Swordtail Black (Xiphophorus helleri black)

Understand the black ornamental form

Swordtail Black (Xiphophorus helleri black) is a black ornamental freshwater livebearer in the family Poeciliidae. The species occurs naturally from Mexico into Central America and reaches a recorded maximum of 14 cm. It suits a keeper who wants an active group fish and can provide a long, mature aquarium with stable, alkaline water, planted shelter and open swimming room. We rate its care level as easy once those foundations are in place, but its adult space requirement and social behaviour still need planning before purchase.

FishBase now lists it as Xiphophorus hellerii. Adults inhabit flowing streams and rivers as well as warm springs, vegetated canals and ponds. Heavily planted areas are therefore a useful reference when arranging cover, although the aquarium must retain open swimming room.

Wild fish eat worms, crustaceans, insects and plant matter. That mixed natural diet explains why aquarium feeding should combine plant-rich dried food with appropriately small live or frozen foods rather than relying on one ingredient.

The black colour is an ornamental characteristic rather than a reason to change the species’ core care. Selective breeding has produced swordtails with many colour patterns and fin shapes, so live animals can differ in shade and fin detail. The gallery images are representative, based on reference photographs of this species, so colours can look slightly different from the live animal.

Choose the dispatch size from the live selector

One purchase supplies one animal of the selected size. Use the live size selector to choose between the 4–5 cm and 5–6 cm labels. These labels describe the fish at dispatch and should not be used as a forecast of its final length.

The size you select is the size supplied, not the adult size. A 4–5 cm fish and a 5–6 cm fish therefore need the same adult aquarium plan. The larger dispatch choice may begin with more body length, but neither label changes the maximum recorded length, group needs or water target.

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

Plan the resident group before selecting quantity. Count the swordtails already in the aquarium, check the adult capacity of the tank and leave biological filtration capacity for every addition. Increasing livestock gradually gives the filter bacteria time to respond to the extra waste and feeding load.

Check the decisive adult requirements

FishBase records a maximum of 14 cm total length for male or unsexed fish. Seriously Fish records 140 mm standard length for accommodation planning. Both figures make clear why a dispatch size of 4–6 cm must not define the permanent setup.

Seriously Fish says an aquarium with a base of 120 × 30 cm or equivalent should be the smallest considered and associates that footprint with approximately 108 litres. Footprint matters because swordtails need both planted shelter and open swimming space; a taller aquarium with a short base is not automatically equivalent.

OATA’s platy and swordtail sheet is group guidance: it recommends at least three swordtails and, when sexes are known, two females for each male. Use that as the social starting point while allowing more room for a larger group.

For routine aquarium water, keep 22–27°C, pH 7.0–8.0 and 10–12 dGH. These figures are the overlap of the Seriously Fish species range and the narrower OATA swordtail guidance.

Decide whether the setup fits

Hold a stable routine water range

The species aquarium profile spans 16–28°C, pH 7.0–8.0 and 10–25 dGH. The OATA swordtail range narrows temperature to 22–27°C and general hardness to 4–12 dGH while keeping the same pH. For routine care, use the overlap: 22–27°C, pH 7.0–8.0 and 10–12 dGH. This remains inside the species range while following the tighter swordtail target.

Test the aquarium rather than estimating from tap-water reputation or appearance. Our aquarium water chemistry guide explains what the readings mean, while the water matcher lets you compare measured values with this target. Check temperature with a thermometer and use liquid or other suitable aquarium tests for pH, hardness and nitrogen waste.

Stability is as important as reaching a number. If a reading falls outside the target, identify whether source water, accumulated waste, temperature control or recent stocking changed before making adjustments. Sudden chemistry changes create another stress, so base any correction on repeated measurements and make it gradually.

Compare the whole set of readings

A temperature of 25°C does not make the aquarium ready if pH is below 7.0, hardness is below 10 dGH or nitrogen waste is present. Likewise, suitable chemistry cannot compensate for a drifting heater. Record the readings together before introduction and again after adding fish. If ammonia and nitrite remain at zero but nitrate rises faster than usual, review the amount fed, the number of residents and the weekly change before increasing livestock. A written test log makes gradual movement easier to recognise than isolated results.

Build a long layout with cover and swimming room

Begin with a base of at least 120 × 30 cm or equivalent and approximately 108 litres. The length gives an active swordtail room to move away from another fish, while the front-to-back depth makes it possible to combine planting with an open route for swimming. Recheck usable volume after substrate, wood, rocks and equipment have been added.

