Koi Kohaku Swordtail (Xiphophorus hellerii)

Decide whether this swordtail fits

The Koi Kohaku Swordtail (Xiphophorus hellerii) is an ornamental poeciliid livebearer. The wild species is native to North and Central America, from the Rio Nantla in Veracruz, Mexico, to northwestern Honduras, and it reaches 14 cm. It suits a keeper who can provide a mature, filtered aquarium with a long swimming area, stable neutral-to-alkaline water and companions chosen for similar conditions. OATA describes most platies and swordtails as undemanding and easy to keep, but that assessment still depends on completing the aquarium and water checks described below.

Keep it at 22–27°C, pH 7.0–8.0 and 10–12 dGH. These routine values are the overlap of the FishBase recorded environment, the Seriously Fish species profile and OATA’s swordtail guidance. OATA’s figures are group guidance, not a species-specific optimum. Seriously Fish recommends an aquarium base of at least 120 × 30 cm, or equivalent, with a volume of about 108 litres.

FishBase places the species mainly in rapidly flowing streams and rivers with heavy vegetation, as well as warm springs, weedy canals and ponds. Its recorded wild foods include worms, crustaceans, insects and plant matter. In the aquarium, translate that mixed diet into balanced dried food with suitable live or frozen variety rather than relying on one ingredient alone.

Choose the dispatch size

Use the live size selector to choose 4–5 cm, 6–7 cm or XL. One purchase supplies one fish from the selected choice. The size you select is the size supplied, not the adult size. XL is a named dispatch grade rather than a stated centimetre measurement, so plan every choice around the same 14 cm adult size and permanent aquarium requirements.

Sex is guaranteed only when the selected option explicitly states male or female. This Koi Kohaku Swordtail listing has no sex-guaranteed option. Do not infer sex from the dispatch length, colour pattern, photograph or visible fin shape when planning a group.

The gallery images are representative of this type of animal rather than photographs of the exact animal supplied, so colours can look slightly different from the live animal. Use the photographs to understand the general ornamental appearance, while making the buying decision from the adult size, water requirements, social plan and aquarium footprint.

Match the fish to your aquarium

Start with the adult aquarium rather than treating a smaller dispatch fish as a reason to use temporary accommodation. A long base, established biological filtration, open swimming space and planted cover address the main needs described for swordtails.

Complete a final fit check

• Measure the base: confirm at least 120 × 30 cm or an equivalent footprint rather than relying on the aquarium’s advertised volume alone.
• Test all three water values: temperature, pH and general hardness must each sit inside the routine range.
• Count the full group: include every existing animal when judging adult space and filtration capacity.
• Map the layout: identify where open swimming room, planted cover and sheltered areas will remain after all décor is installed.

A suitable result depends on all four checks working together. A long aquarium does not correct unsuitable hardness, and matching water does not create room for an overcrowded adult population. Complete each decision before using the dispatch size to choose the individual fish.

Most of the work is preparation: establish the filter, measure the water, reserve the full adult footprint and decide how a group will fit alongside existing fish. If this care pattern is right for your aquarium, you can also browse the livebearers collection without treating every livebearer as automatically compatible.

Plan the aquarium around the adult size

Plan for an adult length of 14 cm. FishBase records a maximum of 14 cm, and Seriously Fish also uses 14 cm in its species profile. OATA’s broad description places swordtails at 8–12 cm, but the species-level figure of 14 cm is the safer basis for permanent space, stocking and equipment decisions.

Use the larger aquarium baseline

Seriously Fish says an aquarium with a base of 120 × 30 cm, or equivalent, should be the smallest considered and associates that footprint with about 108 litres. FishBase notes a minimum aquarium length of 80 cm; use the longer 120 cm recommendation because it also addresses the species’ active swimming and adult body length.

Footprint matters as much as the litre figure. A taller aquarium can hold a similar volume while providing less horizontal travel, so compare the actual base dimensions rather than accepting volume alone. Reserve part of that base for open movement while keeping cover along the sides and rear.

