Black Widow Tetra (Gymnocorymbus ternetzi)

Decide whether this active tetra fits your aquarium

The Black Widow Tetra (Gymnocorymbus ternetzi) is a freshwater American tetra in the family Acestrorhamphidae. It occurs in South America from the Paraguay and Guaporé river basins to Argentina. FishBase records a maximum adult length of 7.5 cm standard length and places adults in the middle and upper water layers. This lively shoaling fish suits a prepared beginner who can provide a mature filtered aquarium, the full social group and compatible companions.

Keep at least 5 together and plan around an aquarium base of 75 × 30 cm with approximately 68 litres. Seriously Fish recommends that footprint for a group, along with open swimming space and quieter planted areas. The adult group, rather than one small dispatch fish, determines whether the aquarium is large enough.

Keep the water at 22–26°C, pH 6.5–7.0 and 5–12 dGH. These routine values are the overlap between FishBase, Seriously Fish and OATA’s tetra and pencilfish ranges; OATA covers the wider group rather than a species-specific optimum. Maintain zero ammonia and zero nitrite, and keep nitrate from exceeding 20 mg per litre above the normal tap-water level.

The species is generally peaceful and lively, but it has a fin-nipping reputation. Seriously Fish says that keeping at least 12 can usually rectify this behaviour because squabbling is generally contained within the group. Treat 5 as the social floor and the larger number as a specific management option, subject to aquarium capacity.

Choose the dispatch size and understand the sold unit

One purchase supplies one Black Widow Tetra in the size chosen on the live size selector. The labels are 1-1.5 cm, 1,5 - 2 cm, 2,5 - 3 cm, 3 - 4 cm, 4 - 5 cm and XL (5 - 6cm). These measurements describe dispatch grades, so use the selector label to understand the approximate size sent.

The size you select is the size supplied, not the adult size. Plan the aquarium, filtration and social group around the recorded adult length of 7.5 cm. A smaller dispatch grade does not reduce the long-term footprint or group requirement, and a larger grade does not indicate sex, life stage or origin.

Plan the whole group before choosing a grade

• Sold unit: each purchase quantity represents one fish of the selected size.
• Social plan: provide at least 5 Black Widow Tetras rather than treating one purchase as a complete shoal.
• Space plan: reserve approximately 68 litres and a 75 × 30 cm base for the group.
• Adult plan: use 7.5 cm for stocking and long-term space decisions.
• Alternatives: compare other fish in the current tetra collection when this footprint or social requirement does not fit.

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

Some gallery images have been digitally enhanced to improve image quality. They show the listed type of animal rather than guaranteeing the exact markings of the individual supplied, so natural colour and pattern can vary.

Choose by the needs of the completed group, not by which dispatch grade looks easiest to place temporarily. Before adding any fish, confirm that the mature filter can process the group’s waste and that the existing community leaves enough open middle and upper water for active swimming.

Build around a long, open swimming area

Use an aquarium with a base of at least 75 × 30 cm and approximately 68 litres for a group. Seriously Fish describes this active species as fairly unfussy about décor, but it benefits from plenty of open swimming room together with quieter areas. A tank can hold enough litres while still being a poor shape, so confirm its actual base dimensions rather than relying on a model name.

Divide the layout into movement and cover

• Open water: preserve a clear route through the middle and upper levels where adults spend most of their time.
• Planted areas: use dense planting in selected parts of the aquarium instead of filling the entire swimming area.
• Quieter areas: provide places where fish can move away from the most active part of the group.
• Floating vegetation: this can dim the light and help the fish show their colour while leaving useful water beneath it.
• Décor: arrange plants and ornaments so they add cover without reducing the recommended footprint.

For an aquarium of about 68 litres, Seriously Fish describes a turnover of four to five times the aquarium volume per hour. That corresponds to roughly 270–340 litres per hour before real-world losses caused by filter media, pipework and maintenance condition. Use the figure as a filter-rating guide, then confirm that circulation remains effective throughout the aquarium.

