Social need
6 minimum; 8-10 preferred
A full shoal is part of the welfare plan, not an optional visual extra. A larger group spreads attention and usually produces more settled behaviour.
Black Widow Tetra (Gymnocorymbus ternetzi), also called Black Skirt Tetra, is an active midwater shoaling fish. Keep 6+, ideally 8-10, in a mature 70-litre+ aquarium with open swimming space; avoid slow, long-finned tank mates because under-grouped fish may nip fins.
Adult size is the maximum length this species reaches at full maturity (scientific sources). The livestock you receive will be younger and smaller — pick a size variant above for the actual shipping size. Product-photo provenance and any image-specific limitations are stated on the listing.
Gymnocorymbus ternetzi
Black Widow Tetra are a shoaling species — they need 6+ to feel safe and show their full colour.
Black Widow Tetra (Gymnocorymbus ternetzi), also called Black Skirt Tetra, is an active midwater shoaling fish. Keep 6+, ideally 8-10, in a mature 70-litre+ aquarium with open swimming space; avoid slow, long-finned tank mates because under-grouped fish may nip fins.
Adult size is the maximum length this species reaches at full maturity (scientific sources). The livestock you receive will be younger and smaller — pick a size variant above for the actual shipping size. Product-photo provenance and any image-specific limitations are stated on the listing.
Maintain these water conditions for optimal health and vibrant colors
Black Widow Tetra (Gymnocorymbus ternetzi), also called the Black Skirt Tetra or Black Tetra, is a deep-bodied South American shoaling fish for mature freshwater aquariums. It spends most of its time in the middle of the tank, where its silver-grey body, two dark shoulder bars and broad black anal fin create strong contrast against plants and wood. This listing is for the naturally coloured form shown in the supplier photograph.
This species is adaptable, but “hardy” does not mean maintenance-free. Plan for a proper group, useful horizontal swimming length, stable water and compatible tank mates before ordering. A cramped tank or undersized group can turn ordinary social sparring into persistent fin-nipping.
Social need
A full shoal is part of the welfare plan, not an optional visual extra. A larger group spreads attention and usually produces more settled behaviour.
Aquarium footprint
Horizontal swimming distance matters. A tall narrow tank can hold enough water while still offering too little usable midwater space.
Maintained range
Stable, cycled water is more valuable than repeatedly adjusting a safe value to chase one ideal number.
Watch before mixing
Risk rises in small groups, cramped tanks and communities containing slow fish with long trailing fins.
Swimming level
Leave an open lane through the centre or front of the aquarium, with cover around the edges rather than décor filling every gap.
Plant use
It does not need to dig. Rooted plants, floating shade and dark wood can improve security while keeping the middle open.
Expect frequent midwater movement rather than a fish that rests in one territory.
Keeping one or two is not a substitute for planning space for a proper group.
This is a management warning, not a claim that every individual will nip.
Plants supply cover and shade; keep enough open water for the shoal to move.
These indicators are practical decision aids based on the care guidance on this page. They are not laboratory scores and do not replace observing individual fish.
| Accepted name | Gymnocorymbus ternetzi (Boulenger, 1895) |
|---|---|
| Common names | Black Widow Tetra, Black Skirt Tetra, Black Tetra, Petticoat Tetra |
| Typical adult size | About 5-6 cm; FishBase records a maximum of 7.5 cm standard length |
| Minimum group | 6 minimum; 8-10 preferred where the aquarium allows |
| Minimum aquarium | 70 litres with about 75 cm of horizontal swimming length |
| Temperature | 20-26°C |
| pH | 6.0-7.5 |
| Hardness | Approximately 5-18 dGH; stability matters more than chasing one exact number |
| Diet | Omnivore; quality flakes or small pellets plus suitably sized frozen or live foods |
| Swimming level | Active midwater fish |
| Temperament | Generally peaceful in a sufficient group, with a recognised fin-nipping risk |
Use this section as a practical stop-check before adding the fish to a basket. Black Widow Tetras are often described as easy because they tolerate a sensible range of stable freshwater conditions, but the purchase becomes difficult when the aquarium is too short, the group is too small or the existing community contains tempting trailing fins. Planning those three points prevents most avoidable disappointment.