Scale space with the whole population

The 120 × 30 cm and approximately 108-litre figures are the starting point for this species, not permission to fill every remaining area with fish. OATA advises that larger groups require more volume and that stocking too quickly can exceed the beneficial bacteria’s capacity. Count the adult size and waste load of every inhabitant before adding a group.

Work through the footprint

Measure the inside base before substrate and décor are arranged. Mark the planted margins, hardscape and open lane on a simple sketch, then check that the long swimming route remains clear. If roots, rocks or dense planting divide the aquarium into short sections, adjust the layout rather than counting the total glass length as fully usable swimming room.

Recheck the same plan against the intended population. Three swordtails plus existing fish create a different demand from one swordtail in an otherwise lightly stocked aquarium, even when both plans use the same 108-litre volume. Larger groups need more volume, and every addition increases the load handled by the filter.

Build the permanent arrangement before the fish arrives. This avoids moving it between increasingly larger aquariums and lets the biological filter, plants, shelter and swimming lane develop as one stable system.

Hold the water inside the overlap

Maintain 22–27°C, pH 7.0–8.0 and 10–12 dGH. The temperature is inside FishBase’s recorded 22–28°C environment, Seriously Fish’s 16–28°C aquarium range and OATA’s 22–27°C swordtail range. The pH range is shared by all three. The 10–12 dGH band overlaps FishBase’s 9–19 dH, Seriously Fish’s 10–25 dGH and OATA’s 4–12 dH.

Measure before choosing the fish

Test the aquarium rather than describing the water from its appearance or from local assumptions. Record temperature, pH and general hardness, then compare the results with the target band. Our water chemistry guide explains what the measurements mean, while the water matcher provides a direct comparison with the stated values.

A result inside one limit does not compensate for another result outside it. For example, 24°C and pH 7.5 fit, but a general hardness of 6 dGH does not fit the complete routine band. Likewise, 11 dGH and pH 7.4 do not make 29°C suitable. Check all three values together.

Keep conditions steady

Use the overlap as a routine operating range, not as a reason to move repeatedly between its lower and upper limits. Choose a point your aquarium can hold consistently, confirm it before introduction and recheck it during routine maintenance. A stable 24°C, pH 7.4 and 11 dGH is one worked example that sits inside every stated limit.

Water quality remains separate from temperature, pH and hardness. Ammonia and nitrite must be zero, and OATA says nitrate should not exceed 20 mg per litre above the normal tap-water level. Test those waste values through the maintenance routine rather than assuming clear water is safe.

Build cover around swimming space

Provide planted cover without filling the entire 120 × 30 cm base. Seriously Fish says décor is not especially critical but notes that the species looks best in a heavily planted aquarium with a dark substrate. OATA also recommends plants or suitable artificial alternatives for shelter and stresses the need for open swimming space, especially for swordtails.

Divide the layout into useful areas

• Sides and rear: use live plants to form cover while preserving sight breaks around the aquarium.
• Centre and front: retain a broad, unobstructed swimming lane.
• Surface: floating plants can diffuse incoming light without occupying the main base area.
• Hardscape: driftwood roots or branches can add shade and structure; a stream-style arrangement may use water-worn rocks and small boulders.
• Substrate: aquarium gravel or sand fits OATA’s setup guidance, while a darker substrate follows the Seriously Fish presentation advice.

If live plants are not used, OATA permits plastic plants and suitable ornaments such as model caves or plant pots to create shelter. Check that every addition leaves enough clear room for active movement and does not turn the recommended footprint into a maze of narrow gaps.

Use measured water movement

Filtration does not need to be particularly strong, although Seriously Fish says the species appears to appreciate some water movement. Position the outlet to create circulation while retaining calmer planted areas. Judge the result by whether the fish can swim normally throughout the aquarium rather than being forced to work continuously against the outlet.

Floating plants and branches can diffuse light, while planted margins give fish somewhere to move away from activity. Together, these features create a practical balance between cover and the long swimming space required by an adult swordtail.

Establish filtration and maintenance

Run a mature biological filter before introducing the fish. The RSPCA explains that biological filtration supports beneficial bacteria that convert toxic ammonia to nitrite and then to safer nitrate. Ammonia and nitrite must both remain at zero mg per litre.

