Assorted Swordtail (Xiphophorus hellerii)

Recognise the Assorted Swordtail and its adult needs

Assorted Swordtail (Xiphophorus hellerii) is a colourful livebearer in the family Poeciliidae, native from Veracruz in Mexico to north-western Honduras. It reaches a maximum recorded length of 14 cm and suits keepers who can provide a long swimming footprint, stable filtered water and carefully screened companions. “Assorted” describes a mixture of ornamental forms rather than one promised colour pattern.

FishBase records a maximum of 14 cm total length for Xiphophorus hellerii. Treat that as the decisive planning length rather than judging the aquarium by the smaller dispatch size. FishBase also records freshwater and brackish environments, rapidly flowing streams and rivers, warm springs, vegetated canals and ponds. Introduced populations occur elsewhere, so never release aquarium fish or water into the wild.

Choose it for the right kind of display

This fish needs a long swimming footprint, stable filtered water and companions chosen for similar conditions. It is generally peaceful, although larger swordtails may be boisterous and males can spend considerable time maintaining a hierarchy when confined. A quiet or cramped aquarium therefore needs a different stocking plan, even when its water values look suitable.

The familiar “sword” is an extension of the lower tail-fin lobe associated with adult males, but ornamental strains may carry other extended fins and females do not have the characteristic sword. Use an explicitly sexed choice whenever sex matters to the group plan; do not infer it from colour, dispatch size or gallery appearance.

Choose a dispatch size without confusing it with adult size

One purchase supplies one animal in the size chosen. The size choices are 3–4 cm, 4–5 cm, 5–6 cm and over 6 cm; check the live size selector when ordering. The size you select is the size supplied, not the adult size.

Compare the selected fish with the animals already in your aquarium, but continue to reserve room for a maximum recorded adult length of 14 cm. A larger dispatch size does not change the minimum footprint, water range or social needs. A smaller size is not a reason to place the fish temporarily in a cramped aquarium.

Sex is guaranteed only when the selected option explicitly states male or female. This Assorted Swordtail listing has no sex-guaranteed option.

The gallery images are representative, based on reference photographs of this species, so colours can look slightly different from the live animal. They illustrate the body form and aquarium presentation rather than promising a particular colour, fin shape or individual fish.

Check whether the aquarium suits this fish

For packing, receipt instructions and claim conditions, read the current delivery and Live Arrival Guarantee policy.

Make the space and water check before choosing this fish

The most useful go-or-no-go check combines adult size, aquarium footprint and water chemistry. The Seriously Fish species profile says an aquarium with base dimensions of 120 × 30 cm, or an equivalent footprint, should be the smallest considered. It equates that footprint to approximately 108 litres and gives an aquarium range of 16–28°C, pH 7.0–8.0 and 10–25 dGH.

Measure the aquarium itself rather than relying only on its advertised volume. A tall aquarium can hold a similar number of litres while giving less horizontal room than a 120 × 30 cm base. Keep open swimming room alongside planted or decorated cover, and allow the filter to move the water without turning the whole aquarium into a powerful current.

Group composition and existing tank mates are the final checks. Swordtails require companionship, while confined males may form persistent dominance hierarchies. Other fish should be peaceful, able to cope with a lively swordtail and suited to the same water. If those conditions cannot be met together, choose a different plan rather than forcing several compromises.

You can also compare related fish through our livebearers collection, but assess each animal by its own adult dimensions and care range.

Shape the aquarium around movement and cover

Begin with the 120 × 30 cm minimum base or an equivalent footprint and divide it into two practical functions: open room for movement and structured areas that provide cover. Décor is flexible, so the aquarium does not need one prescribed style. The important decision is to avoid filling every centimetre with hardscape or leaving the fish without any shelter.

Build a useful layout

• Open room: preserve a clear horizontal route across much of the aquarium.
• Planted margins: place live plants along selected edges or use suitable artificial plants and ornaments for shelter.
• Light diffusion: floating plants and driftwood roots or branches may soften the light and create a more natural feel.
• Substrate: aquarium gravel or sand is suitable; a darker substrate can make a planted display look more settled.
• Water movement: provide some movement without assuming the fish needs an exceptionally strong current.

A 120 × 30 cm base covers 3,600 cm². An “equivalent” aquarium should preserve comparable horizontal usefulness, not merely reach about 108 litres by being unusually tall. Check how much base area remains after wood, rocks, ornaments and dense planting are installed. If the central route becomes narrow, remove or reposition décor before adding fish.

