By Hannah Nielsen
Swordtails look easy — and they are, once you know three things
You're after green swordtails, and almost every guide online tells you the same flat list: hardy, livebearer, easy. All true. None of it prepares you for the three things that actually catch UK keepers out — the male that appears out of a tank of "females", the fish that launches itself onto the carpet overnight, and the single pair that turns into a chase scene.
I'm Hannah Nielsen. I write our livebearer and community-tank guides and I've spent more hours than I'd admit photographing swordtails for this site — which means I've watched a lot of them behave, not just read about them. The green swordtail, Xiphophorus hellerii, is the fish I'd put near the top of any "second tank" list: more presence than a guppy, hardier than most tetras, and available in a genuinely silly number of colour strains.
This is the answer we'd give a customer leaning on the counter asking "are swordtails actually easy?" — yes, with a lid, a sensible group, and hard water you almost certainly already have out of the tap. Here's the green swordtail, properly.
A Marigold swordtail (Xiphophorus hellerii) from our stock — the long lower-tail extension is the male's "sword". Photo: Tropical Fish Co warehouse.
Fun facts — the stuff most UK guides never mention
This is where swordtails get genuinely interesting, because the sword isn't just decoration — it's one of the most-studied ornaments in animal behaviour.
- The female preference for swords is older than the sword itself. In a landmark experiment, biologist Alexandra Basolo gave female Xiphophorus a choice between males with and without artificial swords — and even females of a swordless relative preferred the sworded look. The preference existed before the sword evolved, a "pre-existing bias" that may explain why the sword appeared at all [2].
- Longer swords win, measurably. Across five separate choice tests, females consistently picked the male with the longer sword, with the difference in sword length ranging from 6 to 24 mm [3]. The sword is a real advertising signal, not a random flourish.
- Late-blooming males are normal, not a "sex change". Some genetically male swordtails stay female-looking for months, then develop a sword and gonopodium. They were always male — it's delayed maturation, and it's why you can buy "all females" and find a male weeks later [4].
- They're escape artists by design. Wild swordtails live in flowing streams and are powerful jumpers; a startled fish clears the surface instantly. Seriously Fish flags the need to cover the tank — these are not a fish you keep in an open-top aquarium [4].
- The "green" you rarely see is the wild one. Every red, black, koi and marigold strain on sale is a tank-bred line of one species, X. hellerii. The original wild fish is a modest olive-green with a faint lateral stripe — the colour the Latin name actually refers to [1].
Red, black, tuxedo, koi, marigold, pineapple, lyretail — they are all Xiphophorus hellerii, the same fish Heckel described in 1848. Colour strains are selectively bred lines, not separate species, which is why they interbreed freely and why a mixed tank drifts back toward muddy wild-type over a few generations [1].
Swordtail vs platy vs molly — which livebearer fits your tank?
Swordtails rarely get bought in isolation — they're chosen against the other two big community livebearers. Here's how they actually compare, using the parameters we keep ours at:
| Attribute | Green Swordtail | Platy | Molly |
|---|---|---|---|
| Scientific name | Xiphophorus hellerii | Xiphophorus maculatus | Poecilia spp. |
| Adult size | 12–16 cm (males swordy) | 4–6 cm | 6–12 cm |
| Male sword | Yes | No | No |
| Min tank | 90 L | 54 L | 75 L |
| Temperament | Active, males pushy | Peaceful | Mostly peaceful |
| Hard-water lover | Yes | Yes | Yes (some need salt) |
| Jumper | Strong — lid essential | Mild | Moderate |
| Best for | Bigger community / display | Small first community | Planted hard-water tanks |
If you want the most movement and the showiest males, swordtails win — but they need the most space and the tightest lid. Want the same easy care in a smaller tank? A platy is the swordtail's smaller, calmer cousin. For comparison with the third common livebearer, the molly care guide covers the salt question.
How many to buy, the ratio, and the lid you can't skip
The single most common swordtail mistake is buying a pair. Two swordtails — one male, one female — looks tidy on paper and becomes a relentless chase in a tank. Males pursue females constantly, and with nowhere for her to escape, the female is stressed thin.
Buy a group, skewed female: one male to two or three females. The males' attention spreads out, the chasing diffuses, and the tank settles. Practical Fishkeeping makes the same point — the males "can be real pests if the group isn't large enough to spread out any aggression" [5]. An all-female group is calm and just as colourful, because the colour lives in the strain, not the sex.
On space: 90 litres is our sensible floor for a starter group, and a long tank beats a tall one — Seriously Fish suggests roughly a 120 × 30 cm footprint for a proper group of these fast mid-water swimmers [4].
Swordtails are the jumpiest common livebearer we stock. A startled fish will go straight up and out through any gap — around the filter pipe, through a feeding hole, off the back of an open tank. Fit a tight cover glass or a proper hood before the fish go in. This single step prevents the most common swordtail loss there is.