A heater of roughly 50 W is suggested for this aquarium size when the room is around 20°C. Check the actual aquarium temperature rather than treating heater wattage as a thermostat setting. A different room temperature, aquarium shape or cover can change the heating requirement.

Prepare and mature the biological filter before the fish arrive. Our first tropical aquarium guide explains the preparation sequence. The aquarium is ready for livestock only when testing shows that ammonia and nitrite remain at zero and the filter can support the planned increase in waste.

Hold stable water inside the shared range

Keep the aquarium at 22–26°C, pH 6.5–7.0 and 5–12 dGH. This range sits inside every water range used for the care plan, making it a practical routine target without pushing beyond the species figures. Measure the aquarium rather than repeatedly adding products to chase a number that has not been tested.

Use the nitrogen readings as firm limits

Test at least weekly and record enough results to recognise the aquarium’s normal pattern. If ammonia or nitrite appears, stop treating the tank as stable: check the filter, recent stocking, feeding and maintenance before adding more livestock. Nitrate must be interpreted against the normal tap-water reading because OATA applies the 20 mg-per-litre limit above that baseline.

The aquarium water chemistry guide explains what each reading means. Use the water matcher to compare measured values with the stated range, while retaining the tighter 6.5–7.0 pH target used here.

The RSPCA explains why a working biological filter is essential: fish release toxic ammonia, bacteria convert it to toxic nitrite and then to safer nitrate. Clear-looking water does not replace those tests. Keep the filter running and maintained so its bacterial population continues processing waste between water changes.

Feed an omnivorous midwater fish without leaving waste

Use a good-quality flake or granule as the regular aquarium food. In nature, FishBase records Black Widow Tetras eating worms, small crustaceans and insects, while Seriously Fish describes the species as an unfussy omnivore. Add suitably small live or frozen foods such as daphnia and bloodworm for variety; OATA also names tubifex among suitable small foods for tetras.

Feed once a day and offer only what the fish can eat within a few minutes. That schedule comes from OATA’s guidance for tetras and pencilfish. Watch the whole group during the meal so that feeding remains an observation of access and condition rather than simply a measured amount dropped into the aquarium.

Build each meal around three decisions

Do not answer unequal feeding access by adding excessive food. Uneaten material breaks down and releases waste, which increases the load on the biological filter. Instead, watch whether one fish repeatedly misses food, approaches without eating or has become thinner or fuller than its companions.

A reduction in food intake is one of OATA’s warning signs. If appetite changes, test the water and compare swimming, breathing, colour and group behaviour with the fish’s normal pattern. Continue using foods that match the recorded omnivorous diet rather than depending on leftovers from other aquarium inhabitants.

Use a repeatable weekly maintenance routine

At least once every week, change 25% of the aquarium water. Use the same routine to inspect the aquarium, remove settled waste and compare the latest test results with the established pattern.

Complete maintenance in a safe order

1. Test temperature, ammonia, nitrite and nitrate before disturbing the aquarium.
2. Observe swimming, breathing, colour, fin condition and feeding response.
3. Remove 25% of the water and use a siphon to collect solid waste from accessible substrate.
4. Treat replacement tap water with conditioner to remove harmful chlorine or chloramine before adding it.
5. Check the filter for blockages and confirm that normal water movement returns after maintenance.
6. Record any changed result or behaviour so it can be compared at the next check.

If the filter needs cleaning, do not run it under the tap. Rinse it lightly in the aquarium water removed during the partial change. This reduces the loss of the beneficial bacterial population established in the media. Clean only when required for flow or blockage management rather than dismantling a functioning filter without reason.

Good maintenance protects the readings used elsewhere in the care plan. A missed water change, excess feeding or blocked filter can alter several conditions together, so respond to a change by checking the whole routine rather than concentrating on one number.

Keep a short weekly record

• Water: temperature and test readings.
• Equipment: filter flow and any blockage found.
• Group: every fish seen, feeding and using the expected water level.
• Condition: no new fin loss, scale damage, swelling or weight change.
• Behaviour: no persistent isolation, unusual hiding or increased aggression.