Divide the aquarium into useful areas

• Planted cover: use live plants where practical, especially around the back and ends, to create visual breaks and shelter.
• Open water: preserve a clear swimming lane rather than filling the whole footprint with décor.
• Diffused light: floating plants and driftwood roots or branches can soften the light and add cover.
• Stable footing: aquarium gravel or sand provides the substrate recommended for swordtail setups.

Décor does not need to copy one exact habitat. A heavily planted arrangement with a darker substrate suits the fish, while water-worn rocks and small boulders can evoke the flowing streams occupied by wild swordtails. Every item should leave sufficient swimming width and must not obstruct the filter intake or outlet.

The species appreciates some water movement, but filtration does not need to create an excessively strong current. In an aquarium of about 108 litres, the profile uses a turnover of four to five times the volume per hour, giving a filter-selection reference of roughly 430–540 litres per hour. Judge the final flow by the whole layout: plants and wood can redirect the outlet, while clogging can reduce actual turnover.

For the same approximate volume, a heater reference of 75–125 W assumes a room near 20°C. Choose equipment that can hold the selected temperature steadily in the aquarium’s real room conditions, and confirm performance with a thermometer.

Check the layout from the fish’s route

Look along the full 120 cm length before filling the tank with more ornaments. The fish should be able to cross the aquarium through open water, enter cover at either end and move away without meeting a dead end. If a large branch divides the swimming lane completely, turn or reposition it rather than sacrificing usable length. After the filter starts, watch whether floating plants collect against the outlet and whether stems bend flat in the current. Adjust the outlet direction while retaining some water movement and keeping the intake clear.

Mature the filter before adding livestock

A Swordtail Black needs a well-established biological filter. The RSPCA explains that biological filtration supports bacteria that convert toxic ammonia into nitrite and then into safer nitrate. Both ammonia and nitrite must read zero before the fish is introduced.

If this is your first setup, follow our first tropical aquarium guide to prepare and mature the aquarium before livestock arrives. Adding several fish to a newly matured filter at once can increase waste faster than the bacterial population can respond, so build the population slowly and continue testing after each addition.

Use a repeatable maintenance rhythm

Treat replacement tap water with a conditioner that removes chlorine or chloramine before it enters the aquarium. Chlorine or chloramine can harm fish and can also damage the bacteria living in filter media. Cleaning media in removed tank water limits bacterial loss while still removing debris.

A 25% water change at least once a week is the baseline, not permission to ignore test results between changes. If ammonia or nitrite appears, pause further livestock additions, check that the filter is operating and reduce the waste entering the system. Nitrate trends also show whether the normal maintenance rhythm is keeping pace with feeding and stocking.

Use the first month to confirm capacity

• First day: confirm the heater and filter continue running after introduction and test if behaviour changes.
• First week: test at least once, complete the planned 25% water change and compare nitrate with the starting result.
• After each addition: repeat ammonia and nitrite checks rather than assuming the previous result still applies.
• Before adding more: wait until feeding, waste removal and test results have settled into a predictable pattern.

This sequence links stocking decisions to the filter’s measured performance. Clear water alone is not a substitute for zero ammonia and nitrite.

Feed one measured, varied meal each day

Swordtails are omnivores. Wild stomach contents include aquatic and terrestrial invertebrates, organic detritus, algae and other plant material, while FishBase also records worms, crustaceans, insects and plant matter. Aquarium food should reflect that variety without turning every meal into a large mixture.

Build the diet around two parts

• Daily foundation: use a good-quality flake or granule with a high algae content.
• Small varied additions: offer suitably sized live or frozen foods such as daphnia, small bloodworm, tubifex or Artemia.

Feed only what the fish can finish within a few minutes, once a day. Begin with a modest portion and watch whether every piece is taken. If food remains after a few minutes, remove what you can and reduce the next portion. If it disappears immediately, do not automatically keep adding more; body condition, water tests and the absence of leftovers give a better guide over time.

Uneaten food breaks down and adds waste, which can raise ammonia, nitrite and nitrate. Overfeeding therefore creates a water-quality problem as well as an unbalanced diet. A larger group may need food distributed across more than one point so active fish do not monopolise the meal, but the total should still be consumed within the same short feeding window.

Use feeding time to notice appetite and access. A fish that repeatedly hangs back may need food placed where it can reach it without entering the busiest part of the group. Reduced intake is also one of the changes that should prompt a water test and closer observation.

