Seriously Fish and Wikipedia both point out that this is a misspelling. The fish was named after Karl Bartholomaeus Heller, an Austrian botanist who found it in Mexico, and FishBase lists the accepted name as Xiphophorus hellerii Heckel, 1848. Search for either spelling and you are looking at the same animal.

One last piece of background. Seriously Fish notes that the ornamental swordtails in the hobby were shaped by selective breeding, including deliberate crosses with the platies Xiphophorus maculatus and X. variatus. Fancy lines like this one may therefore carry some platy ancestry. It changes nothing about how you keep them.

What one unit of this listing gives you

This listing sells single fish. Each unit you add to the basket is one Koi Shiro Utsuri swordtail, and there are no options to pick from: no size grades and no choice of male or female. The sex of each fish is not selected and not promised.

With swordtails that is worth thinking about before you order, because the balance of the sexes shapes how a group behaves. The care sheet from the Ornamental Aquatic Trade Association (OATA), the UK trade body, recommends keeping at least three swordtails together, ideally two females to each male, to reduce stress. An unsexed listing cannot guarantee that split, so here is how we would approach it:

• Starting from scratch, order three or more. A group gives you room for the mix to land unevenly; two fish could easily be two males.
• Already keeping swordtails? A newcomer is easier to absorb into an established group, where one extra male or female matters less.
• Have a fallback for surplus males before you need it, whether that is a larger tank, a second tank or a local club or shop that will take them.

Young swordtails can also be slow to show their sex. Males only develop the sword and a finished gonopodium as they mature, and Wikipedia reports that the presence of a well-endowed male can hold back the maturity of young males close by. So a fish that looks female for weeks can still turn into a late male, and a confident sexing on a young fish is never a sure thing.

There is no size option here, so plan the aquarium around grown fish rather than the size that arrives. OATA puts typical adult swordtails at 8–12 cm. FishBase records maximum total lengths of 14 cm for males and 16 cm for females, while Seriously Fish quotes 140 mm standard length. Those top figures are records rather than what most aquarium fish reach, but they make the point clearly enough: this is a fish for a proper community tank, not a nano.

Before you place the order, run through this short checklist. If every line is a yes, you are ready for swordtails:

• The tank holds at least 80 litres, and ideally has a base of around 120 × 30 cm.
• It has been running long enough to be mature, with ammonia and nitrite reading zero.
• Your tap water tests at pH 7.0 or above and is not very soft.
• The heater holds a steady temperature between 22 and 28 °C.
• Existing fish are not shy, slow species that a busy swordtail would harass.
• You know what you will do with extra males and with fry, because a mixed group will breed.

Where the wild green swordtail lives

All swordtail colour lines trace back to a wild fish from southern Mexico and northern Central America. FishBase gives the native range as the Río Nantla in Veracruz, Mexico, south to north-western Honduras. Seriously Fish lists south-eastern Mexico, Guatemala, Belize and Honduras, at altitudes from sea level to around 1,500 m.

Across that range it lives in a fair variety of water. FishBase describes adults as found mainly in rapidly flowing streams and rivers with heavy vegetation, and also in warm springs and their outflows, weedy canals and ponds. Seriously Fish adds that the water runs from clear to turbid. FishBase classes the species as freshwater and brackish, and benthopelagic, which in FishBase terms means it lives and feeds near the bottom and in the open water above it.

The brackish listing is a record of where the species has been found, not an instruction to add salt. Both Seriously Fish and OATA give freshwater aquarium ranges, and that is how we would keep a koi line.

A wild fish looks nothing like a koi swordtail. The wild form is the one the hobby sells as the green swordtail, and Wikipedia describes the male's sword as yellow with a black lower edge. Seriously Fish notes that the ornamental forms are not present in wild fish; the reds, blacks, albinos and koi patterns are all the work of breeders.

Three things from the habitat carry straight into the aquarium:

1. Plants. Wild swordtails favour heavily vegetated water, so a planted tank suits them.
2. Movement. They come from flowing streams and rivers, and Seriously Fish recommends moderate water movement, though not heavy filtration.
3. Hard, alkaline water. Every source we used gives a pH of 7.0–8.0, so these are not fish for soft, acidic water.

The species has also spread a long way from home. FishBase lists feral populations in Natal and the eastern Transvaal in South Africa and in Lake Otjikoto in Namibia, and flags the swordtail as a potential pest. Wikipedia adds Madagascar, Puerto Rico, Hawaii and eastern Australia. OATA is clear on the rule that follows: never release aquarium fish or plants into the wild. It is illegal, and for the fish it usually means a slow death. The green swordtail is assessed as Least Concern on the IUCN Red List (FishBase, assessment dated 2018).

