Berlin Swordtail (Xiphophorus hellerii)

Choose the correct animal and dispatch size

The Berlin Swordtail (Xiphophorus hellerii) is a livebearing freshwater fish in the family Poeciliidae. FishBase records its native range from Mexico to north-western Honduras and a maximum length of 14 cm. This active social fish suits a prepared keeper with a mature, filtered aquarium, measured mineral-bearing water and enough horizontal swimming space for its adult size. Plan at least 108 litres with a 120 × 30 cm base, keep the routine water within 22–27°C, pH 7.0–8.0 and 10–12 dGH, and assess every intended companion before introduction.

Understand the purchase unit

One purchase supplies one animal from the size selected. The live selector may show 4–5 cm, 5–6 cm and 6–8 cm, and it is the place to choose the required size. Quantity two means two individual animals from the same selected size; it does not create a pair or a particular social ratio.

The size you select is the size supplied, not the adult size. These measurements are commercial dispatch sizes rather than ages, life stages or final dimensions. A 4–5 cm fish therefore needs the same permanent adult aquarium plan as a larger dispatch size.

Keep sex separate from size

Sex is guaranteed only when the selected option explicitly states male or female. This Berlin Swordtail listing has no sex-guaranteed option. Neither dispatch size nor quantity changes that boundary, so plan a social group that can accommodate any sex supplied.

The gallery images are representative, based on reference photographs of this species, so colours can look slightly different from the live animal. Use the gallery to recognise the named form while allowing for individual variation. You can also browse the current livebearer collection when comparing animals with different adult-space and water requirements.

Plan the aquarium around the adult swordtail

Adult dimensions and swimming space decide whether this fish fits your aquarium. Seriously Fish uses 140 mm standard length for aquarium planning and says a base measuring 120 × 30 cm, or an equivalent footprint, should be the smallest considered. Its corresponding volume is approximately 108 litres. FishBase separately records a maximum of 14 cm total length for male or unsexed fish. Plan around the 14 cm adult figure and the full recommended footprint rather than the dispatch size.

Check both footprint and volume

A nominal 108-litre aquarium is suitable only when its base offers the required swimming room. A tall, short tank may hold the same number of litres but does not provide the same horizontal route. Substrate, décor and the air space above the water also reduce usable volume, making the external capacity only the starting point for a stocking decision.

This is an active social fish rather than a choice for a compact desktop aquarium. Seriously Fish notes that males may form dominance hierarchies in confined spaces, while OATA recommends companionship for swordtails. More fish also require more filtration capacity and adult swimming room.

Match the keeper to the decisions

Our store suitability assessment is intermediate. The everyday tasks are straightforward, but success depends on coordinating adult space, mineral content, mature biological filtration, social pressure and measured feeding. A prepared keeper who tests water and observes behaviour can meet those needs; a newly filled aquarium or a setup chosen around dispatch size cannot.

Before choosing this animal, confirm that the permanent aquarium can provide its adult footprint, that every intended companion shares compatible water values and that additional fish will not push filtration or social space beyond capacity. Those decisions matter more than choosing between adjacent dispatch sizes.

Build open swimming space with useful cover

Arrange the aquarium around a clear route through the main swimming area, then place cover along the back and sides. Seriously Fish says décor is not especially critical and describes heavily planted layouts, floating plants, driftwood roots and branches as suitable choices. These elements diffuse light and provide sheltered positions while preserving the open base length needed for movement.

Divide the footprint deliberately

Filtration does not need to create an especially strong current, although the fish appreciates some water movement. Aim for circulation that carries waste towards the filter and moves the surface while still leaving calmer positions behind plants or wood. If every fish must swim continuously to hold position, diffuse or redirect the outlet rather than reducing essential filtration.

Use aquarium-safe gravel or sand. OATA recommends substrate and cover for swordtails, with live plants preferred where practical; artificial plants and suitable ornaments can provide shelter when live planting is not used. Check that openings and gaps cannot trap a fish at its current size or after growth.

Test the finished layout

1. Look along the full 120 cm planning length and confirm that décor has not divided it into cramped pockets.
2. Check that sheltered areas remain accessible and do not create narrow dead ends.
3. Confirm that water reaches the whole aquarium rather than bypassing areas behind dense décor.
4. Leave enough access to siphon waste, inspect equipment and observe every animal.

A good layout balances shelter with visibility. Planting should not hide maintenance problems or stop you seeing whether an individual is feeding, breathing normally and using the aquarium confidently.