Adjust the portion from what happens next

Vary the food type without increasing the total daily amount. Small live or frozen items complement the plant-rich dried foundation; they do not require an extra unrestricted meal.

Acclimatise with a prepared aquarium

Complete the aquarium checks before the delivery window. OATA’s swordtail guidance requires suitable water quality before introduction, so the tank should already be mature, within 22–27°C and reading zero for ammonia and nitrite. Do not use the arrival day to make major layout or water-chemistry changes.

Follow the arrival sequence

1. Have someone present to receive the parcel because live fish cannot be left unattended.
2. Start filming continuously while the parcel is still sealed and the outer box is visible.
3. Open the parcel in the same continuous take and show the fish while it is still inside the sealed bag.
4. Float the bag in the aquarium for 20–30 minutes.
5. Release the fish without pouring the bag water into the aquarium.

Read our delivery and Live Arrival Guarantee policy for the current arrival and claim requirements.

Keep the first assessment factual. Note whether the fish is upright, swimming with control and breathing consistently. A newly introduced fish may choose cover, so use the planted areas and open swimming route to observe it without repeatedly disturbing the aquarium.

Continue with the established daily feeding and weekly maintenance rhythm rather than making several reactive changes at once. If behaviour changes, test the water first; measured ammonia, nitrite, temperature and pH readings are more useful than guessing from appearance alone.

Keep the first day measurable

Once the fish is transferred, leave the layout in place and observe from outside the aquarium. Check that the filter outlet still moves water, the heater remains within 22–27°C and existing residents are not preventing access to cover. Record swimming, balance and breathing so later changes have a clear comparison. If several fish show unusual behaviour, test the aquarium promptly. If only the newcomer withdraws into plants but remains upright and controlled, give it room to settle while continuing observation.

Respond to changes in behaviour or appearance

Learn the fish’s normal swimming, colour, temperament, breathing, body condition and feeding response. A change from that baseline is the signal to inspect the aquarium and test the water. Early action is most useful when it identifies a measurable cause such as ammonia, nitrite, temperature drift, crowding or a blocked filter.

Do not treat every hiding fish as the same problem. One individual may be responding to a dominant tankmate, while several affected fish can point towards a wider environmental change. Compare what is happening across the group and note whether it began after stocking, maintenance, equipment adjustment or a change in feeding.

When a health change persists, test the aquarium and contact us with the readings, aquarium size, stocking details and clear description of the signs. Those details make it easier to distinguish a water-quality emergency from a compatibility or health concern without guessing at a treatment.

Separate environmental and social clues

• Several fish affected together: prioritise ammonia, nitrite, temperature and filter checks because the whole aquarium may have changed.
• One fish repeatedly displaced: watch which resident approaches it and whether it can reach food, cover and open water.
• A change after maintenance: confirm replacement water was conditioned and that the filter and heater restarted correctly.
• A change after stocking: compare the timing with the new waste load and group interactions.

Write down the sequence rather than changing food, décor, temperature and chemistry together. One measured action at a time makes the response easier to assess.

Screen every tankmate for water, pace and temperament

OATA describes platies and swordtails as generally peaceful, but large swordtails can be boisterous and may chase timid tankmates. FishBase reports aggression between males, while the species profile explains that males in confined spaces can spend considerable time maintaining dominance positions. Space and group structure therefore matter even when the wider community is peaceful.

Apply four checks before mixing fish

1. Water: the other species should remain within 22–27°C, pH 7.0–8.0 and 10–12 dGH as part of its own normal care.
2. Adult size: compare mature dimensions rather than dispatch sizes and ensure every resident fits the 120 × 30 cm footprint and total capacity.
3. Swimming style: choose fish able to share space with an active, sometimes boisterous livebearer.
4. Temperament: allow timid residents places to withdraw and monitor male swordtails for persistent chasing or exclusion.

A matching temperature alone does not make a combination suitable. A soft, acidic-water fish fails the chemistry screen, while a very timid fish may struggle even when the numerical ranges overlap. Individual behaviour can still vary, so reassess after introduction rather than treating any species name as a universal guarantee.

Browse the livebearers collection to compare animals with related care patterns, but check the adult size, water range and temperament of each exact choice. For a stocking and compatibility assessment, ask us with the aquarium dimensions, measured water values, filter, current residents and planned quantities.