Tank size and layout for a group of swordtails

Swordtails are larger and more active than platies, and the tank should allow for that. OATA's minimum for swordtails is 80 litres. Seriously Fish recommends a base of 120 × 30 cm, which works out at roughly 108 litres, and that is the figure we would build around for a group that includes grown males. Footprint matters more than height: a long tank gives far more swimming room than a tall one holding the same volume.

For layout, Seriously Fish suggests a heavily planted set-up with a dark substrate, and says floating plants and driftwood are appreciated. For wild-type fish it describes a stream-style tank with water-worn rocks and small boulders. OATA asks for aquarium gravel or sand, ideally with live plants for cover, and accepts plastic plants and suitable ornaments where live plants are not used.

Here is how we would put that together for a Koi Shiro Utsuri group:

The cover is not just decoration. Males in a group sort themselves into a pecking order, and females get a lot of attention from them, so planting that lets a fish slip out of sight for a while keeps the whole tank calmer. Spread the cover across the tank rather than piling it in one corner, so no single male can sit over all of it.

A practical note on the colour: on a black-and-white fish, the white areas tend to stand out most against darker surroundings, which is one more reason the dark substrate Seriously Fish suggests works well for this line. That is a matter of display rather than welfare, and the fish will be just as healthy over a lighter gravel.

Finally, give the tank time. No fish does well in an aquarium that has not finished maturing, and OATA's own lifespan note ties several years of life to a well-matured set-up with good water quality.

Water: temperature, pH and hardness, source by source

The three sources we used agree on pH and broadly on temperature, and disagree on hardness. Here they are side by side, so you can see where our recommended figures come from:

The care specifications on this listing use the FishBase ranges: 22–28 °C, pH 7.0–8.0 and 9–19 dGH. For everyday keeping we would set the heater in the middle of that temperature band rather than at either end. Seriously Fish's lower figure of 16 °C is a long way below what a tropical community tank runs at, and we would not treat it as a target. The species does live from sea level up to around 1,500 m, so a wide tolerance is not a surprise, but tolerance and comfort are different things.

On hardness, the honest answer is that the three ranges only overlap at 10–12 dGH, and each sits at a different point. What they all make clear is that this is not a soft-water fish. Test your tap water before you order. If it is already moderately hard and alkaline, a swordtail will be happy in it without any adjustment. If your water is very soft, think carefully about whether a hard-water livebearer is the right choice, or ask us for advice first.

Carbonate hardness (KH) deserves a mention because it is the figure many keepers never test. KH is the water's buffering capacity: the higher it is, the harder it is for pH to drop between water changes. OATA gives 3–10 dKH for platies and swordtails. In a tank with very low KH, the waste a busy group of livebearers produces can pull the pH down below the 7.0 that every source treats as the floor, so if your test kit reads KH near zero, that is worth fixing before the fish arrive rather than after.

Clean water matters as much as the numbers. OATA's targets are:

• ammonia: zero
• nitrite: zero
• nitrate: no more than 20 mg per litre

To hold those, OATA strongly recommends a partial water change of 25% at least once a week. Match the temperature of the new water to the tank, and treat tap water with a dechlorinator before it goes in. Swordtails breed freely and a group grows quickly, so keep an eye on stocking: the same weekly change that suits five fish will not hold a tank that has quietly become twenty-five.

Stability is worth more than chasing an exact figure. A pH that sits steadily at 7.4 is better for a swordtail than one that swings between 7.0 and 8.0 because of rushed adjustments. Test, change water on a fixed day, and only correct a reading when it is actually outside the ranges above.

Feeding an unfussy omnivore

In the wild, FishBase records swordtails feeding on worms, crustaceans, insects and plant matter. Seriously Fish calls the species a generalised omnivore and says that in the aquarium it is unfussy and will accept just about anything offered. That makes feeding easy, but it also makes overfeeding easy, so a little structure helps.

Both care sources point the same way. Seriously Fish recommends a balanced diet built on good-quality dried food, backed up with small live and frozen foods such as daphnia, brine shrimp (Artemia) and chironomid larvae, which are sold as bloodworm. OATA recommends a good-quality flake or granule with a high algae content, plus live or frozen foods such as daphnia, small bloodworm or tubifex. The plant matter in the wild diet is the reason an algae-rich staple earns its place.