Wild-form presentation can use water-worn rocks and small boulders to suggest flowing water, while ornamental forms can be kept in a heavily planted display. Neither choice changes the need for filtration, stable chemistry or open space. Choose smooth, securely placed décor that leaves room to maintain the substrate and inspect the fish clearly.

As the group grows or more fish are added, revisit the layout rather than assuming the original arrangement still works. Persistent crowding around one gap, repeated displacement from cover or difficulty crossing the aquarium indicates that the usable space is poorer than the litre figure suggests. Simplifying the hardscape can be more useful than adding another ornament.

Hold stable water values inside the aquarium range

Keep the aquarium at 22–27°C, pH 7.0–8.0 and 10–12 dGH. These routine values are the overlap of the Seriously Fish species profile and OATA’s platy-and-swordtail group guidance. Hold conditions steadily within these values rather than moving between their extremes.

OATA’s platy-and-swordtail sheet gives 22–27°C for swordtails, pH 7.0–8.0, general hardness 4–12 dH and carbonate hardness 3–10 dKH. Seriously Fish gives the wider aquarium boundaries of 16–28°C and 10–25 dGH for Xiphophorus hellerii; their shared routine ranges are 22–27°C and 10–12 dGH.

Measure your aquarium and tap water before trying to alter either. Our water chemistry guide explains what the readings mean, while the water matcher helps compare measured values with the stated ranges. Stability within suitable values is safer than repeatedly adding products to chase one precise number.

If pH, hardness or temperature sits outside the routine range, resolve the mismatch before introducing the fish. If ammonia or nitrite is detectable, postpone the introduction and restore biological filtration. A reading at the edge of a range also deserves closer observation because routine water changes, room temperature and stocking can move it further.

Mature and size the filter before fish arrive

A working biological filter is essential. The RSPCA explains that fish release toxic ammonia; established beneficial bacteria convert it first to nitrite, which is also toxic, and then to safer nitrate. A filter therefore needs time to establish and must remain running and maintained as part of the aquarium’s life-support system.

For an aquarium of approximately 108 litres, the species profile uses a turnover of four to five times the volume each hour and gives a filter rating of about 430–540 litres per hour. Treat the printed rating as an equipment-planning figure. Filter media, pipework and blockages influence actual movement, so inspect the aquarium rather than assuming the number on the box settles every decision.

Complete these checks before introduction

1. Set up the aquarium, filter, substrate, shelter and open swimming area.
2. Mature the biological filter before adding livestock.
3. Test ammonia and nitrite and proceed only when both read zero mg/l.
4. Confirm temperature, pH and hardness against the planned range.
5. Check that water moves around the aquarium without an unnecessarily forceful current.

If this is your first setup, follow our first tropical aquarium guide before choosing an introduction date. Adding several fish quickly increases waste and feeding at the same time. The bacterial population must increase to process that extra load, so build the aquarium population slowly.

Watch for blocked intakes, reduced output and debris accumulating where water barely moves. Correct the cause without stripping all established media at once. A mature filter is valuable because of its bacterial population, not simply because the casing or sponge looks visually clean.

Use weekly maintenance to prevent waste from accumulating

Complete a partial water change of 25% at least once a week. Use a siphon to remove solid waste from the gravel or sand, especially from sheltered areas where debris gathers. Treat replacement tap water with a conditioner that removes chlorine or chloramine before it enters the aquarium.

Test the water at least weekly. Ammonia and nitrite should remain at zero mg/l, while nitrate should stay no more than 20 mg/l above the normal tap-water level. Record the results so a gradual change is visible before it becomes a health problem. A result that differs sharply from the usual pattern deserves an immediate recheck and an inspection of feeding, stocking and filter flow.

Protect the biological filter

• Check the filter for blockages during routine maintenance.
• When cleaning is needed, rinse media lightly in water removed from the aquarium.
• Do not run it under the tap because any chlorine or chloramine present may kill the beneficial bacterial population established in the media.
• Restore normal flow after cleaning and check that the filter restarts correctly.

Use the fish as an additional warning system, not as a replacement for tests. Surface hanging, bottom sitting, erratic swimming, altered breathing or a sudden lack of feeding interest can accompany deteriorating conditions. Test first whenever behaviour changes, even if the scheduled weekly check was completed recently.

