Longfin White Widow Tetra (Gymnocorymbus ternetzi)

Meet the Longfin White Widow Tetra

The Longfin White Widow Tetra is a pale, long-finned form of Gymnocorymbus ternetzi, the hardy black widow tetra from South America. It keeps the species' deep, diamond-shaped body but combines two traits bred for the aquarium: a pearly white (leucistic) colour with dark eyes, and extended, flowing fins. The dorsal fin rises tall and sweeps back, the anal fin trails in a long skirt, the pelvic fins stream behind and the tail lobes are drawn out, so a shoal looks almost veiled as it moves.

Seriously Fish lists long-finned and leucistic forms as man-made varieties selectively bred for the trade, and notes that every widow tetra on sale is captive bred. The wild fish comes from slow-moving creeks with dense overhanging vegetation in the Rio Paraguay and Guaporé basins of Brazil, Bolivia and Argentina. Those calm, plant-edged waters are a good guide to keeping the long-finned form well: gentle flow, soft edges and plenty of cover.

Compare it with the other widow tetras

All of them are one species with the same care, so they can share a shoal. FishBase places the genus in the family Acestrorhamphidae, the American tetras, while Seriously Fish and OATA still use the older family Characidae. See the whole group on our Gymnocorymbus category page or browse all tetras for sale.

Know what one purchase supplies

One purchase is one live Longfin White Widow Tetra at a medium-large size, priced per fish. It is a shoaling fish, so choose the number you need for a group. The photographs show the colour and fin form, not the individual fish; they were prepared from a reference image of this form, and fin length, sheen and body shape vary a little from fish to fish.

Long fins are delicate in transit, and a small nick or a slightly frayed edge on arrival usually regrows in clean water. Unpack promptly, keep the lights low and follow the acclimatisation steps below. Our delivery and Live Arrival Guarantee page explains dispatch days and what to do if anything is wrong on arrival.

Order the shoal together

• Six: the practical minimum for a shoal; FishBase and OATA say at least five.
• Twelve: the shoal size Seriously Fish recommends to keep nipping within the group.
• One order: adding the whole shoal at once means the fish settle together instead of newcomers being chased.

Give long fins room to flow

The body grows to about 6–7.5 cm: Seriously Fish gives 60 mm standard length and FishBase 7.5 cm standard length, both measured without the tail. The long fins add noticeably to the overall size and to the space the fish needs to turn and swim without catching them on décor.

Seriously Fish recommends a base of at least 75 × 30 cm, about 68 litres, for a group, FishBase a minimum aquarium length of 60 cm, and OATA at least 80 litres for tetras of 5 cm or more. For a shoal of twelve long-finned fish, the larger figure gives the fins room.

Check space and current

1. Length first: a long tank with a base of at least 75 × 30 cm gives a shoal a clear swimming run.
2. Gentle current: OATA recommends a gentle current for smaller tetras; long fins tire quickly in strong flow, so spread the filter outlet or point it at the glass.
3. Whole community: count every fish at adult size, using our stocking calculator.

If you are setting up from scratch, our first tropical tank guide explains how to mature the filter before the fish arrive.

Plant for shade and soft edges

Seriously Fish describes the widow tetra as an active fish that wants open swimming space with some quieter areas, and says it shows its best colours with dense planting and floating plants that dim the light. For a long-finned form, choose décor with soft edges: fine-leaved and broad-leaved plants, smooth wood and rounded stones rather than sharp rock or rough ornaments that can split fins.

Divide the tank by function

OATA recommends gravel or sand and, ideally, live plants for cover. Place the aquarium away from draughts, heat sources and direct sunlight, and cover filter intakes with a sponge so trailing fins cannot be pulled against them.

Match water values to the published ranges

Seriously Fish calls captive-bred widow tetras unfussy about water chemistry, but its range differs from FishBase and OATA. Aim for the values all three accept, and keep them steady.

Separate habitat values from waste readings

If your tap water is hard and alkaline, as it is in much of the UK, the fish can still do well: FishBase and OATA both accept up to pH 8.0. Steady water is better than water adjusted up and down. Compare your own readings with these ranges using our water matcher, and see the water chemistry guide for what each value means.

Keep water clean to protect the fins

Long fins are the first thing to suffer when water quality slips. Ragged edges, milky margins or fins that fray back towards the body can all follow a build-up of waste, so clean, well-filtered water is the best protection. For a 68-litre tank, Seriously Fish suggests a filter rated at 270–340 litres per hour and a heater of roughly 50 W in a room at about 20°C; spread the filter return so the flow stays gentle.

Follow a weekly maintenance sequence

1. Test ammonia, nitrite and nitrate first; any reading of ammonia or nitrite above zero needs action before anything else.
2. Change at least 25% of the water each week, as OATA recommends.
3. Dechlorinate the new water and match its temperature before adding it.
4. Siphon food and waste from the substrate and from among the plants.
5. Rinse filter media lightly in removed tank water, never under the tap, and check the intake sponge.
6. Look over every fish's fins while the lights are on and note any change.

Our guide to aquarium filtration explains how to keep good flow without strong currents.

Feed a varied diet in fair shares

The widow tetra is "a most unfussy, omnivorous species", according to Seriously Fish, which recommends flakes and granules with regular small live and frozen foods such as Daphnia and bloodworm. FishBase records that wild adults live in the middle and upper water layers and eat worms, small crustaceans and insects. Long fins add drag, so watch that every fish in the shoal gets its share when faster tank mates join in at feeding time.