Set compatible water values before the fish arrives

Keep the aquarium at 22–27°C, pH 7.0–8.0 and 10–12 dGH. These routine values are the overlap of the species aquarium range from Seriously Fish and OATA’s platy and swordtail guidance. Seriously Fish gives 16–28°C, pH 7.0–8.0 and 10–25 dGH for Xiphophorus hellerii; OATA gives 22–27°C, pH 7.0–8.0 and 4–12 dGH for the group, not a species-specific optimum.

OATA’s platy and swordtail guidance also gives 3–10 dKH, sets ammonia and nitrite at 0 mg/l and says nitrate should not exceed 20 mg/l above the normal tap-water level. Apply those nitrogen-waste targets to routine maintenance.

Measure before adjusting

Test the tap water and aquarium with the same method so the comparison is meaningful. Our water chemistry guide explains what the main measurements mean, while the water matcher helps compare measured values with the care ranges. Enter test results rather than estimates.

Work from a real example

If tap water measures pH 7.5 and 11 dGH, it sits within the shared aquarium guidance. Keeping it stable is preferable to dosing towards pH 7.0 or a particular hardness number. If the aquarium then shows ammonia or nitrite above 0 mg/l, treat that as a filtration or loading problem rather than trying to disguise it by changing pH.

Record temperature, pH, hardness, ammonia, nitrite and nitrate before introduction. Recheck after additions because more livestock and food increase the waste processed by the biological filter.

Mature and maintain the biological filter

A working biological filter is essential. The RSPCA explains that beneficial bacteria convert toxic ammonia into nitrite and then into safer nitrate. Water in a new aquarium may look clear before that biological process is ready, so confirm ammonia and nitrite at 0 mg/l before adding this fish.

Prepare the aquarium in advance with the help of our first tropical tank guide. Increase livestock gradually after maturation. OATA warns that adding too many fish too quickly can exceed the bacterial population’s ability to process the extra waste, causing ammonia and nitrite to rise.

Size flow for the real volume

For an aquarium of approximately 108 litres, Seriously Fish suggests turnover of four to five times the volume each hour and a filter rated around 430–540 litres per hour. Treat the rating as a selection aid rather than a guaranteed delivered flow: media, pipework and accumulated debris can reduce actual circulation.

OATA strongly recommends a 25% partial water change at least once every week and testing at least weekly. Use the measured nitrate rise, feeding level and stocking load to decide whether more frequent maintenance is needed. Siphon accumulated solid waste while leaving enough established biology in the filter to continue processing dissolved waste.

Protect beneficial bacteria

Do not run filter media under the tap because chlorine or chloramine may kill the beneficial bacterial population. Avoid replacing every mature medium at once. If one component must be changed, retain established media and monitor ammonia and nitrite closely while the replacement matures.

Surface movement supports oxygen exchange, but it does not replace waste processing. Keep the filter running continuously, investigate an unexplained loss of flow and make equipment checks part of every water-change routine.

Feed once daily and measure the portion

Xiphophorus hellerii is an omnivore. Seriously Fish reports that wild fish consume aquatic and terrestrial invertebrates, organic detritus, algae and other plant material. In the aquarium it recommends a balanced diet based on good-quality dried food with small live or frozen foods such as Daphnia, Artemia and bloodworm.

OATA advises feeding swordtails once a day, offering only what they can eat within a few minutes. Its group guidance recommends a good-quality flake or granule with a high algae content, supplemented by small live or frozen foods. This provides a clear routine: use a complete prepared food as the staple, keep an algae component in the rotation and add suitable small foods for variety.

Give each food a defined role

Start with a modest portion and watch the entire feeding period. Food should be eaten within a few minutes rather than collecting on the substrate or entering hidden spaces. Remove persistent leftovers and reduce the next portion; decaying food adds waste that the filter must process.

Adjust from observation

• If food remains after the group loses interest, offer less at the next feed.
• If one fish takes everything, spread small portions across more than one point while keeping the total controlled.
• If an individual repeatedly misses food, check whether social pressure or a health change is keeping it away.
• If nitrate rises faster than usual, review portion size alongside stocking and maintenance.

A missed feed does not need to be corrected with an oversized meal. Resume the measured routine and assess appetite alongside swimming, breathing and body condition. Consistent small decisions protect both nutrition and water quality.

Plan companionship without assuming sex

OATA recommends keeping at least three platies or swordtails together and suggests two females for each male to reduce pressure on females and other inhabitants. This is broad social guidance rather than a guaranteed outcome for every group. Because this offer does not guarantee sex, do not treat a quantity of three as a planned one-male, two-female trio.