Increase attention after adding fish because both waste production and feeding rise. Do not respond to a poor result by adding more livestock. Restore zero ammonia and nitrite, review the biological load and resume additions only after stable readings return.

Feed a balanced omnivore diet without leaving waste

Wild stomach analyses describe Xiphophorus hellerii as a generalised omnivore that consumes aquatic and terrestrial invertebrates, organic detritus, algae and other plant material. In the aquarium, provide a varied diet rather than relying on one treat or one animal-based food.

Use a good-quality flake or granule with a high algae content as the routine food. Add small live or frozen foods such as Daphnia, small bloodworm, tubifex or Artemia for variety. Quality dried products, freeze-dried bloodworm and freeze-dried Artemia are also included in the species feeding guidance.

Set the portion by eating time

Feed once a day and offer only what the fish consume within a few minutes. Begin with a small amount, watch it being eaten and stop before food reaches the substrate uneaten. This method scales the portion to the fish present instead of relying on a fixed number of flakes or granules that may be wrong for the chosen dispatch size and group.

Overfeeding creates two linked problems: uneaten food breaks down into waste, and the filter must process more pollution. Extra food is therefore not a safe response to poor appetite. Check ammonia, nitrite, temperature and behaviour first. When all fish eat promptly and no food remains, keep the routine consistent rather than increasing the amount for faster growth.

Introduce fish only after the aquarium passes its checks

Prepare the aquarium before the fish arrive. Confirm that the footprint, cover, open room and group plan are suitable, then test the water. Ammonia and nitrite must both read zero mg/l, and temperature, pH and hardness must sit within the planned ranges.

Increase the aquarium population slowly. Every added fish raises waste production, and feeding adds another load for the biological filter. A filter matured for a lighter population needs its bacterial population to grow as stocking increases. Adding too many fish at once can cause new tank syndrome, with unhealthy ammonia or nitrite levels.

Use observation as part of the introduction

1. Before adding: record ammonia, nitrite, nitrate, temperature, pH and hardness.
2. After adding: confirm steady swimming, normal breathing and intact fins and scales.
3. At feeding: offer the normal small ration and note whether each fish shows interest.
4. During the first week: watch for constant chasing, hiding, surface hanging, bottom sitting or erratic movement.
5. Before another addition: repeat the water tests and proceed only after stable zero ammonia and nitrite readings.

Do not use a single good reading as permission to add the rest of a planned community immediately. The aquarium must process the extra waste over time. Keep the first introduction modest, maintain the 25% water change at least once a week and use repeated results to judge stability.

Group behaviour matters alongside chemistry. Active exploration and feeding are useful signs, while one fish being continually displaced or pursued calls for a review of space, cover and group composition. Rearranging the stocking plan may be more effective than adding décor to an aquarium that is already crowded.

Respond to changes in health or behaviour early

Learn the normal appearance and behaviour of each fish so changes stand out. Healthy fish should have clear, bright eyes, undamaged fins, intact scales and an appropriate swimming and feeding pattern. Compare each animal with its own recent behaviour rather than expecting every ornamental colour form to look identical.

Pair each warning sign with an immediate check

Also inspect for lost fins or scales, ulcerations, bumps and changes in body condition. Several fish changing at the same time makes water quality and shared environmental conditions an urgent first check. One fish changing while others remain normal still deserves prompt attention.

Record the readings, the signs observed and when they began. Contact us for health guidance after testing the water, particularly when a visible lesion, altered breathing or persistent refusal to feed is present. Do not choose a treatment solely from one symptom because different problems can produce similar behaviour.

Continue routine maintenance while investigating, but avoid cleaning all established filter media or making multiple uncontrolled changes together. A clear record of one action and its result makes it easier to judge whether the aquarium is returning to its normal pattern.

Plan the group before choosing the number of fish

OATA recommends keeping at least three platies or swordtails together because they require companionship. Its suggested mixed-sex structure is two females to each male, with larger groups added in the same ratio to reduce pressure on females and other inhabitants. Base that ratio only on choices explicitly labelled by sex; colour, gallery appearance and dispatch size are not reliable ways to determine sex.

Do not assume that ordering three assorted fish creates the recommended ratio. When the size choice does not identify sex, plan enough aquarium space and cover to observe and manage the resulting group dynamics. If a precise mixed-sex ratio is essential, choose an offer that explicitly labels the relevant sex.