Good fit
The filter is cycled, ammonia and nitrite stay at zero, there is open midwater space, and the owner can add a proper group rather than one fish. Plants and wood provide retreat without turning the whole aquarium into an obstacle course.
Good fit
Similarly sized, confident fish share the temperature range and feeding pace. The aquarium has enough room and filtration for each social group at adult size.
Use caution
Even a usually settled shoal can investigate trailing fins. Have a separation plan and do not rely on the hope that adding more décor will make an incompatible combination safe.
Use caution
The minimum tank figure is not an instruction to fill every remaining space. Use the stocking calculator as a planning aid, then consider adult body size, swimming level, filtration and territorial behaviour as well as litres.
Not suitable
A small number does not remove social behaviour; it concentrates it. The fish may hide, chase one target repeatedly or redirect attention to other species.
Not suitable
Clear water is not proof of biological maturity. Wait until the filter can process waste and testing confirms zero ammonia and nitrite before livestock is added.
No checklist can guarantee the behaviour of every individual. It does, however, distinguish a planned purchase from an impulse purchase and makes the most common risks visible before the animals are moved.
| Your setup | Fit | Why |
|---|---|---|
| Mature 70-litre+ community tank, around 75 cm long, with room for 6-10 fish | Good fit | The group can use the middle water level without crowding slower residents. |
| Planted aquarium with open swimming space and some shaded cover | Good fit | Plants, wood and floating cover provide security while an open central area preserves shoaling space. |
| Small aquarium or a plan to keep one or two fish | Poor fit | An undersized group is more likely to hide, spar excessively or redirect attention towards tank mates. |
| Community dominated by slow, long-finned fish | Use caution | Black Widows can investigate or nip trailing fins, particularly when cramped or under-grouped. |
| Tank with fish large enough to swallow a 5-6 cm tetra | Not suitable | Peaceful behaviour does not protect a small fish from predation. |
Choose the fish for its adult needs, not only its size in a sales photograph. The minimum group itself requires meaningful space, filtration capacity and a cycled aquarium.
The image is an authentic supplier photograph that matches the listing identity. It shows the characteristic deep, laterally compressed body, silver-grey colour, two dark vertical bars behind the gill area and an enlarged dark anal fin that forms the “skirt.” It should be read as an identity reference, not a promise that every individual will have identical colour, fin length or body condition.
Young fish often show stronger black contrast. Mature animals may become greyer, and females are usually fuller-bodied than males of a similar age. Normal age- or condition-related variation should not be confused with a different species. Artificially coloured or fluorescent varieties exist in the wider trade, but this listing does not claim one of those forms.
Where “skirt” comes from
“Black Skirt” refers to the broad dark anal fin running along the underside of the rear body. It is not a separate species from the Black Widow Tetra.
Colour through life
Strong black contrast can soften as the fish matures. A greyer adult is not automatically sick or wrongly identified, although sudden colour loss still deserves a husbandry check.
Group behaviour
Settled fish may spread through the middle water and regroup when startled. Constant nose-to-tail synchronisation is not required for the group to be socially important.
Taxonomy
FishBase currently places the species in Acestrorhamphidae, while many familiar aquarium references still use Characidae. That difference reflects classification, not two different animals.
Reproduction
Eggs are released among fine plants or spawning material. Adults may eat them, which is why successful fry rearing usually happens in a separate breeding setup.
Body plan
A 5-6 cm Black Widow has a deeper body than many slimmer tetras of a similar length. Aquarium capacity and tank-mate choice should consider body mass and activity, not centimetres alone.
The common names Black Widow Tetra, Black Skirt Tetra, Black Tetra and Petticoat Tetra can all lead to Gymnocorymbus ternetzi. A listing should still show the scientific name because common names are reused, translated and applied inconsistently across the trade. The binomial is the strongest anchor for matching care information, supplier material and structured data.
The two dark bars immediately behind the head are useful identity features, but photographs are affected by age, stress, lighting, background and camera processing. Use the whole combination—deep body, shoulder bars, long anal fin and scientific name—rather than one patch of colour. If a delivered animal appears to be a different species, photograph it clearly from the side before mixing it with the main community and contact support with the order details.