How much? OATA's guidance is simple: feed only what the fish will eat within a few minutes, once a day. Food still drifting about after that time is excess, and it ends up as ammonia and nitrate.

A week for a small group might look like this:

• Most days: a pinch of algae-rich flake or small granules, eaten within a few minutes.
• Two days a week: swap the dry food for frozen daphnia or brine shrimp, thawed in a little tank water first.
• Once a week: small bloodworm as a treat rather than a staple.
• Weekly: look at the gravel the morning after feeding. Leftovers mean you are giving too much.

Feeding time is also how you check on individual fish. A swordtail that stops rising for food, or hangs back while the rest of the group eats, is telling you something. Note it and look again at the next meal; persistent loss of appetite is one of the first things to report if you ask us for advice.

In a busy community tank, make sure the food reaches the fish that need it. Bolder fish reach the food first, and a second pinch at the other end of the tank lets females and lower-ranking fish eat without a fight. That also matters for pregnant females, which are carrying a brood that grows over several weeks.

Males, females, pecking order and tank mates

Swordtails are lively rather than aggressive towards other species, but between themselves the males take rank seriously. Seriously Fish notes that males form dominance hierarchies in confined spaces. In practice one male usually ends up dominant and the others give way to him. Space, cover and the right ratio are what keep that from becoming a problem.

Sword length is not just for show. Seriously Fish reports that females show preferences for male traits such as sword length, and Wikipedia summarises research showing that the elongated tail fin significantly affects a male's mating success. The same Wikipedia article reports that a well-endowed male can delay the maturity of young males near him, which is one reason a group can contain a male that develops much later than the rest.

Getting the ratio right

OATA recommends at least three swordtails kept together, with two females to each male, to reduce stress. Females spread across several fish share the attention of the males; a single female with two males gets very little peace. As this listing does not let you choose the sex, buy enough fish that the mix has room to work, and be ready to adjust if it lands male-heavy.

The signs that the balance is wrong are easy to read once you know them. Females that spend most of the day pressed into the plants, a female with split fins, or one male that never leaves the back corner because another patrols the front are all telling you the same thing. Adding females, adding cover or moving a male to another tank are the usual fixes, and they work far better than waiting to see whether things settle.

Choosing companions

OATA describes swordtails as generally peaceful and compatible with other species, with one warning: large swordtails may chase more timid tank mates. Combine that with the water figures and you get a sensible shortlist:

• Good fits: active community fish of similar size that are content in hard, alkaline water at 22–28 °C.
• Use caution with: shy, slow species that are easily pushed off food or harried by a bold 14 cm fish.
• Avoid: soft, acidic-water specialists, and predators large enough to treat a swordtail as food.

Other swordtails and platies

Every colour line of swordtail is the same species, so a Koi Shiro Utsuri will happily breed with a black, red or green swordtail sharing its tank. Platies are close relatives too: Seriously Fish records that breeders have crossed swordtails deliberately with Xiphophorus maculatus and X. variatus. In a display tank that may not bother you. If you want your fry to resemble their black-and-white parents, though, keep the breeding fish away from other swordtail colours and from platies.

When you add swordtails to an existing community, add them after the tank's quieter residents have settled, and make sure the layout has plenty of broken sight lines so that no fish, old or new, is in view of a rival all day.

Pregnancy, fry and keeping the pattern going

Swordtails are livebearers. Fertilisation is internal, and the female gives birth to free-swimming young rather than laying eggs. Wikipedia calls them some of the easiest fish for an amateur to breed, provided the tank conditions are right, and in a mixed group you should expect fry whether you planned for them or not.

The numbers

• Pregnancy: 24–30 days, according to both FishBase and OATA.
• Brood size: FishBase gives 20 to 200 young; OATA's care sheet says 20–80. Treat 200 as the top of the recorded range, not a typical brood.
• Sperm storage: Seriously Fish notes that females can store viable sperm for months. A female that has lived with males can produce more than one brood without a male present.
• Maturity: Wikipedia gives sexual maturity at around three months; FishBase gives eight to twelve months. The sources disagree, so we give both.

Late in pregnancy, Wikipedia notes, a dark gravid spot appears near the female's anal fin, which is the usual sign that a birth is not far off.

Protecting the fry

If you want to raise a brood, OATA suggests keeping the pregnant female in a suitably large breeding net, which keeps the newborn fry apart from the other fish. A thickly planted corner gives a few fry a chance in the main tank too, but a net or a separate tank is the reliable route. Return the female to the group once she has given birth, so she is not confined for longer than necessary. Feed the young on food small enough for them to swallow, little and often.