Follow the arrival sequence before release

Read the current delivery and Live Arrival Guarantee policy before the parcel is due. Someone must be at the address to receive live fish, and the parcel must be opened and inspected on the same day it arrives.

Use this arrival sequence

1. Receive the parcel in person. Live fish cannot be left unattended.
2. Start filming before opening. Film every delivery from the sealed parcel onward, with the outer box visible.
3. Open it in one continuous take. Show the fish still inside the sealed bag before anything is disturbed.
4. Inspect the parcel the same day. Retain the continuous video with the order details.
5. Float the bag. Float the bag for 20–30 minutes before releasing the fish.

If there is a live-arrival concern, contact us the same day with the order number and continuous unboxing video. The video must begin with the parcel sealed, show it being opened and show the fish in the sealed bag before release.

Check the aquarium separately

Before adding the fish, OATA advises confirming that ammonia and nitrite are zero. Add livestock only at a rate the mature filter can support, while retaining the social plan of at least 5 Black Widow Tetras. Increasing fish numbers and feeding increases the bacterial capacity required from the filter; adding too much waste too quickly can lead to unhealthy ammonia and nitrite.

After release, observe swimming, breathing, colour, feeding and interaction with the group. Establish what is normal for these fish during the first week so later changes are easier to recognise. Avoid interpreting one behaviour alone; compare it with the water tests and the condition of the rest of the aquarium.

Act on changes before they escalate

Check the fish every day for changes in swimming, colour, breathing, appearance, body condition, temperament and feeding. OATA identifies these as useful early indicators because each aquarium and fish develops a normal pattern that can be compared over time.

Pair each observation with a useful response

Healthy fish should have clear, bright eyes, undamaged fins, intact scales and no ulcerations or bumps. These checks work best when combined with regular testing, because ammonia, nitrite, temperature and filter performance can affect several fish together.

Do not choose a treatment from a single general sign. Begin with measured water values, equipment checks, recent maintenance and a comparison of affected and unaffected fish. Keep notes or clear side-on images so changes in body condition or fin damage can be followed accurately.

If you remain concerned about health after testing the water, contact us with the aquarium dimensions, group size, companions, temperature, ammonia, nitrite, nitrate and a short description of when the change began. Prompt, specific observations are more useful than waiting for several signs to become severe.

Plan growth around the recorded maximum

Plan for an adult length of 7.5 cm standard length. This is the FishBase maximum used throughout the care plan, stocking decisions and search fields. Standard length excludes the tail, so use the figure as a serious space reference rather than treating it as the full visual extent of the fish.

Seriously Fish uses 6 cm standard length in its aquarium profile. Keep 7.5 cm as the planning figure rather than averaging the two values or substituting a smaller dispatch grade. The difference does not change the recommendation for an aquarium base of 75 × 30 cm and approximately 68 litres for the group.

Separate dispatch size from long-term planning

• Dispatch grade: describes the approximate length sent for the chosen selector label.
• Adult length: determines long-term space, stocking and filtration planning.
• Social requirement: remains at least 5 fish regardless of dispatch grade.
• Sex and life stage: cannot be inferred from a size label.

Juveniles can show more intense black colour, and Seriously Fish notes that this colour fades as the fish grows. Adult fish can therefore provide a softer contrast than younger animals without becoming a different species. Evaluate sudden colour change alongside breathing, appetite, water quality and behaviour rather than using shade alone.

The familiar Black Skirt Tetra and Black Tetra names also refer to Gymnocorymbus ternetzi. Keep the scientific name attached to the care plan when comparing aquarium advice, because common names can vary between shops and regions.

Manage the shoal before judging temperament

Keep at least 5 Black Widow Tetras together. FishBase uses 5 or more for aquarium keeping, and OATA says tetras should be kept in groups of at least five because single fish or groups that are too small often become stressed. More can be beneficial, but the aquarium must have the footprint, filtration and water quality to support the larger group.

Fin nipping needs a more specific response. Seriously Fish says this behaviour can usually be rectified by keeping a small shoal of at least 12 specimens; with this number, squabbling is generally contained within the group. This is permission to use a larger group as a management approach, not an instruction to place 12 fish in an aquarium that cannot support them.