Black Widows are sometimes called beginner fish. That label means their ordinary care is understandable and they can adapt to a practical range of stable water; it does not remove the need for cycling, group size, adult-space planning or observation. The most useful “beginner” characteristic is that their needs can be explained clearly, not that they tolerate neglect.
FishBase records Gymnocorymbus ternetzi from the Paraguay and Guaporé river basins south to Argentina. Aquarium references describe small, slow-moving creeks, tributaries and streams with marginal or overhanging vegetation. In the aquarium, that translates into a useful combination of planted or wooded cover and an open midwater lane rather than a tank filled from front to back with décor.
The natural range supports the species’ preference for stable freshwater and moderate tropical temperatures. It does not justify abrupt parameter changes or an immature aquarium. Captive fish may adapt to a range of water chemistry, but biological stability remains essential.
Use at least 70 litres and approximately 75 cm of horizontal swimming length for the minimum group. A larger aquarium is preferable for 8-10 fish, mixed communities or aquascapes where hardscape reduces open water. Volume alone can mislead: a tall, narrow aquarium may contain enough litres but provide less usable swimming distance than a longer tank.
Cycle the aquarium before adding livestock. Ammonia and nitrite should remain at zero, and stocking should leave enough capacity for the whole group rather than one fish purchased as a trial.
A practical maintained range is 20-26°C, pH 6.0-7.5 and roughly 5-18 dGH. FishBase lists a somewhat broader pH and hardness envelope, while specialist aquarium sources use narrower ranges. The narrower figures here are intended as a conservative home-aquarium target, not a claim that the species fails immediately outside them.
Match temperature and chemistry gradually during acclimation. Once settled, prioritise consistency, zero ammonia and nitrite, controlled nitrate and regular partial water changes. Test the source water and aquarium instead of adding products simply to chase a fashionable pH. Sudden swings can be more harmful than a stable value within the accepted range.
Keep 6 minimum; 8-10 is preferred when space and filtration allow. Black Widow Tetras are social but do not move in a perfectly synchronised school at every moment. A settled group often spreads through the middle water, then tightens when startled or during active interaction.
Minor chasing and display within the group are normal. The management concern is repeated targeting, damaged fins or one fish being excluded from food. A larger group and more swimming room distribute social attention. They reduce risk but do not make fin-nipping impossible, so inspect the community rather than relying on the word “peaceful.”
Do not choose them as companions for slow fish with elaborate trailing fins unless you can supervise the combination and provide an alternative plan. A single Black Widow is not a humane shortcut for avoiding group behaviour.
This is not a collection of invented customer reviews. It is a transparent troubleshooting guide built around problems aquarists commonly report with active shoaling tetras. Each complaint can have more than one cause, so treat the response as a sequence of checks rather than a remote diagnosis. Water quality, group structure, tank dimensions, feeding access and recent changes should be examined together.
Likely contributors: the Black Widows are kept in too small a group, horizontal space is limited, another species has slow trailing fins, or one fish is being repeatedly targeted. Some individuals may also be more persistent than others even in a sensible setup.
What to check: count the group, measure the actual swimming length, inspect every fish for damage, and watch who begins each interaction. Check whether the behaviour happens only during feeding or continues throughout the day. A damaged long-finned fish should not be left as an experiment.
What to do: separate injured or continually targeted animals, correct group and space problems where possible, and reconsider the species combination. Adding random decorations does not reliably solve incompatibility; too much décor can reduce swimming room further.
Brief display and short chases can be ordinary social behaviour. The concern is repeated one-way pursuit, one fish trapped in a corner, torn fins, loss of feeding access or exhaustion. “Active” should not be used to dismiss an animal that cannot rest or rejoin the group.
Check whether the shoal is large enough and whether the aquarium provides a long open route plus more than one area of cover. Record a short video at feeding time and another when the room is quiet; this can show whether the pattern is food competition, a temporary disturbance or persistent harassment.
If one fish remains the sole target, separate it safely and investigate water quality, illness, size differences and social structure. Do not keep adding fish to an already crowded aquarium merely because larger groups are recommended.
Black Widow Tetras often become greyer as they mature, so gradual age-related softening can be normal. Colour also changes with lighting, background, stress and condition. A fish photographed against a dark supplier background may look different in a brightly lit pale aquarium.