Choose capacity before quantity

Start with the 108-litre, 120 × 30 cm species setup and assess how much of its capacity is already used. More swordtails need more swimming room and filtration capacity. The recommendation for companionship does not override adult stocking limits.

Seriously Fish notes that groups of males can form dominance hierarchies in confined spaces and may spend substantial time maintaining their positions. Brief interaction is different from one fish being chased continually, excluded from food or held in a corner. Observe who starts an interaction, who moves away and whether both animals resume normal swimming.

Recognise adult sexual features carefully

Adult males tend to be smaller and possess a gonopodium, while the characteristic sword is an extension of the lower caudal-fin lobe. Females do not usually have that male sword, although ornamental strains may carry other extended fins. These adult biological features can help manage an established group, but they do not change the sex terms of this offer.

Swordtails are livebearers, and Seriously Fish notes that females may store viable sperm for months. Plan aquarium capacity with the possibility of young in mind whenever sexes mix or a female has previously encountered a male. Do not rely on removing a visible male as a certain way to prevent later births.

Prepare the aquarium and first week in advance

Complete the permanent setup before the fish arrives. Confirm the required footprint, stable temperature, suitable pH and hardness, continuous filter flow, ammonia at 0 mg/l and nitrite at 0 mg/l. Check that existing inhabitants leave enough adult capacity rather than calculating from the selected dispatch size.

Use a before-and-after checklist

OATA advises increasing fish numbers slowly so the beneficial bacteria in the mature filter can adjust as waste and feeding rise. Avoid combining this introduction with several other major changes. Replacing filter media, rearranging the whole aquarium and adding multiple animals together makes it harder to identify the cause of a problem.

Read our current delivery and Live Arrival Guarantee policy while planning receipt. Arrange for someone to receive the parcel and prepare to film continuously from the sealed parcel through opening, with the fish still inside the sealed bag. Float the bag for 20–30 minutes before releasing the fish, and never pour the bag water into the aquarium.

Record the starting point

Write down the temperature, pH, hardness, ammonia, nitrite and nitrate readings taken before introduction. Repeat the key waste tests during the first week and compare them with that baseline. A rising nitrate trend may call for maintenance, while any ammonia or nitrite needs prompt investigation of filter performance, feeding and total load.

Watch from a distance as well as at feeding time. A newly introduced animal needs access to swimming space, cover and food without continuous pursuit. Early observation lets you identify social pressure before it affects condition.

Respond to behaviour and health changes early

OATA recommends watching swimming, colour, temperament, breathing, appearance, body condition and feeding. Learn the normal pattern for each individual, then act when a change persists or several signs occur together. A single observation is most useful when compared with test results and recent events.

Pair each sign with the first check

Begin with objective measurements rather than guessing from appearance alone. Ammonia and nitrite should remain at 0 mg/l, and nitrate should stay no more than 20 mg/l above the tap-water baseline. Confirm temperature and pH with working equipment, then inspect whether the filter has stopped or lost flow.

If water tests are outside their targets, correct the husbandry cause while avoiding abrupt swings. Review overfeeding, sudden stocking increases, blocked flow and recent media changes. Keep notes of readings and behaviour so a pattern is easier to interpret.

Escalate persistent concerns

Contact us when a health change persists, worsens or occurs alongside abnormal breathing, loss of balance, marked weight change or refusal to feed. Accurate test results, the aquarium volume, temperature and a short timeline make further guidance more useful.

Continue routine observation after the first week. Weekly testing and maintenance are minimum habits, while daily feeding provides a regular opportunity to confirm that each fish can reach food and move normally through the aquarium.

Screen every community companion individually

OATA describes platies and swordtails as generally peaceful and compatible with other species, but notes that large swordtails can become boisterous and may chase timid tankmates. Use that as a screening rule rather than a universal approval. Compare adult dimensions, water values, activity, feeding access and the space already occupied before adding any companion.

Apply the same five checks every time

1. Adult space: fit every inhabitant at adult size within the aquarium’s footprint and filtration capacity.
2. Water: select one stable temperature, pH and hardness range shared by all inhabitants.
3. Activity: make sure a quieter fish can still rest and move without continual pursuit.
4. Feeding: confirm that slower animals can receive suitable food without overfeeding the whole tank.
5. Social needs: preserve the group size, territory or shelter required by each species.

A temperature match alone is not enough. Two fish may share 24°C while requiring different hardness, activity levels or social space. Equally, a large volume does not solve every conflict if one fish controls the only open route or feeding point.

