False Black Tetra

About

Gymnocorymbus thayeri is the tetra that gets sold as a black skirt tetra and is not one. The two are sister species - Benine et al.'s 2015 revision of the genus recovers G. ternetzi and G. thayeri as each other's closest relatives - and the thing that separates them is the black itself. The revision's diagnosis for G. ternetzi rests on 'the presence of a dense field of dark chromatophores spread homogeneously over the posterior one half of the body vs. the lack of such pigmentation in all congeners': that smoky rear half, the skirt the English name is built on, belongs to G. ternetzi alone. G. thayeri is a paler fish. Aquarium Glaser, importing it, describe 'a golden or silver basic colour and an attractive, copper-red eye' and sell it as the silver widow tetra. If you need certainty rather than an impression, the revision gives three countable characters for G. thayeri: two to four scale rows over the anal-fin base instead of five or six, an anal fin whose distal margin is straight or only slightly convex instead of strongly convex, and pelvic-fin rays i,7 instead of i,6. It is also the bigger fish, which is the part that matters for a stocking plan. The largest G. thayeri the revision measured was 80.9 mm SL, against 53.1 mm for the largest G. ternetzi, while FishBase puts the two level, at 7.4 cm SL against 7.5 cm, and so does not corroborate the difference. On the revision's own measurements, a shop tank of 4 cm juveniles is offering a fish that outgrows the cousin it is being sold as. They do not overlap in the wild either: G. ternetzi is a Rio Paraguay fish, while G. thayeri occurs in the Corantijn and Amazon basins and the rivers of north-eastern Brazil, with old records from Guyana and Trinidad unconfirmed and Orinoco records probably referring to G. bondi.

Care

Treat it as a warmer, softer black skirt tetra that needs more room. Eight or more in a tank from 100 litres with 80 cm of length, planted around the edges and open through the middle. FishBase gives this species 23-27 °C, pH 5.5-7.5 and hardness up to 20 °dGH, against 20-26 °C and pH 6.0-8.0 for G. ternetzi, so aim at the Amazon end of the scale - around 24-27 °C and pH 6-7 - rather than at hard alkaline tap water. Take those numbers as the best available rather than as corroborated: there is no Seriously Fish profile and no care sheet of any kind for this species, so FishBase is the only source behind every husbandry figure here, and this entry is rated medium confidence for that reason. On temperament, be careful what you carry over: our black skirt tetra entry flags that fish as an occasional fin-nipper, and no source we could find makes the same claim for G. thayeri, so we do not repeat it here. It is still a deep-bodied Gymnocorymbus in a shoal, so watch a new group around long-finned tankmates rather than assuming either way. Keep the tank covered.

Diet

Adaptable community omnivore. Quality dry food as the base with regular frozen or live daphnia, bloodworm and mosquito larvae. Nothing published describes its wild diet specifically.

Telling males from females

What works for this species, most reliable first.

• Belly depthA hint only · only on well-fed adults, and only as a comparison

  For most tetras, barbs and rasboras this is all there is: a conditioned female is deeper through the belly than a male of the same length.

Also in these lists

Similar species

Same category and the closest water parameters — if this species isn't quite it, start here.

Sold in Lithuania

These Lithuanian shops listed this species in their public catalogue (checked August 8, 2026). Stock moves — ask before you travel.

These links are unpaid: we get nothing for them and ask for nothing.

Frequently asked questions

What tank size does a False Black Tetra need?

A False Black Tetra needs an aquarium of at least 100 litres (26.4 US gallons) with a footprint at least 80 cm (31.5 in) long — for this species, swimming length matters as much as raw volume.

What water parameters does a False Black Tetra need?

False Black Tetra does best at 22–28 °C (71.6–82.4 °F), pH 5.5–7.5 and a general hardness of 1–20 °dGH.

What group size does a False Black Tetra need?

False Black Tetra is a social species — keep a group of at least 8 so natural behaviour emerges and stress stays low.

Is the False Black Tetra suitable for beginners?

Yes — False Black Tetra is one of the easier species to keep and forgives typical first-tank mistakes, as long as the aquarium is cycled and maintained.

Sources

• FishBase — Gymnocorymbus thayeri (False black tetra)
• Plazi TreatmentBank — Gymnocorymbus thayeri, in Benine et al. 2015 (Zootaxa 3956)
• Plazi TreatmentBank — Gymnocorymbus ternetzi, in Benine et al. 2015 (Zootaxa 3956)
• Zootaxa 3956(1) — Benine et al. 2015, revision of Gymnocorymbus (abstract)
• Aquarium Glaser — Gymnocorymbus cf. thayeri
• Wikipedia — Gymnocorymbus thayeri