Work through the screen in order

Suppose a candidate fish fits 24°C but requires water below pH 7.0: stop at the chemistry check rather than trying to average the two requirements. If another candidate shares 22–27°C, pH 7.0–8.0 and 10–12 dGH, continue to adult size and behaviour instead of declaring it suitable immediately. Then consider whether the full adult community can use the 120 × 30 cm base without crowding and whether a timid swimmer can withdraw from a boisterous swordtail. Passing all four checks makes the plan reasonable, but observation after introduction remains essential.

Plan the group without assuming sex

Use at least three swordtails as the social starting point. When the sexes of the fish in the aquarium are known, OATA recommends two females for each male to reduce stress on females and other residents. More can be kept in the same ratio only when the aquarium has enough adult space, filtration and cover.

Build an exact ratio only from options explicitly labelled male or female. Do not infer the sex that will arrive from dispatch size, colour or a gallery scene. For fish already in the aquarium, adult males possess a gonopodium and typically show the lower tail extension, while females have a triangular anal fin. Ornamental strains can carry other extended fins, including in females, so the tail outline alone is not a purchase guarantee.

Watch how the group uses the whole aquarium. Short displays that end with both fish returning to normal activity are different from persistent pursuit that keeps one fish hiding or away from food. If one fish cannot rest or feed, reassess group composition, cover and usable swimming length rather than simply adding more animals.

Swordtails are livebearers and mixed-sex groups may produce live young. Females can store viable sperm for months, so separating sexes later does not immediately remove that possibility. Plan stocking capacity around the adult group and potential population growth without relying on breeding as a way to balance behaviour.

Expand only after the first group settles

A plan for six known-sex swordtails begins with two males and four females, but the ratio alone does not establish capacity. First confirm that the aquarium exceeds the smallest footprint where the whole community requires it, the filter keeps ammonia and nitrite at zero and each fish can feed and withdraw. Additions should be gradual so both the social hierarchy and biological filter can respond. If persistent pursuit already confines one fish to cover, adding another animal is not the first remedy; reassess swimming length, visual breaks and group composition.

Answer the key buying and care questions

How big does Swordtail Black get?

FishBase records a maximum of 14 cm total length for male or unsexed fish. Plan the permanent aquarium around that adult figure rather than the dispatch size.

What size aquarium does Swordtail Black need?

Seriously Fish says an aquarium with a base of at least 120 × 30 cm or equivalent should be the smallest considered and associates that footprint with approximately 108 litres.

How many Swordtail Black should I keep together?

OATA’s platy and swordtail guidance recommends at least three swordtails. When sexes are known, plan two females for each male and increase aquarium capacity for a larger group.

Is Swordtail Black suitable for beginners?

We rate it easy when the keeper can provide a mature filtered aquarium, stable water at 22–27°C, pH 7.0–8.0 and 10–12 dGH, and enough adult swimming space.

What does Swordtail Black eat?

Feed a good-quality flake or granule with a high algae content, varied with small live or frozen foods such as daphnia, bloodworm, tubifex or Artemia. Offer only what it finishes within a few minutes once daily.

What temperature and pH does this fish need?

Keep it at 22–27°C, pH 7.0–8.0 and 10–12 dGH. This routine range is the overlap of Seriously Fish species guidance and OATA swordtail group guidance.

Is it aggressive or likely to nip fins?

Males can be aggressive towards each other, and confined groups may form dominance hierarchies. Large swordtails can also be boisterous and may chase timid tankmates, so provide adult space, cover and open swimming room and monitor persistent pursuit.

What kinds of animals can it live with?

Choose robust, peaceful fish whose adult size and normal water range match 22–27°C, pH 7.0–8.0 and 10–12 dGH. Screen every candidate for swimming pace and temperament because timid tankmates may be chased.

How does this fish arrive alive in the UK?

Start filming continuously while the parcel is still sealed and before floating the bag. Someone must be present to receive it; show the sealed outer parcel, open it in the same take with the fish still in its sealed bag, float the bag for 20–30 minutes, then release the fish without pouring the bag water into the aquarium.

Check the care references

• FishBase: Xiphophorus hellerii — accepted identity, family, distribution, environmental range, biology and maximum recorded length.
• Seriously Fish species profile — aquarium dimensions, water conditions, layout, feeding, behaviour and sexual dimorphism.
• OATA platy and swordtail care sheet — UK group guidance for water, social planning, feeding, maintenance and health observation.
• RSPCA aquarium environment guidance — biological filtration and nitrogen-waste control.