If this is your first setup, follow our first tropical aquarium guide before adding livestock. OATA warns that increasing the population too quickly can leave too few nitrifying bacteria to handle the added waste, causing ammonia and nitrite to rise. Increase stocking slowly and test after the waste load changes.

Set the equipment for the adult aquarium

For an aquarium of about 108 litres, Seriously Fish uses a turnover of four to five times the volume per hour and suggests a filter rated at approximately 430–540 litres per hour. Its heater estimate is approximately 75–125 W for maintaining its wider temperature range in a room around 20°C. Select equipment against the actual aquarium volume and room conditions, then confirm the result with measurements.

Complete the weekly routine

• Test ammonia and nitrite at least once per week and keep both at zero.
• Check nitrate against OATA’s limit of no more than 20 mg per litre above the normal tap-water level.
• At least once every week, complete a partial water change of 25%.
• Treat replacement water with tap-water conditioner to remove chlorine or chloramine before adding it.
• Check the filter for blockages and loss of normal flow.
• If the filter needs cleaning, do not run it under the tap. Rinse it lightly in tank water removed during the partial water change.

Respond to changes promptly

Watch for surface hanging, bottom sitting, erratic swimming, altered breathing, unusual hiding or aggression, darker or paler colour, damaged fins or scales, white spots, fluffy growths, weight changes or reduced feeding. If you are concerned, test the water and contact us for further guidance.

Feed a balanced omnivorous diet

Feed a balanced omnivorous diet once a day, offering only what the fish can eat within a few minutes. Seriously Fish describes the species as a generalised omnivore whose wild diet includes aquatic and terrestrial invertebrates, organic detritus, algae and other plant material. FishBase also records worms, crustaceans, insects and plant matter.

Choose the staple

Use a good-quality dried product as the regular food. OATA recommends quality flake or granules with a high algae content for platies and swordtails, while Seriously Fish recommends balancing quality dried products with small live and frozen foods. Choose a suitably sized flake or granule that provides a mixed nutritional base rather than feeding only one frozen or live food.

Add suitable variety

• Small Daphnia
• Artemia
• Small bloodworm or chironomid larvae
• Tubifex, where offered as an appropriately sized live or frozen item
• Freeze-dried bloodworm or Artemia as part of the wider rotation

Match each item to the fish’s mouth and make the full daily portion disappear within a few minutes. An enthusiastic feeding response is not a reason to add more after the planned portion has been eaten.

Check the result, not just the appetite

• Food remains after a few minutes: remove the excess and reduce the next portion.
• One fish misses the meal: observe whether chasing or the feeding position is preventing normal access.
• Interest in food falls: test water quality and compare swimming, breathing and body condition with the fish’s normal behaviour.
• Waste values rise: review the amount offered, uneaten food and the recent increase in stocking or feeding.

Keep the schedule at once daily while varying the food across the rotation. Variety means changing suitable ingredients over successive feeds, not combining every item into an oversized single portion. The few-minutes limit still applies when live, frozen or freeze-dried foods are used.

Use water quality to check the portion

Overfeeding leaves uneaten food that breaks down and releases waste into the aquarium. If food remains after a few minutes, reduce the next portion rather than extending the feeding period. Continue the weekly ammonia, nitrite and nitrate checks so a gradual rise in waste is noticed before it becomes a larger problem.

Body condition and interest in food are useful daily observations. Reduced intake or a noticeable increase or decrease in body condition belongs with the other health-change checks: test the water first and review whether the portion, food size and aquarium conditions remain appropriate.

Plan the group before purchase

OATA suggests keeping at least three platies or swordtails together. Where the sexes are known, its suggested mixed-sex ratio is two females to each male to reduce stress on females and other aquarium inhabitants. More can be kept in that ratio when the aquarium volume, filtration and total population can support them.

Do not build the plan from appearance

Only count a fish as a particular sex when the selected choice explicitly guarantees it. Dispatch size, colour and gallery details do not turn an unsexed purchase into a known male or female. If the intended group depends on a precise ratio, establish how each guaranteed choice will contribute before completing the plan.