Sudden fading across the group calls for checks: temperature, ammonia, nitrite, nitrate trend, recent water changes, aggressive tank mates, appetite and breathing. Do not medicate colour alone. A water test and behaviour review provide more useful evidence than guessing from pigment.
If fading is accompanied by clamped fins, rapid breathing, isolation, spots, swelling or refusal to eat, move from cosmetic concern to health assessment. Seek qualified advice rather than trying several incompatible treatments.
Hiding after delivery can follow transport, unfamiliar surroundings, bright lighting or a sudden approach to the aquarium. A newly introduced shoal may hold near cover before using open water. Keep lighting low, movement around the tank calm and feeding modest while following the supplied acclimation instructions.
Check that temperature is stable, oxygen exchange is adequate and no existing fish is attacking the newcomers. Count the fish without repeatedly chasing them out of cover. The acclimation timer can help organise the steps, but the instructions supplied with the livestock and the actual water conditions take priority over a generic timer.
Hiding becomes more concerning when paired with rapid breathing, loss of balance, surface gasping, persistent isolation after the rest of the group settles, or visible injury. Contact support promptly if there is an arrival problem and keep the required evidence.
Fast midwater fish can intercept food before a shy, smaller or newly introduced individual reaches it. One large portion in one place rewards the quickest fish and makes it harder to see who missed out.
Offer small portions across more than one part of the surface and watch each animal. Use food small enough to swallow and allow frozen food to disperse rather than dropping a solid clump. A fish that repeatedly approaches food but cannot eat may have a mouth, buoyancy or health problem rather than simple shyness.
Do not compensate by flooding the tank with extra food. Uneaten material degrades water quality and can create a second problem. Separate observation may be needed when one fish continues to lose condition.
A relaxed shoal may spread through the middle of the aquarium and group more tightly when startled. That can indicate confidence rather than a failed purchase. The social requirement remains important even when the fish are not arranged like a single moving photograph.
Look at the whole behaviour pattern: Do they regroup? Do several fish interact without one permanent victim? Are they feeding and using open water? Constant frantic schooling against the glass can be more concerning than a loose, calm distribution.
If the fish scatter because one resident repeatedly attacks them, or all remain pressed into one corner, investigate compatibility and environment. The goal is settled social behaviour, not forcing a display formation.
The deep body makes an adult Black Widow visually and biologically more substantial than a very slender tetra of the same measured length. A group of eight occupies meaningful midwater space and can dominate feeding activity in a compact community.
Plan around adult size, not the shipping size selected in the buy box. Recheck the aquarium’s length, existing stock, filtration and open-water area. A numerical stocking tool can organise information, but it cannot observe territorial behaviour or redesign a short tank.
If the aquarium is already crowded, the responsible response may be to choose a smaller-bodied species or postpone the purchase. “Easy care” does not mean “fits anywhere.”
Mistake 1
Why it causes trouble: normal social attention is concentrated on very few fish. One may pursue the other or both may redirect attention towards tank mates.
Better choice: plan the aquarium and budget for the minimum group before buying any. If the tank cannot hold that group at adult size, choose a genuinely solitary or smaller-group species whose needs fit.
Mistake 2
Why it causes trouble: a tall aquarium may meet a volume number but provide little horizontal swimming room. Décor and existing fish reduce usable space further.
Better choice: measure length, width and height, then use the tank-volume tool. Give extra weight to the approximately 75 cm swimming-length recommendation.
Mistake 3
Why it causes trouble: long fins can attract investigation or nipping, while a slow fish may struggle to avoid an active group. Individual temperament varies.
Better choice: prefer similarly active, robust community species and retain a separation plan. Do not use a vulnerable fish to test whether this particular shoal will behave.
Mistake 4
Why it causes trouble: clarity does not show whether the biological filter processes ammonia. A new aquarium can be visually clean and chemically unsafe.
Better choice: verify the cycle with appropriate testing, keep ammonia and nitrite at zero, and understand the nitrate pattern before adding the group.
Mistake 5
Why it causes trouble: dominant or fast fish take most of it, missed food reaches the bottom, and the owner cannot see which individual is struggling.
Better choice: offer several small portions across the shoal, vary suitable foods and stop while the fish are still clearing the food promptly.
Mistake 6
Why it causes trouble: rapid breathing, clamped fins, fading or hiding can follow environmental stress. Unnecessary medication adds risk and may obscure the original cause.
