Description
The swordtail is defined by its sword: an extension of the lower caudal fin rays that only males develop, and which in an adult can be almost as long as its own body. It is not merely an ornament. It is one of the best-studied cases of sexual selection in freshwater fish: females consistently choose males with longer swords, and experiments suggest that preference already existed in the genus before the sword itself evolved.
You will also see it called swordtail or green swordtail. Do not confuse it with the swordfish proper, Xiphias gladius, a marine fish weighing hundreds of kilos with no relation to this twelve-centimetre livebearer.
It is vigorous and among the longest-lived in its family, but there are two things the shop rarely tells you and you should know before taking one home: it jumps, and it hybridises with the platy.
Behavior
Males compete with each other constantly. They never come to serious aggression, but they chase, display sideways with the sword spread and establish a hierarchy that keeps shifting. With females the harassment is continuous, and that is where the decision that most affects their welfare lies:
- One male per two or three females. That ratio spreads the pressure.
- A lone pair is the worst combination: the female ends up exhausted.
- Males only works in long tanks with hiding places, but you need real space.
And a warning usually learned the hard way: it is an excellent jumper. Without a lid, or with filter gaps and cable holes left open, sooner or later it turns up on the floor. It is probably the commonest mistake with this species, and it has nothing to do with water parameters.
It lives well with corydoras, otocinclus, rasboras, cherry barb, molly and nerite snails: they all tolerate the hard, alkaline water it needs, which is the real key to compatibility rather than temperament.
Avoid housing it with platy, because they hybridise readily; adult angelfish and large cichlids, which eat the fry and sometimes small adults; betta, because the swordtail is fast and restless and ends up nipping its fins; and soft, acidic water tetras such as cardinal or rummy-nose, which fail not on temperament but because the two will never both be comfortable in the same water.
With shrimp: adult Neocaridina coexist without trouble, but newborn shrimplets get eaten. If you want a breeding shrimp colony, this is not your fish.
Habitat
Xiphophorus hellerii inhabits fast-flowing rivers and streams on the Atlantic slope of Central America, from Veracruz in Mexico to northern Honduras. It occurs from sea level up to around 1,500 metres, and that range explains why it tolerates a wider temperature span than you would expect of a tropical fish.
The water is hard, alkaline and well oxygenated, with dense bankside vegetation and beds of gravel and sand. This is not a backwater fish: it lives where the water moves.
That has a practical consequence often overlooked: it appreciates more flow than most community fish. A tank with generous filtration and some surface movement suits it far better than the still water set up for bettas or gouramis.
It has become naturalised in dozens of countries outside its native range, from the United States to Australia, where it competes with local wildlife. Never release surplus fish into a natural watercourse: if you end up with too many fry, pass them to another hobbyist or a shop.
Habitats
Fish sharing this habitat
Estos peces aparecen en uno o mas habitats asociados a esta ficha. No implica compatibilidad directa en acuario.
Details
🌊 Water parameters
🐠 Minimum tank size
Breeding
A livebearer. The female gives birth to fully formed fry able to swim from the first second, in broods of twenty to eighty every four to six weeks. And there is a detail that surprises many: she can store sperm for months, so a female bought pregnant will keep producing broods with no male in the tank at all.
The fry are independent, but the adults hunt them. With dense-rooted floating plants or a good clump of moss a fair number survive on their own; if you want to breed seriously, you will have to separate them.
On sex change, you will often read that females turn into males. The reality is more nuanced: usually these are late-maturing males that spend months looking like females and then develop a gonopodium and sword. Functional sex change in females that have already bred is documented, but rare.
Important warning: the swordtail crosses with the platy (Xiphophorus maculatus) with great ease, and the hybrids are fertile. Keep both species together and within a few generations you will have neither pure swordtails nor pure platies. That is the main reason not to mix them, however compatible they look in every other respect.
Diet
An omnivore with a stronger plant component than is usually assumed. In its home rivers it spends much of the day rasping aufwuchs — the film of algae, microorganisms and detritus coating stones and wood — and it does not lose that habit in the aquarium.
- Staple: quality flake or granules with plant content.
- Plant matter: spirulina, nori or blanched courgette, two or three times a week.
- Protein supplement: frozen daphnia, brine shrimp or mosquito larvae, a couple of times a week.
- Fry: the same food crushed, or brine shrimp nauplii.
The commonest mistake is treating it as a strictly carnivorous fish and feeding only protein flake: it ends up with digestive problems and dull colour. It eats a fair amount for its size, so split the ration into two feeds rather than one.
What about your tank?
Enter your tank's values to see only the compatible species that fit them. Litres apply to fish.
Compatible fish
General guidance based on water parameters, behaviour and known exceptions; always review the individual animals' and aquarium's specific needs.
They share water: pH 7–7.5 and 22–27 °C.
They share water: pH 7–7.5 and 24–28 °C.
They share water: pH 7 and 22–28 °C. They only meet at the very edge, so the tank has to hold that value.
They share water: pH 7–8 and 22–26 °C.
They share water: pH 7–7.5 and 22–25 °C.
They share water: pH 7–7.5 and 22–24 °C.
They share water: pH 7–8 and 22–28 °C.
They share water: pH 7–7.5 and 25–28 °C. They only meet at the very edge, so the tank has to hold that value.
They share water: pH 7–7.5 and 24–28 °C.
They share water: pH 7.5–8 and 24–28 °C.
They share water: pH 7.5–8 and 24–28 °C.
They share water: pH 7–7.2 and 22–26 °C.
They share water: pH 7–7.5 and 22–26 °C.
They share water: pH 7–8 and 22–27 °C.
They share water: pH 7 and 26–28 °C. They only meet at the very edge, so the tank has to hold that value.
They share water: pH 7 and 22–26 °C. They only meet at the very edge, so the tank has to hold that value.
They share water: pH 7–7.5 and 23–28 °C.
They share water: pH 7–7.5 and 23–27 °C.
Compatible invertebrates
General guidance based on water parameters, behaviour and known exceptions; always review the individual animals' and aquarium's specific needs.
They share water: pH 7–8 and 22–28 °C.
They share water: pH 7–8 and 22–25 °C.
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They share water: pH 7–7.5 and 22–27 °C.
Sources
- FishBaseFroese, R. and D. Pauly. Editors. 2026. FishBase. World Wide Web electronic publication.
- Seriously FishSeriously Fish. Xiphophorus hellerii. Ficha de especie; consultada 2026-08.
Plan your next aquarium
Is this fish right for you? Plan a suitable aquarium before setting it up.