Allow for male social behaviour

FishBase describes males as aggressive towards each other. Seriously Fish adds that groups of males in confined spaces tend to form dominance hierarchies and may spend a significant amount of time maintaining their positions. Provide the full adult footprint, planted sight breaks and open swimming space, then monitor how the actual group uses them.

Repeated chasing, one fish being displaced from normal swimming areas or a marked change in hiding should trigger a review of space and group composition. OATA also notes that large swordtails can be boisterous and may chase timid tankmates, so observe the group rather than relying only on the species’ generally peaceful description.

Review the group at clear checkpoints

• Before addition: confirm the adult footprint, group total and zero ammonia and nitrite.
• After introduction: check whether every fish can use open water, reach food and move into cover.
• During the first weekly test: compare waste values with the readings taken before the population increased.
• Before another addition: confirm that behaviour and filtration have remained stable rather than adding several fish in quick succession.

Do not treat a ratio as a substitute for space or observation. Even when known sexes match OATA’s suggested two-females-to-one-male arrangement, the aquarium must still support the adult group and any existing inhabitants. When sexes are not guaranteed, plan around an unknown composition and respond to the behaviour that develops.

Allow for live young in mixed-sex plans

Swordtails are livebearers and give birth to live young. Seriously Fish describes internal fertilisation through the male gonopodium and notes that females can store viable sperm for months. A mixed-sex arrangement can therefore affect future stocking pressure. Reserve filtration and space for the population you intend to maintain instead of treating unexpected young as having no effect on capacity.

Prepare for arrival and acclimatisation

Before the parcel arrives, confirm that the aquarium is mature, the filter is running normally and temperature, pH and hardness are inside the target band. Test ammonia and nitrite and proceed only when both are zero. OATA advises increasing the fish population slowly because the beneficial bacteria must increase as stocking and feeding rise.

Follow the arrival steps in order

1. When you receive the parcel, start filming while it is still sealed and the outer box is visible.
2. Open the parcel on camera in one continuous take, keeping the fish inside the sealed bag so its condition can be seen before anything is disturbed.
3. Open and inspect the parcel on the day it arrives.
4. Float the bag for 20–30 minutes before releasing the fish.
5. Release the fish without pouring the bag water into the aquarium.

Someone must be at the delivery address to receive the parcel because live fish cannot be left unattended. The fish is hand-bagged in pure oxygen and placed in an insulated poly box, with a heat pack when the weather calls for it. A licensed live-animal courier carries the parcel.

For a Live Arrival Guarantee claim, contact us the same day with the order number and the continuous unboxing video showing the sealed parcel, its opening and the fish still inside the sealed bag. Read the full delivery and Live Arrival Guarantee policy before the delivery date.

Check settling behaviour

Continue the established water-testing and maintenance routine after introduction. Watch swimming, breathing, colour, temperament, body condition, fins, scales and feeding. A change such as surface hanging, bottom sitting, erratic swimming, unusually fast or slow gill movement, increased hiding or reduced interest in food is a reason to test the water promptly.

Screen every companion

Choose peaceful fish that share 22–27°C, pH 7.0–8.0 and 10–12 dGH, fit the completed aquarium and can live alongside an active swordtail. OATA describes platies and swordtails as generally peaceful and compatible with other species, but it does not turn any named fish into a universal companion. Screen the exact animals and aquarium together.

Apply four checks

Do not select a companion only because both animals are described as peaceful. Water chemistry, adult space and actual behaviour remain separate decisions. A fish can share the same pH yet still be a poor fit for the available swimming room or activity level.

Invertebrate compatibility is not part of this plan. If an aquarium includes shrimp, snails or another kind of animal, ask us for a stocking and compatibility assessment using the exact species, numbers, adult sizes, water measurements and aquarium dimensions.

Manage chasing and social pressure

Treat male rivalry and boisterous chasing as the main documented social concerns. FishBase describes males as aggressive towards one another, while Seriously Fish says male groups in confined spaces form dominance hierarchies. OATA adds that large swordtails may chase more timid tankmates.

