Black Widow Tetra (Gymnocorymbus ternetzi)

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.

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.

Quick facts

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

Before you buy

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.

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.

Is a Black Widow Tetra right for your aquarium?

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.

Identity, appearance and the supplier photograph

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.

Interesting facts

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.

Natural range and habitat

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.

Aquarium size and setup

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.

• Swimming space: keep a clear central or front-to-middle lane for the shoal.
• Cover: use rooted plants, wood and optional floating plants around the margins.
• Substrate: sand or smooth gravel both work; this is a midwater species and does not require a specialised bottom.
• Flow: moderate circulation is suitable. Avoid turning the entire aquarium into a high-velocity channel.
• Lid: use a secure cover because startled tetras can jump.
• Filtration: size the filter for the complete community and maintain it without destroying all biological media at once.

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.

Water parameters and routine care

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.

Shoal size, behaviour and fin-nipping risk

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.

Common keeper complaints and likely causes

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.

Common mistakes and better choices

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.

Tank mates

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.

Feeding

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.

A practical weekly care routine

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.

1. Daily

   Observe before feeding

   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.

2. At meals

   Check access, not only appetite

   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.

3. Through the week

   Check equipment and temperature

   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.

4. Maintenance day

   Test and perform an appropriate partial water change

   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.

5. During cleaning

   Protect swimming room and cover

   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.

6. After maintenance

   Compare behaviour with the baseline

   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.

Health, acclimation and welfare checks

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.

Troubleshooting: normal, check or act

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.

A sensible troubleshooting order

1. 1

   Observe without chasing

   Identify whether the issue affects one fish, the whole shoal or several species. Note breathing, posture, location, appetite, fins and interactions.

2. 2

   Check the environment

   Measure temperature and relevant water values, inspect filter flow and surface movement, and compare with the aquarium’s normal results.

3. 3

   Review recent changes

   New livestock, food, maintenance, décor, filter cleaning, power interruption and source-water changes can supply important context.

4. 4

   Protect vulnerable animals

   Separate persistent aggressors or injured fish safely when required. Use equipment and water appropriate to the animal rather than an unheated emergency container.

5. 5

   Contact support with the essentials

   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.

Breeding overview

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.

Delivery and the first seven days

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.

1. Before arrival

   Prepare a stable destination

   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.

2. At the door

   Receive and inspect without unnecessary delay

   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.

3. Acclimation

   Follow the supplied method

   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.

4. First hours

   Reduce stimulation and observe breathing

   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.

5. Day 1

   Offer modest food only when the group is settled enough to respond

   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.

6. Days 2-3

   Look for a developing pattern

   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.

7. Days 4-7

   Move from arrival checks to the normal care baseline

   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.

Why buy Black Widow Tetra from Tropical Fish Co?

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.

Common questions before buying

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.

Sources and identification notes

• FishBase: Gymnocorymbus ternetzi — accepted name, range, maximum recorded length and environmental data.
• Seriously Fish: Black Widow Tetra — aquarium dimensions, temperature, pH, hardness, diet, behaviour and breeding summary.
• Fishkeeper: Black Widow Tetra — UK-oriented size, temperature, water, shoal, feeding and compatibility guidance.
• Aquarium Industries: Black Widow Tetra — supplier-sector confirmation of common name, schooling behaviour and diet.

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.

Related fish and useful tools

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.

Planning tools