Neon Swordtail (Xiphophorus hellerii)

Choose the Neon Swordtail, an iridescent form of a classic livebearer

The Neon Swordtail is an aquarium-bred colour form of the swordtail, Xiphophorus hellerii, one of the best-known livebearing fish in the hobby. "Neon" is a shop name for its shine rather than a scientific term: our photographs show a fish with a silvery head, a warm orange body and a broad band of turquoise-blue scales running from the gill cover to the tail, split by a thin red line. The dorsal fin is orange with red streaks and spots, and the male's lower tail extends into a long yellow sword edged in black.

Under the colour it is an ordinary swordtail, which is good news for a UK fishkeeper. Swordtails are hardy, active and easy to feed, they suit the moderately hard to hard tap water found across much of the country, and they give birth to live young. Seriously Fish notes that selective breeding has produced many ornamental forms with colours and fins never seen in wild fish, and that some strains also carry genes from the platies X. maculatus and X. variatus.

Check the names on the label

Tell it apart from our other swordtails

The Green Swordtail we sell is the same species in its natural colours: an olive body with a red stripe. The Neon Swordtail replaces most of that olive with orange and carries far stronger blue iridescence. Other colour lines include the Red Simpson Swordtail, with its tall dorsal fin, and the patterned Koi Kohaku Swordtail. All share the same care, and all interbreed freely, so keep one line per tank if you want the colours to stay true.

Know exactly what you receive

One purchase supplies one live Neon Swordtail of medium size, a young fish that is still growing. Sex is not chosen or guaranteed. Our photographs show a male; females are larger and deeper-bodied, have a normal fan-shaped anal fin and never grow a sword. Colour and shine vary from fish to fish and are usually paler for a few days after a journey.

Decide how many to order

• Three fish: the smallest group OATA recommends, ideally one male and two females.
• Five or six fish: a lively display with two females to each male, as OATA advises.
• One male only: the calmest choice if you do not want fry, but you will need to ask us about sexed fish, as this listing does not guarantee sex.

Read our delivery and Live Arrival Guarantee information before you order, and have the aquarium running, heated and matured before the fish arrive.

Plan the aquarium around the adult size

Swordtails grow bigger than many community fish. OATA gives an adult size of 8–12 cm for swordtails. Seriously Fish gives the species a maximum of 140 mm standard length, measured without the tail, and FishBase records 14 cm for males and 16 cm for females. Males are the smaller sex, but the sword adds several centimetres to their overall length.

OATA asks for at least 80 litres for swordtails, and FishBase gives a minimum aquarium length of 80 cm. Seriously Fish recommends a base of 120 × 30 cm, about 108 litres. We suggest the larger figure when you keep more than one male, because extra length lets a lower-ranking male stay out of the top male's way.

Run a three-step space check

1. Measure the length: swordtails are fast swimmers that use the full length of the tank.
2. Count the adults: plan for fish of 8–12 cm, not the medium-sized youngsters you receive.
3. Allow for fry: a mixed-sex group breeds readily, so check the numbers in our stocking calculator before the population grows.

Arrange open water with planted edges

Seriously Fish describes wild swordtail habitats from sea level to around 1,500 m altitude, mostly less than 1.5 m deep and often without aquatic plants, with adults in faster current and juveniles in quiet margins. FishBase, by contrast, says the species prefers heavily vegetated habitats. In the aquarium both points are useful: give the fish an open, well-oxygenated swimming lane, and plant the sides densely.

Divide the tank by function

A cover glass or lid is sensible. OATA calls a secure lid optional but advisable for platies and swordtails, and fast, active fish are better kept safely inside the tank.

Set hard, alkaline water

Swordtails come from hard-water rivers and do best in moderately hard to hard, alkaline water. Our recommendation uses the values where the three sources agree, and states clearly where they do not.

Read the sources side by side

Many UK households, especially in the south and east of England, have hard tap water that suits swordtails with no treatment beyond a dechlorinator. If your water is soft, do not keep swordtails in it as it is; talk to us about raising hardness gradually. Compare your readings in our water matcher and read the water chemistry guide to understand what each test means.

Filter and maintain a busy livebearer tank

Seriously Fish recommends a filter turning the water over four to five times an hour, which for a 108-litre tank means roughly 430–540 litres per hour, and a heater of about 75–125 W in a room near 20°C. Swordtails eat well and multiply quickly, so the filter must keep up with a growing population.

Follow a weekly routine

1. Test ammonia, nitrite, nitrate and pH before maintenance; OATA advises testing at least once a week.
2. Change 25% of the water at least weekly, as OATA recommends, with dechlorinated water at the same temperature.
3. Siphon waste and uneaten food from the gravel.
4. Rinse filter sponges in removed tank water, never under the tap.
5. Count the fish and look for new fry so the stocking level does not creep up unnoticed.

Feed algae-rich food once a day

Seriously Fish describes the swordtail as a generalised omnivore that eats invertebrates, detritus, algae and plant material in the wild and accepts most foods in the aquarium. OATA recommends a quality flake or granule with a high algae content as the staple, fed once a day and only as much as the fish eat within a few minutes.

Build one measured meal

• Staple: algae-rich flake or small granules, every day.
• Variety: small live or frozen foods such as Daphnia, brine shrimp (Artemia) and bloodworm a few times a week, in place of part of the staple, not on top of it.
• Portion: what the group clears in a few minutes; remove leftovers.
• Fry: powdered flake or newly hatched brine shrimp in small, frequent feeds.