Build one measured meal

• Dry staple: a good flake or small granule, small enough to swallow whole.
• Frozen or live: Daphnia, bloodworm or brine shrimp a few times a week, in place of one dry meal.
• Quantity: what the shoal eats within a few minutes, once a day (OATA).
• Placement: spread food across the surface, away from the filter outlet, so it does not sweep past the slower fish.

Overfeeding does the fins no favours: leftover food breaks down into the waste that fin rot follows. Remove anything uneaten after a few minutes.

Settle the shoal in its first week

Before the fish arrive, confirm that ammonia and nitrite read zero and that temperature, pH and hardness are in range. Use OATA's floating-bag method with the lights off: float the bag for 10 minutes, add small amounts of tank water over up to 20 minutes, then net the fish gently into the tank, letting in as little bag water as possible. Handle long-finned fish with a soft, fine net. Our guide to acclimatising new fish covers each step.

Use a first-week checkpoint plan

Small splits from the journey usually close in clean water. Fins that keep getting worse, or a fish that hides alone and stops feeding, need a water test and a look at the tank mates.

Choose companions that leave fins alone

The long-finned form brings a two-way question. The widow tetra itself has a reputation as a fin nipper, which Seriously Fish says can usually be fixed by keeping at least 12 together. At the same time, its own long fins are a target for other nippers, so the safest communities are calm ones.

Screen companions with suits and avoid

Our tank mates checker compares temperature and adult size, and our tetra guide suggests calm community mixes.

Watch fins and body for early warning signs

Check the shoal at every feed while the fish are out in the open. OATA lists the changes worth noticing in any fish: swimming, colour, temperament, breathing, appearance, body condition and feeding. In a long-finned fish, the fins are the early warning.

Pair each observation with an action

If signs continue after the water is corrected, contact us with your test results and tank details.

Breeding longfin widow tetras

Seriously Fish describes the species as quite easily bred in a separate, dimly lit tank of about 18 × 12 × 12 inches, with java moss, spawning mops or a mesh base to protect the adhesive eggs. A group of about six of each sex can spawn together, or a conditioned pair can be moved to a spawning tank at about 28–30°C with slightly acidic water after conditioning at about 24–25.5°C; they usually spawn the next morning. Remove the adults once eggs appear. The eggs hatch in 18–36 hours, the fry swim freely a few days later and take infusoria first, then microworm or brine shrimp nauplii, in dim light.

Males are slimmer and a little smaller than females, with more pointed dorsal and anal fins, according to Seriously Fish; in a long-finned form the extended fins can make that fin difference harder to judge, so body shape is the easier guide.

Answer Longfin White Widow Tetra buying questions

Is the Longfin White Widow Tetra a different species?

No. It is a selectively bred form of Gymnocorymbus ternetzi, the black widow tetra, combining the pale white (leucistic) colour with long fins. Seriously Fish lists both long-finned and leucistic forms as man-made varieties bred for the trade.

Will other fish nip its long fins?

They can. Keep it away from known fin-nippers such as tiger barbs and serpae tetras, and keep the widow tetras themselves in a full shoal so they squabble among themselves rather than with other fish.

Do longfin widow tetras nip fins themselves?

The species has a reputation as a bit of a fin nipper. Seriously Fish says this can usually be fixed by keeping a shoal of at least 12, when any squabbling generally stays within the group.

How many should I buy?

Six at the very least, and twelve if you can. FishBase and OATA give five or more as the minimum, and Seriously Fish recommends at least twelve to curb nipping.

How big does it grow?

About 6–7.5 cm in body length, plus its long fins. Seriously Fish gives 60 mm standard length and FishBase 7.5 cm standard length; neither includes the tail.

What tank and water does it need?

A tank with at least a 75 × 30 cm base (about 68 litres, Seriously Fish), or 80 litres for a full shoal (OATA), with a gentle current. Aim for 22–26°C, pH 6.5–7.0 and 5–12 dGH, where the sources overlap.

Can I mix it with other widow tetras?

Yes. Black, gold, white and long-finned widow tetras are all one species and can share a shoal. Long-finned fish do best when none of the shoal is short-finned and boisterous enough to chase them.

How do I keep its fins in good condition?

Keep the water clean with weekly 25% water changes, avoid sharp décor and strong currents, keep a full shoal and avoid fin-nipping tank mates. Ragged fin edges with white margins can mean fin rot, so test the water at once.

Consult the care references

• Seriously Fish: Gymnocorymbus ternetzi, Black Widow Tetra — man-made long-finned and leucistic forms, distribution, habitat, size, aquarium base, décor, water, filter and heater sizing, diet, fin-nipping and shoal size, companions, sexing and breeding.
• FishBase: Gymnocorymbus ternetzi, Black tetra — family Acestrorhamphidae, distribution, environmental ranges, maximum length, wild diet, group size and minimum aquarium length.
• OATA: How to look after tetras and pencilfish — UK group guidance on water quality, gentle current, aquarium size, maintenance, acclimatisation, feeding and warning signs.
• Tropical Fish Co delivery policy — current dispatch days and Live Arrival Guarantee terms.