Tank mates — who shares a swordtail tank
The rule with swordtails is simple: same water, same pace. They want hard, alkaline water and they move fast, so the best companions are other hard-water community fish that won't be outpaced or out-eaten.
- Platies — the natural first choice. Same genus, same water, same easy temperament; they fill the lower- mid levels while swordtails patrol the open water.
- Mollies — same hard-water needs and a similar size, so nobody bullies anyone. Check the molly guide for the occasional salt question, but they share a tank with swordtails happily.
- Guppies — fine with female swordtails and short-finned guppy strains, but watch fancy long-finned males: a swordtail will out-compete them at feeding and may nip those trailing tails.
- Corydoras and peaceful danios — corys work the substrate, danios the surface, and neither competes with a swordtail for the same water. A solid spread across all levels of the tank.
Avoid the obvious mismatches: large or aggressive cichlids, fin-nippers like tiger barbs, slow long-finned fish (fancy guppies, male bettas), and anything small enough to become a snack. For the broader hard-water shortlist, the livebearers hub is the place to browse.
Watch — a swordtail community in motion
Reading about how fast swordtails cruise is one thing; seeing it makes the "give them a long tank" advice click into place.
The classic Red swordtail (Xiphophorus hellerii) — the strain most people picture when they hear "swordtail". Photo: Tropical Fish Co warehouse.
When your swordtails arrive — our UK delivery protocol
Swordtails ship well — they're robust, hard-water fish — but two species-specific things matter on arrival: they're sensitive to a sharp water-chemistry change, and they'll bolt the moment the bag opens [7].
- Open in a dim, quiet room. Low light keeps them calm and reduces the panic-jump reflex when the bag comes open.
- Lid off the tank only at the last second — and back on the instant the fish are in. Have the cover glass within reach before you start.
- Float the sealed bag 15 minutes to equalise temperature.
- Drip-acclimate ~30 minutes at one to two drops a second. Swordtails don't need the 45-minute deep drip a soft-water tetra needs, but they do dislike an abrupt swing in hardness, so don't rush it.
- Net them in — don't pour the bag water into your tank.
- Lights off for an hour, and no feeding for 12 hours. Resume normal feeding the next morning, lightly.
Swordtails evolved in hard, mineral-rich Central American streams, so most British tap water suits them with nothing more than a dechlorinator. Thames Water classes all of its supply as hard [6], and the same is true across much of England — the very thing soft-water keepers fight is exactly what a swordtail wants. No remineralising, no buffering, no fuss.
Live arrival guarantee: if a fish arrives in poor condition, photograph the unopened bag within two hours and we'll refund or replace at our cost.
What to look for when you buy a swordtail
Healthy swordtails advertise their condition before you commit. Whether you're buying from us or anywhere else, this is the 30-second check that separates a fish that'll thrive from one that's already struggling:
- A straight, strong sword (on males). A male's sword should be straight and intact, not bent, split or stubby. A deformed sword often points to poor early water quality or inbreeding.
- A straight spine and full body. Hollow-bellied or curve-spined swordtails are a sign of stunting in "old" water — the exact condition Practical Fishkeeping warns shortens their lives [5]. Walk away.
- Active mid-water swimming. Healthy swordtails cruise the open water confidently. A fish hanging at the surface gasping, or sitting on the bottom, is stressed or sick.
- Clean, upright fins — no clamping. Clamped fins held tight to the body signal stress or the early stage of disease.
- No white spots or dusty sheen. Check under bright light for ich (white salt grains) or velvet (a fine gold dust) before you buy.
If you're stocking a new tank, add the female group first and let them settle for a week before the male goes in. The females establish the space, and the single male arriving into an established group has far less of a chase reflex than a male and females dropped in together.
UK swordtail community — where strains get traded
One thing retail rarely tells you: the most interesting swordtail lines move through hobbyist hands, not shop tanks. Livebearer keepers are an active, generous bunch in the UK.
- Federation of British Aquatic Societies (FBAS) — the umbrella body for dozens of regional fishkeeping clubs, many with livebearer enthusiasts who line-breed and trade strains at meets [8].
- r/Aquariums and r/livebearers on Reddit — quick, UK-aware answers on water chemistry and stocking, with weekly stocking threads.
- Regional aquatic society auctions — where rare swordtail lines (true lyretails, clean koi patterns) change hands at cost rather than through specialist retail.
If you catch the livebearer bug, a local society is the cheapest route to better genetics — and to people who'll happily talk swordtails for an hour.
Ready for more?
If swordtails are your entry into livebearers, the natural next reads are the platy care guide — the smaller, calmer cousin that shares their water — and the molly care guide for the third member of the hard-water community trio.
Comparing them with the most popular livebearer of all? Our guppy buying guide covers sex ratios, breeding control and hard-water care for the fish swordtails are most often kept alongside.
Already know you want swordtails? The full in-stock range lives on the livebearers hub, and the wider tropical fish for sale page is the place to build the rest of the community around them.