Keeping a line

We cannot tell you how strongly the Shiro Utsuri pattern will show in any brood, and we would be wary of anyone who claimed to know for an unsexed group of patterned fish. What keeps a colour line going is selection: breed from the fish whose pattern you like best, keep other colours of swordtail and platies out of the breeding tank, and rehome surplus young through a local shop or club. For comparison, a solid line such as the black swordtail shows how far breeders have taken the species from its wild green colouring.

Plan for numbers before they arrive. With broods every month or so from each female, a small group can outgrow its tank within a season, and fry need homes just as much as the adults do.

Health: what a fit swordtail looks like

Swordtails are robust fish when their water is right. When one does go downhill, check water quality, crowding and stress from a poor male-to-female balance. OATA's care sheet gives a straightforward picture of a healthy fish and of the early signs that something is wrong.

If you see any of the warning signs, test the water before anything else. Ammonia or nitrite above zero, or nitrate creeping past 20 mg per litre, explains a lot of clamped fins, and a water change is the first treatment. Check the heater as well: a temperature that has drifted outside 22–28 °C stresses a tropical fish quickly.

Next, watch the social side for ten minutes. A female constantly shadowed by two or three males, or one male driven into a corner by another, will lose condition even in perfect water. More cover, a better ratio or a larger tank fixes that where medication will not.

A few practical habits keep problems small:

• Look over every fish at feeding time. That is when they come out in the open and you see fins, eyes and appetite together.
• If you run a quarantine tank, use it for new fish before they join an established group.
• Do not medicate on a guess. Identify the problem, then treat, and remove carbon from the filter if the medicine's instructions say so.
• Keep a simple log of test results and water changes. Patterns jump out of a log long before they show in the fish.

Look after the basics and swordtails repay you. OATA notes that these fish can live several years in a well-matured set-up with good water quality. If you are unsure about anything you see, email us at hello@tropicalfishco.co.uk and describe what the fish is doing; a short video helps.

When your swordtail arrives

Please read our delivery and live-arrival information before you order. The aquarium should be fully ready before the parcel is on its way: filter running, water tested, and the temperature already where the fish will live. Someone needs to be at the delivery address to take the parcel in, because live animals cannot wait on a doorstep.

Your swordtail travels in an oxygen-filled bag inside insulated packaging, with heat packs added when the weather calls for them, carried by a tracked courier licensed to transport live animals. We email you the confirmed delivery date and the tracking details.

Have these ready on the day, before the courier arrives:

• a phone or camera charged and ready to film;
• a clean fish net that fits the bag and your tank;
• a bucket for the used bag water, so it never goes near the aquarium;
• your test kit, in case a reading needs checking before the fish go in;
• the tank lights switched off.

Film the unboxing

Our live-arrival policy depends on one continuous video. Start recording while the parcel is still sealed, open it in the same take, and show the fish in its still-sealed bag. If a fish arrives dead, dying or clearly in poor condition, 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, step by step

1. Turn the tank lights down or off.
2. Float the sealed bag in the aquarium for 20–30 minutes so the water inside reaches tank temperature. This only matches temperature; it does nothing for pH or hardness, so if your water is very different from typical swordtail water, ask us before the fish arrive.
3. Open the bag and net the swordtail across into the tank.
4. Throw the bag water away. Never pour it into your aquarium.

Over the first week, keep the lights low to begin with and watch breathing, appetite and behaviour. Give a new fish time to find its way around before you judge how it is doing. If you are adding to an existing group, keep an eye on how the resident males react, and make sure the newcomer is getting food. A small first feed tells you plenty: a fish that eats on the first or second day is usually settling well.

Further reading

These are the sources behind every fact on this page. They are worth reading in full if you want to go further with swordtails.

• FishBase: Xiphophorus hellerii — accepted name, maximum sizes, water ranges, distribution and reproduction.
• Seriously Fish: Xiphophorus hellerii — aquarium size, layout, diet, behaviour and the spelling of the name.
• Wikipedia: Green swordtail — the sword, mating research, naming history and introduced populations.
• OATA: How to look after platies and swordtails — UK trade care sheet covering tank size, water quality, feeding, ratios and health checks.
• Wikipedia: Koi — the koi varieties, including Kohaku and Shiro Utsuri, that give koi swordtails their names.