Assess the aquarium, not a label

Watch established inhabitants at feeding time and during normal movement. If the swimming space is already busy, adding an active swordtail can increase competition even when the nominal litre count looks adequate. A larger footprint and separated feeding points may improve access, but they do not guarantee harmony.

Compatibility remains conditional because individual behaviour varies. Use the screening rule before introduction and continue observing afterwards, with a practical separation plan if one animal cannot feed, rest or move normally.

Manage dominance and reproductive pressure

Social compatibility includes interactions within the swordtail group. Seriously Fish records that males may form dominance hierarchies in confined spaces. OATA recommends companionship and a female-heavy ratio when sexes are known, but this offer does not provide a chosen sex. Build the aquarium so any resulting combination can be observed and managed.

Distinguish interaction from sustained pressure

Footprint is the first management tool. The 120 × 30 cm species minimum provides a starting point for the adult plan, while groups and communities may need more. Plants, roots and branches provide cover, but leave enough open water for every fish to move without entering a narrow bottleneck.

Allow for livebearing biology

Swordtails reproduce through internal fertilisation, and females may retain viable sperm for months. A group can therefore produce young after an apparently simple change in composition. Treat future population growth as part of compatibility because additional fish increase feeding, waste, swimming-space and filtration demands.

Do not select companions or group quantity on the assumption that a particular sex will arrive. If the aquarium cannot absorb additional reproductive or dominance pressure, choose a lower-risk stocking plan before purchase rather than relying on later identification.

Use an intervention threshold

Step in when a fish is continually excluded from food, held in one part of the aquarium, injured or losing condition. First check water quality because environmental stress can occur alongside social problems. Then assess footprint, group composition and total stocking, and separate fish where one cannot resume normal activity.

The most reliable community is one planned around measurable overlap and observed behaviour. General peacefulness supports consideration, but the final decision belongs to the actual aquarium and its inhabitants.

Answer the key buyer questions

How big does a Berlin Swordtail get?

The Berlin Swordtail reaches 14 cm, the maximum length recorded by FishBase. Plan the aquarium around that adult figure rather than the selected dispatch size.

What size aquarium does a Berlin Swordtail need?

A Berlin Swordtail needs at least approximately 108 litres with a 120 × 30 cm base or an equivalent footprint, according to Seriously Fish. Allow more adult capacity for a group or community.

How many Berlin Swordtails should I keep together?

OATA recommends at least three swordtails and suggests two females per male when sexes are known. This offer does not guarantee sex, so allow enough adult space for any combination supplied.

Is the Berlin Swordtail suitable for beginners?

The Berlin Swordtail has an intermediate store suitability rating. It suits a prepared keeper who can provide at least 108 litres, a 120 × 30 cm base, mature filtration, measured water and careful social planning.

What does Xiphophorus hellerii eat?

Feed Xiphophorus hellerii once daily with a good-quality complete flake or granule containing an algae component, plus small live or frozen foods such as Daphnia, Artemia or bloodworm. Offer only what it eats within a few minutes.

What temperature and pH does this fish need?

Keep this fish at 22–27°C, pH 7.0–8.0 and 10–12 dGH, the overlap of Seriously Fish species guidance and OATA group guidance. Keep ammonia and nitrite at 0 mg/l.

Is it aggressive or likely to nip fins?

Males may form dominance hierarchies in confined spaces, and OATA says large swordtails can be boisterous and may chase timid tankmates. Screen companions for temperament, adult space and feeding access, and intervene if chasing prevents normal movement or feeding.

What kinds of animals can this fish live with?

It can be considered for a peaceful community when every inhabitant shares compatible temperature, pH, hardness, adult space, activity and feeding needs. Avoid pairing a large, boisterous swordtail with timid tankmates that cannot withdraw from chasing.

How does this fish arrive alive in the UK?

Start filming every delivery continuously while the parcel is still sealed, then record it being opened with the fish still 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.

Check the public care references

• FishBase: Xiphophorus hellerii — scientific identity, family, distribution, environmental values, wild diet and maximum recorded length.
• Seriously Fish: Xiphophorus hellerii — aquarium footprint, volume, water range, décor, filtration, diet, behaviour and reproductive biology.
• OATA: How to look after platies and swordtails — UK group guidance covering companionship, water quality, feeding, maintenance, introduction and observation.
• RSPCA aquarium environment guidance — biological filtration and the conversion of ammonia through nitrite to nitrate.