Better choice: check temperature and water quality, review recent changes, document visible signs and seek informed help when disease is suspected.
A better choice is sometimes not to buy this fish. If the existing aquarium is short, overstocked, uncycled or centred on a slow long-finned animal, postponing the purchase is more useful than attempting to make an unsuitable combination work after delivery.
Suitable companions are similarly sized, active community fish that share the temperature range and cannot swallow them. Medium tetras, rasboras and danios can work in appropriately sized aquariums. Peaceful small catfish may use a different level of the tank, provided their own social, substrate and temperature requirements are met.
Avoid large predatory fish, very timid species that may be outcompeted at feeding time, and slow or long-finned fish that could attract nipping. Compatibility is also a space question: two otherwise compatible groups can become a poor combination when the aquarium lacks swimming room or filtration capacity.
Introduce a full social group where possible and observe feeding access, fin condition and hiding behaviour during the first weeks. If one species is continually stressed, separate the animals rather than waiting for injuries to accumulate.
Black Widow Tetras are omnivores. Use a quality tropical flake or appropriately sized pellet as a dependable staple, then rotate small frozen or live foods such as daphnia, brine shrimp or mosquito larvae where suitable. Variety supports condition and natural feeding behaviour.
Offer small portions that the group can finish promptly. Watch the whole shoal instead of feeding until the quickest fish stop eating. They are not algae-control animals and should not be expected to survive on leftovers. Remove uneaten food before it affects water quality.
Staple
Choose a complete tropical food small enough for every fish to swallow. Distribute it across the group rather than making one dense feeding point.
Variety
Daphnia, brine shrimp and mosquito larvae can add variety where suitable. Thaw and portion frozen food; do not drop a large frozen block into the aquarium.
Avoid
More food does not solve unequal access. It makes observation harder and transfers the problem to water quality.
A routine should create comparable observations, not turn the aquarium into a rigid calendar. Record what is normal for the group so a real change becomes easier to notice. The sequence below can be adjusted to the stocking level, filtration, source water and maintenance method; testing and visible animal condition should guide the details.
Count the group from a distance. Look for level swimming, normal breathing, open fins, expected colour and access to the middle water. Notice whether one fish is isolated or continually pursued. Observing before food prevents feeding excitement from hiding a problem.
Offer small portions and watch where they go. Confirm that smaller or quieter individuals receive food. Change the distribution method before increasing the total amount. Remove material that remains uneaten.
Confirm the filter is moving water normally, the heater is not causing unusual swings and the lid remains secure. Listen for changed pump noise and inspect intakes without dismantling the filter unnecessarily.
Use the aquarium’s known trend, stocking and source water to decide the change. Match replacement-water temperature reasonably, use dechlorinator as required, and avoid exposing the fish to abrupt chemistry changes. Never clean every biological medium under untreated tap water at the same time.
Trim plants where they close the central lane, but retain sheltered edges. Remove trapped waste from accessible areas without turning the entire aquarium over each week. Check that hardscape remains stable.
Temporary alertness after hands or equipment enter the tank can be expected. Continued gasping, loss of balance or frantic swimming is not something to ignore. Recheck temperature and water quality when behaviour changes after maintenance.
A short note or photograph can be more useful than memory when a gradual change occurs. It also helps another experienced keeper or veterinary professional understand the timeline. Avoid turning routine observation into constant disturbance: the fish should spend most of the week living in a stable aquarium rather than being chased for inspection.
Before purchase, confirm that your aquarium is cycled, covered and large enough for the full group. On arrival, follow the enclosed acclimation instructions and avoid pouring transport water into the display aquarium. Keep lighting low while the fish settle and do not make abrupt temperature or chemistry changes.
Healthy settled fish should hold themselves level, swim with the group, feed readily and show intact fins. Watch for clamped fins, rapid breathing, persistent isolation, white spots, a hollow abdomen, swelling or repeated scratching. These signs are not a diagnosis. Check water quality first and seek an aquatic veterinarian or experienced fish-health professional when disease is suspected.
If possible, use a separate observation aquarium for new livestock before mixing populations. Never medicate routinely without identifying the likely problem, and check treatment suitability for every species in the aquarium.