Account for male hierarchy

In confined spaces, groups of males may form dominance hierarchies and spend substantial time maintaining their positions. Occasional movement or display is different from one fish being chased without relief. Persistent pursuit, repeated displacement from feeding or cover, damaged fins or constant hiding means the social arrangement needs attention.

Swordtails are livebearers and give birth to live young. Females may store viable sperm for months, so separating sexes later does not guarantee that no young will follow. Plan the aquarium’s capacity before deliberately maintaining a mixed-sex group, and do not treat the initial number of adults as the only future stocking load.

More fish require more volume, filtration capacity and maintenance. The minimum 120 × 30 cm footprint is a starting point for the species, not permission to add an unlimited group. Let adult size, behaviour and repeated water tests set the final limit.

Screen every tank mate for water, space and temperament

Swordtails are generally peaceful, but large individuals can be boisterous and may chase more timid tank mates. Confined males can also devote considerable time to hierarchy. Compatibility therefore depends on the whole aquarium rather than a universal list of species names.

Use four checks for every proposed companion

Passing one check does not cancel a failure in another. Two species may share pH yet differ sharply in activity or space needs. A large aquarium may still be unsuitable if one group continually displaces the other. Likewise, peaceful behaviour does not solve incompatible hardness or temperature requirements.

Observe the aquarium at ordinary swimming times and while feeding. A workable community lets each fish move, shelter and eat without persistent pursuit. Repeated hiding, missed food, damaged fins or a sudden change in temperament requires a review of the combination as well as a water test.

Honest uncertainty remains until the intended numbers, adult sizes, sex composition, aquarium dimensions and measured water are considered together. Request a stocking and compatibility assessment with those details if the combination is unclear. Include the aquarium’s base dimensions and volume, current inhabitants with numbers and sizes, filter, temperature, pH, hardness, ammonia, nitrite and nitrate.

Do not use the appearance of two fish together in a gallery scene as a compatibility decision. The social plan comes from the actual group, aquarium and water, while one purchase on this offer supplies one animal.

Check the answers to common Assorted Swordtail questions

How big does an Assorted Swordtail get?

FishBase records a maximum adult length of 14 cm total length, so plan its aquarium and companions around that size rather than the smaller dispatch size.

What size aquarium does Xiphophorus hellerii need?

Use a base of at least 120 × 30 cm or an equivalent footprint, approximately 108 litres, with filtration, open swimming room and planted or decorated cover.

How many Assorted Swordtails should I keep together?

OATA recommends at least three swordtails and suggests two females per male for a mixed-sex group; use only explicitly sexed choices when that ratio is essential.

Does this fish suit beginners?

OATA describes swordtails as generally easy to keep, but the Assorted Swordtail still needs a mature filtered aquarium, a 120 × 30 cm base, stable water and a planned social group.

What does Xiphophorus hellerii eat?

Feed good-quality flake or granules with a high algae content once a day, offering only what it eats within a few minutes and adding small live or frozen Daphnia, bloodworm, tubifex or Artemia for variety.

What temperature and pH does this fish need?

Keep it at 22–27°C, pH 7.0–8.0 and 10–12 dGH; these values are the overlap of Seriously Fish guidance for the species and OATA guidance for swordtails.

Is it aggressive or likely to nip fins?

It is generally peaceful, but males are aggressive towards one another and confined males may maintain dominance hierarchies; large swordtails can also be boisterous and chase timid tank mates.

Which animals can live with it?

Choose peaceful fish that share 22–27°C, pH 7.0–8.0 and 10–12 dGH, fit the adult stocking plan and will not be overwhelmed by lively or occasionally boisterous behaviour.

How does it arrive alive in the UK?

Film every delivery continuously from the sealed parcel as it is opened, keeping the fish visible inside the sealed bag; someone must be present to receive it. Float the bag for 20–30 minutes before releasing the fish, and never pour the bag water into the aquarium.

References for identity and care planning

• FishBase: accepted identity, family, distribution, environmental range and maximum recorded total length.
• Seriously Fish: species aquarium footprint, approximate volume, water range, décor, feeding, behaviour and reproductive biology.
• Ornamental Aquatic Trade Association: platy and swordtail group guidance for water quality, social planning, feeding, introduction and maintenance.
• RSPCA: general aquarium welfare guidance on biological filtration and nitrogenous waste.