Plant-based food matters for livebearers. A diet of meaty food alone, offered in large portions, pollutes the water and does not match the mixed diet the species eats in the wild.

Introduce the fish to a mature aquarium

Before the fish arrive, confirm that the filter is mature, ammonia and nitrite are zero and the water is 22–27°C at pH 7.0–8.0. Follow OATA's floating-bag method: float the sealed bag for 10 minutes, then add small amounts of tank water over up to 20 minutes, and release the fish without pouring the bag water into your aquarium. Our guide on how to acclimatise new fish explains each step.

Use a first-week checkpoint plan

Keep the right group and tank mates

Swordtails are generally peaceful, but OATA notes that large swordtails can be boisterous and may chase timid tank mates. Seriously Fish describes groups of males forming dominance hierarchies in confined spaces and spending a lot of time maintaining their rank, and FishBase notes that males are aggressive towards each other. OATA's answer is simple: keep at least three, with two females to each male.

Screen each companion

Run any planned mix through our tank mates tool, browse more livebearers, the swordtail category and our Xiphophorus hellerii swordtails, or look through our community tank fish. Our green swordtail guide covers the species in more depth, and swordtails also feature in our list of the best beginner tropical fish.

Breed swordtails or keep the numbers steady

Swordtails are livebearers. The male fertilises the female internally with his gonopodium, the modified anal fin, and Seriously Fish notes that females can store viable sperm for months, so a female may produce several broods after a single mating. FishBase gives a gestation of 24–30 days and 20–200 young per brood, with maturity at 8–12 months. OATA gives 24–30 days and 20–80 fry.

Raise a brood

1. Spot the signs: a heavily pregnant female becomes very round and darker behind the belly.
2. Protect the fry: OATA suggests moving the female into a breeding net; dense floating plants also give fry cover, because adults eat them.
3. Feed little and often: rear the fry on specialist fry food or powdered flake.
4. Plan homes: every brood adds fish, so decide in advance where surplus youngsters will go.

FishBase also records that swordtails tend to undergo sex reversal from female to male under certain conditions. An older female that develops a gonopodium and a short sword is a known event in this species, not a sign of illness.

Watch for problems early

OATA lists the warning signs for platies and swordtails: clamped fins, flicking against objects, shimmying, hanging at the surface or sitting on the bottom, colour change, white spots or fluffy growths, lost fins or scales, a changed breathing rate and reduced appetite.

Pair each sign with an action

Never release unwanted swordtails or their fry. Seriously Fish lists feral swordtail populations in 34 countries or territories, and FishBase notes that several countries report adverse ecological effects after introduction.

Answer Neon Swordtail buying questions

What is a Neon Swordtail?

It is an aquarium-bred colour line of the swordtail, Xiphophorus hellerii, a livebearer from Mexico and northern Central America. "Neon" is a trade name for its strong iridescence: our photographs show an orange body with a band of shining turquoise-blue scales and a thin red line along the flank, a red-marked dorsal fin and, in males, a long yellow sword edged in black.

Is it a different species from the Green Swordtail?

No. The Neon Swordtail and the Green Swordtail are the same species, Xiphophorus hellerii. The Green Swordtail is the olive, wild-type colour; the Neon Swordtail is a selectively bred colour form with the same care needs.

How big does a Neon Swordtail grow?

OATA gives adult swordtails as 8–12 cm. Seriously Fish gives the species a maximum of 140 mm standard length, and FishBase records 14 cm for males and 16 cm for females. Males are the smaller sex, and the sword adds to their overall length.

What size aquarium does it need?

OATA asks for at least 80 litres for swordtails, and FishBase gives a minimum aquarium length of 80 cm. Seriously Fish recommends a base of 120 × 30 cm (about 108 litres), which we suggest for a group with more than one male.

What water does it need?

Hard, alkaline water: 22–27°C and pH 7.0–8.0, where OATA, FishBase and Seriously Fish all agree. For hardness the sources differ (OATA 4–12°dH, FishBase 9–19, Seriously Fish 10–25 dGH); moderately hard water of about 10–12 dGH satisfies all three, and harder water suits it well.

How many should I keep, and in what ratio?

Keep at least three, with two females to every male, as OATA advises. Males form a pecking order and can chase each other, so one male with several females is the calmest start.

What does it eat?

Swordtails are omnivores. Feed a quality flake or granule with a high algae content once a day, only as much as is eaten in a few minutes, and add small live or frozen foods such as Daphnia, brine shrimp or bloodworm.

Will Neon Swordtails breed in my aquarium?

Very likely, if you keep both sexes. Females give birth to live young after 24–30 days; FishBase records 20–200 young per brood. Dense plants or a breeding net help fry survive, because adults eat them.

How many fish come with one purchase?

One live fish of medium size. Sex is not chosen or guaranteed; our photographs show a male, and females have no sword.

Consult the care references

• Seriously Fish: Xiphophorus hellerii — size, aquarium base, water values, natural habitat, diet, male hierarchy, sexing, sperm storage and ornamental strains of the swordtail.
• FishBase: Xiphophorus hellerii — maximum recorded lengths, environment values, distribution, minimum aquarium length, brood size, gestation, maturity and sex reversal.
• OATA: How to look after swordtails & platies — UK trade-body guidance for platies and swordtails: adult sizes, tank volumes, water values, sex ratio, feeding, breeding, acclimatisation and health signs.
• Tropical Fish Co delivery policy — current dispatch days and Live Arrival Guarantee terms.