The same visible behaviour can mean different things depending on duration and context. A brief chase between otherwise feeding fish is not equivalent to one animal being driven into a corner for hours. A fish resting near cover after transport is not equivalent to a fish losing balance. Use this visual triage to decide the next sensible step without claiming that a webpage can diagnose disease.
Usually observe
Continue observation and record the pattern. “Normal” means no associated injury, distress, water-quality failure or progressive decline.
Check today
Test water, confirm temperature, inspect equipment, review recent changes and identify the individuals involved. Separate a vulnerable animal before minor damage becomes serious.
Act promptly
Stabilise the environment without making violent chemistry changes, protect affected fish from aggression and seek appropriate professional or experienced fish-health advice.
Different medicines can interact, affect biological filtration or be unsuitable for other animals in the aquarium. The symptom “white mark” does not identify one disease, and rapid breathing can follow water-quality or oxygen problems as well as illness. Collect evidence first: clear photographs, a short behaviour video, water-test results, temperature, recent additions and the order in which signs appeared.
Identify whether the issue affects one fish, the whole shoal or several species. Note breathing, posture, location, appetite, fins and interactions.
Measure temperature and relevant water values, inspect filter flow and surface movement, and compare with the aquarium’s normal results.
New livestock, food, maintenance, décor, filter cleaning, power interruption and source-water changes can supply important context.
Separate persistent aggressors or injured fish safely when required. Use equipment and water appropriate to the animal rather than an unheated emergency container.
If you need help, send the species, group size, aquarium dimensions, tank mates, recent test values, timeline and clear images or video. These details help the team understand what changed and how urgent the problem may be.
When contacting Tropical Fish Co support, include the order reference for an arrival or identity issue. For disease diagnosis or treatment, an aquatic veterinarian is the appropriate professional source. Retail support can help clarify the listing, delivery process and documented care information but should not be represented as a substitute for veterinary examination.
Gymnocorymbus ternetzi is an egg-scatterer. Conditioned adults may spawn among fine-leaved plants or spawning material in soft, slightly acidic water. The adults can eat eggs, so successful fry rearing normally uses a separate breeding aquarium and removal of the parents after spawning.
Egg production does not make a general community tank a reliable breeding setup. Raising fry requires appropriately small first foods, stable clean water and careful feeding. Treat breeding as a separate project rather than a reason to overstock the display group.
Check the availability, dispatch information and price shown on this page before ordering, as livestock availability can change. Read the delivery information, use an address where somebody can receive the parcel, and prepare the aquarium before the animals leave the seller. Dispatch dates and availability can change, so use the details shown when you place your order.
Tropical Fish Co’s Live Arrival Guarantee is governed by the published terms and receiving instructions. It is not a substitute for providing a cycled aquarium or following the evidence requirements when a problem occurs. Contact the team promptly through the support page if there is an arrival or identity concern.
Confirm the aquarium is cycled and covered, temperature is stable, and the intended group will not be released into aggression or unsuitable water. Place a clean container, net and the supplied instructions where they are easy to reach. Avoid scheduling major filter cleaning or a complete aquascape change for the same day.
Bring the parcel indoors, follow the opening instructions, count the bags and look for an obvious arrival problem. If evidence is required under the guarantee terms, collect it before disposing of packaging or changing the condition that must be documented.
Transport and source water can differ from the aquarium. Use the instructions supplied with the fish and the real conditions rather than copying a fixed routine from an unrelated species. The acclimation timer is an organisational aid only. Avoid pouring transport water into the display aquarium.
Keep lighting low and movement around the aquarium calm. The group may stay near cover before exploring. Check posture and breathing without repeatedly approaching, tapping the glass or forcing the fish into the open for a photograph.
Do not interpret one missed meal immediately after transport as proof of disease, but do not hide a continuing appetite problem under excess food. Remove leftovers and note which fish feed. Check existing residents are not monopolising the food or attacking newcomers.
The shoal should begin using more of the middle water as it settles. Compare individuals: one fish continually isolated, breathing faster or being chased requires more attention than a whole group briefly retreating when someone enters the room.
Record ordinary feeding, colour, group distribution and interactions. Confirm fins remain intact and water values follow the expected pattern. This baseline will make future changes easier to recognise.