Read the interaction rather than one short chase

Adults naturally use the middle and upper water layers, so judge space where the group actually swims. A community can have unused substrate while its midwater zone is crowded. Include all active groups when assessing the 75 × 30 cm footprint, filter capacity and feeding competition.

Temperament is best assessed as a combination of group size, visible fin condition, access to food and the response of companion fish. The label “generally peaceful” remains useful, but it does not erase the documented fin-nipping reputation.

Screen every companion before mixing

Choose companions that share the water range, have compatible temperaments and leave enough adult space for every social group. OATA’s broad rule is that tetras and pencilfish mix best with fish of similar size and temperament. Seriously Fish names several potential companions for Black Widow Tetras, but each combination still depends on the complete aquarium.

Use named companions as candidates, not guarantees

Use caution with slow or long-finned fish because this tetra has a fin-nipping reputation. A larger Black Widow group can usually contain its squabbling more effectively, but that does not make every long-finned companion dependable. Inspect fins and feeding access after any introduction.

Do not turn a named companion into a universal pairing. Two fish can share a temperature range while competing for the same swimming zone, and a tank large enough for one group may not have enough room for two. Check adult dimensions, social numbers, filter capacity and temperament together.

Apply one rule to every proposed community

Keep only combinations in which all animals fit the same measured water, the aquarium supports every adult group and no resident is being persistently targeted or excluded from food. Individual and group behaviour can vary, so continued observation remains part of the compatibility decision.

Answer the main questions before purchase

How big does a Black Widow Tetra get?

FishBase records a maximum adult length of 7.5 cm standard length. Plan the aquarium around 7.5 cm even when the selected dispatch fish is smaller.

What size aquarium does a Black Widow Tetra need?

Seriously Fish recommends a 75 × 30 cm base and approximately 68 litres for a group. Preserve open swimming room as well as quieter planted areas.

How many Black Widow Tetras should I keep together?

Keep at least 5, as FishBase and OATA advise. Seriously Fish says at least 12 can usually rectify fin nipping by containing squabbling within the group.

Is a Black Widow Tetra suitable for beginners?

Yes, it can suit a prepared beginner. Seriously Fish calls it one of the best choices for newcomers, but it still needs a mature filter, a proper group and weekly maintenance.

What does a Black Widow Tetra eat?

Feed quality flake or granules, with small daphnia, bloodworm or tubifex for variety. OATA’s tetra guidance says to feed once daily, giving only what is eaten within a few minutes.

What temperature and pH does this fish need?

Keep it at 22–26°C, pH 6.5–7.0 and 5–12 dGH. These are the overlapping FishBase, Seriously Fish and OATA ranges.

Is it aggressive or likely to nip fins?

It is generally peaceful and lively but has a fin-nipping reputation. Seriously Fish says a group of at least 12 can usually contain squabbling within the group.

Which fish can it live with?

Screen most livebearers, danios, rasboras, other tetras, Corydoras, smaller Loricariids, robust dwarf cichlids and angelfish for shared water, adult space and temperament. No named pairing is automatic.

How does this fish arrive alive in the UK?

Start filming the sealed parcel before opening it or floating the bag. Have someone receive it, film the continuous opening and fish in the sealed bag, then float the bag for 20–30 minutes before release; fish are hand-bagged in pure oxygen inside an insulated box, with a heat pack when weather calls for it.

Consult the public care references

• FishBase: Gymnocorymbus ternetzi — scientific identity, family, distribution, environment, adult length, wild diet, swimming level and aquarium group figure.
• Seriously Fish: Black Widow Tetra — aquarium footprint, water ranges, décor, filtration, feeding, behaviour, compatibility and colour development.
• OATA: How to look after tetras and pencilfish — group care, feeding, testing, maintenance, introduction and health observations.
• RSPCA aquarium environment advice — biological filtration and nitrogen-waste processing.
• Tropical Fish Co delivery policy — receiving, continuous unboxing video, packing and bag-floating steps.