Watch the whole aquarium

Normal assessment means looking beyond the most visible fish. Check whether every animal can use open water, reach food and move into cover. Surface hanging, bottom sitting, erratic swimming, increased hiding, altered aggression or reduced feeding all belong on the observation list.

If one fish repeatedly drives another from the same area, review the adult stocking level, group composition and distribution of cover. Planted margins and driftwood can break sight lines, but they do not add litres or replace the required swimming footprint. Preserve the open lane while creating more than one sheltered area.

Separate brief activity from a repeated pattern

Record where chasing happens, which fish begins it and whether the pursued fish returns to normal swimming and feeding. A repeated pattern matters when the same animal is driven from food, open water or cover, or begins hiding more than normal. Use those observations to review the group rather than judging the aquarium from a single pass across the tank.

Check the layout without sacrificing swimming room. Moving a branch or strengthening a planted margin can alter a sight line, but filling the central lane with extra décor removes useful space. Reassess the population and aquarium footprint when structural changes do not stop repeated displacement.

Add fish slowly

OATA warns that rapid stocking can overload the biological filter as feeding and waste increase. Additions should therefore be assessed both socially and biologically: confirm zero ammonia and nitrite, introduce population changes slowly, and observe behaviour before making another change.

Use cautious wording when predicting an individual fish’s temperament. The species is generally peaceful, yet male aggression and boisterous chasing are specifically described. A successful community plan comes from matching water and space, providing structure, and responding to the behaviour actually seen.

Answer common buyer questions

How big does a Koi Kohaku Swordtail get?

It reaches 14 cm. Plan its permanent aquarium, filtration and stocking around that adult length rather than the dispatch size.

What size aquarium does Xiphophorus hellerii need?

Use a base of at least 120 × 30 cm or equivalent, associated with approximately 108 litres. Seriously Fish says this should be the smallest aquarium considered.

How many Koi Kohaku Swordtails should I keep together?

OATA’s swordtail group guidance suggests at least three, using two females to each male when the sexes are known. Larger groups require more aquarium volume.

Is the Koi Kohaku Swordtail suitable for beginners?

OATA describes most platies and swordtails as undemanding and easy to keep. It still needs a mature filter, zero ammonia and nitrite, weekly testing and stable water.

What does Xiphophorus hellerii eat?

Feed a balanced quality flake or granule once a day, in an amount eaten within a few minutes. Add small Daphnia, Artemia, bloodworm or other suitable live and frozen 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 routine figures are the overlap of FishBase, Seriously Fish and OATA’s swordtail ranges. Hold them steadily and test the aquarium regularly.

Is this swordtail aggressive or likely to nip fins?

It is generally peaceful, but FishBase describes males as aggressive towards each other. Male groups may form dominance hierarchies in confined spaces, and large swordtails may chase timid tankmates; chasing is the stated concern to monitor.

What kinds of animals can it live with?

Choose peaceful fish that share 22–27°C, pH 7.0–8.0 and 10–12 dGH, fit the adult stocking plan and can cope with an active swordtail. Assess each proposed companion rather than relying on a universal tank-mate list.

How does this fish arrive alive in the UK?

Start filming the sealed parcel before opening it or floating the bag. The fish is hand-bagged in pure oxygen, packed in an insulated box with a heat pack when weather calls for it and carried by a licensed live-animal courier; be present to receive it, open it that day, keep filming through the sealed-bag inspection, then float the bag for 20–30 minutes before release.

Consult the public references

• FishBase: Xiphophorus hellerii — scientific identity, family, native distribution, recorded environment, maximum length, wild habitat, diet and male behaviour.
• Seriously Fish: Xiphophorus hellerii — aquarium footprint, water conditions, filtration rate, layout, feeding, social behaviour and reproduction.
• OATA: How to look after platies and swordtails — group husbandry, water quality, social planning, feeding, maintenance and health observations.
• RSPCA aquarium environment advice — biological filtration and the conversion of ammonia through nitrite to nitrate.