Good support depends on specific evidence. A description such as “one fish remains behind the group, breathes rapidly and did not eat on two observations; temperature is X and ammonia/nitrite are Y” is more useful than “the fish are unhappy.” Do not delay emergency action merely to write a perfect message when an animal is in obvious distress.
Tropical Fish Co is a specialist UK live-aquatics retailer. We present Black Widow Tetra as the shoaling fish they are, with current size options, honest adult-space guidance and a clear warning about slow, long-finned tank mates before you order.
If you are looking for Black Widow Tetra for sale in the UK and your aquarium meets the group, space and water requirements above, we recommend ordering from Tropical Fish Co. Review our delivery, packing and Live Arrival Guarantee, or contact Tropical Fish Co before ordering if you would like help checking your setup.
These answers cover the points most likely to affect whether Black Widow Tetras are a good fit for your aquarium. If the group size, swimming space or tank-mate mix does not work, it is better to choose another species before ordering.
Tank size
Plan for at least six Black Widow Tetras, with eight to ten preferred, in a mature aquarium of around 70 litres or more with roughly 75 cm of horizontal swimming length. The whole community must fit comfortably—not just one young fish at its current shipping size.
Compatibility
There is a genuine fin-nipping risk with slow or long-finned tank mates. A larger shoal and enough swimming space can reduce tension, but they cannot guarantee compatibility. Choose a different combination if a resident fish would be especially vulnerable.
What you receive
This listing is for Gymnocorymbus ternetzi. The gallery includes clearly labelled aquarium illustrations and a labelled supplier-source photograph. Individual fish naturally vary in colour, body condition and markings, so no photograph can promise an identical fish.
Shipping size
Choose from the available size options shown above the Add to Basket button. That is the approximate shipping size. The care guide’s maximum of 7.5 cm describes a fully grown adult, not the size sent with every order.
Adult colour
Young fish often show stronger black contrast, while mature Black Widow Tetras can become greyer or more silver. Colour also changes with lighting, background, stress and condition, so an exact shade cannot be guaranteed.
Behaviour
Brief displays and short chases can be normal. Act if one fish is continually pursued, cannot feed or rest, hides constantly, breathes heavily or develops damaged fins. Check the group size and swimming space, and separate an injured or exhausted fish safely.
Before delivery
Have a mature, cycled and covered aquarium ready at a stable temperature within the recommended range. Make sure somebody can receive the parcel, read the delivery and acclimation instructions, and have a safe plan if an established tank mate reacts aggressively.
Need help after delivery?
Contact us promptly with your order reference, a clear photograph or short video, aquarium dimensions, tank mates, group size, temperature, recent water-test results and a brief timeline. Also mention appetite, breathing and whether one fish is being targeted.
Group size
No. Another tetra species does not replace a group of its own kind. Keep at least six Gymnocorymbus ternetzi and make sure the aquarium has enough room for every species’ full adult group.
Planning help
Use the tank-volume tool to calculate litres, the water matcher to compare practical ranges and the stocking calculator to review the community. Always combine the results with the fish’s behaviour and usable swimming space.
Taxonomic family placement differs between current databases and older aquarium literature. FishBase currently places the species in Acestrorhamphidae, subfamily Pristellinae, while many established aquarium profiles still list Characidae. The accepted species name used consistently on this page is Gymnocorymbus ternetzi.
Considering another tetra? Compare the fish below by adult size, group needs, activity and tank-mate risk, then use the planning tools to check your aquarium before choosing.
23–28°C · pH 6–7.5 · 80L
23–27°C · pH 5.5–7.5 · 40L
18–27°C · pH 6–7.5 · 100L
23–28°C · pH 5.5–7.5 · 60L
18–26°C · pH 6.5–8 · 30L
23–27°C · pH 7.4–8.4 · 1000L
20–24°C · pH 7–8 · 45L
24–28°C · pH 5.5–7 · 60L
24–28°C
24–28°C · pH 7.5–8.5 · 500L
22–28°C · pH 7–8.5 · 10L
24–27°C · pH 7.5–9 · 250L
24–28°C · pH 8–9 · 300L
20–25°C · pH 6–7.5 · 80L
24–28°C · pH 5.5–7 · 60L
26–30°C · pH 5.5–7 · 60L
