Black Widow Tetra (Gymnocorymbus ternetzi): Complete Guide to Care, Breeding, and Varieties

The Black Widow Tetra (Gymnocorymbus ternetzi), also affectionately known as ‘pez monjita’ or internationally as the “Black Skirt Tetra,” has established itself as a favorite among aquarium hobbyists. Its popularity stems from its distinctive appearance, its generally peaceful nature, and the relative ease of its care, making it an excellent choice for both beginners and experienced aquarists.

Originating from South America, this tropical freshwater fish has seen its fame increase exponentially with the development of fluorescent transgenic varieties (known as fluorescent tetras, GloFish®, or transgenic varieties) (Pan et al., 2008), which add a vibrant and colorful touch to any aquarium.

If you are thinking of adding this fascinating fish to your tank or already enjoy its company, this complete guide will provide you with all the necessary information about its diet, reproduction, care for the Black Skirt Tetra, aquarium requirements, compatibility, and the different varieties available.

Characteristics of Black Widow Tetra

Taxonomy

• Kingdom: Animalia
• Phylum: Chordata
• Class: Actinopterygii
• Subclass: Neopterygii
• Infraclass: Teleostei
• Superorder: Ostariophysi
• Order: Characiformes
• Family: Characidae
• Genus: Gymnocorymbus
• Scientific name: Gymnocorymbus ternetzi
• English name: Black Widow Tetra, Black Tetra, Petticoat tetra, and blackamoor.

Anatomy and Morphology

The Black Skirt Tetra belongs to the Characidae family. It has a rhomboid-shaped body, which is tall and laterally compressed.

• Coloration: The wild form has a silvery base with two distinctive vertical black stripes behind the operculum (gill cover). The rear half of the body, including the anal and dorsal fins, is deep black, although this color tends to fade to grayish with age (usually after the first year).
• Size: They reach an average length of 5-6 cm (approximately 2 inches), although some varieties, like the long-finned ones, can be slightly larger.
• Fins: Its long, flowing anal fin is notable, resembling a skirt (hence its common English name “Black Skirt Tetra”).
• Sexual Dimorphism: Differentiating males and females can be subtle. Generally, females are slightly larger and have a rounder body, especially noticeable during the spawning season. The female’s anal fin is usually parallel to the abdomen line, while the male’s may be wider at the front.
• Lifespan: With proper care, the Black Skirt Tetra can live between 3 and 7 years in an aquarium.

Common Varieties of the Black Widow Tetra

Besides the classic form, several popular varieties exist:

1. Classic/Wild Widow Skirt Tetra: The coloration described above (silvery with black).
2. Albino Black Skirt Tetra: Lacks dark pigmentation, presenting a white or pale pink body with red eyes. A “semi-albino” variant with black eyes also exists.
3. Long-fin Black Widow Tetra (Veil): Developed through selective breeding, this variety possesses notably longer, flowing fins. They can be more susceptible to fin nipping by other fish.
4. Transgenic Black Skirt Tetra (Fluorescent / GloFish®): Resulting from genetic engineering, these tetras incorporate genes from marine organisms (like jellyfish or corals) that give them bright fluorescent colors (red, green, blue, purple, orange) under actinic or blue light. They are just as hardy as the natural variety, but it is crucial to acquire them from responsible sources.
5. Warning about Dyed Fish: It is essential to avoid buying artificially dyed Black Skirt Tetras. This cruel practice involves injecting dyes or dipping the fish in coloring solutions, which severely weakens their immune system, drastically reduces their lifespan, and makes them highly susceptible to diseases. Always opt for natural, albino, or ethically bred transgenic varieties.

According to Frankel (2004), wild Black Widow Tetras exhibit two phenotypes associated with body bands:

• Black Tetra: smoky gray coloration with two prominent black vertical bands located directly behind the operculum.

• White Widow tetra: lighter coloration and lack of vertical bands.

Naturally occurring color variations of Black Tetras can range from white to pink; however, aquarists have developed different color lines, as well as long-finned varieties (Sharpe, 2020).

Natural Habitat

Gymnocorymbus ternetzi is native to the Paraguay and Guaporé river basins in South America, covering areas of Brazil, Paraguay, and Argentina. It prefers slow-moving and calm waters, such as streams, minor tributaries, and marshy areas with abundant vegetation. In its natural environment, the dense vegetation along the banks and aquatic plants provide them with shelter and shaded areas, while they forage for food near the surface.

The distribution of Gymnocorymbus ternetzi has expanded to many continents thanks to the aquarium industry. In some countries, they are considered invasive species due to their release into the natural environment.

Diet

Black Widow Tetras are omnivores and not picky eaters. They will readily accept a varied diet:

• Base Food: Good quality flakes or granules for tropical fish. If you use artificial feeds for your Black Skirt Tetras, including Vitamin C in the feed at a rate of 50 mg/kg is the optimal dose to enhance growth, survival, feed efficiency, FCR, and reduce stress (Dhewantara et al., 2023).
• Supplements: Regularly offer live or frozen food such as brine shrimp (Artemia), daphnia, mosquito larvae, tubifex.
• Vegetables: You can occasionally supplement with some commercial vegetable-based food or spirulina.

Feed 1-2 times a day in small amounts that they can consume in a few minutes. It is important to note that Kuhn et al. (2023) reported that increased water temperature affects the feeding of Black Widow Tetra (Gymnocorymbus ternetzi).

Feeding Black Widow Tetra Larvae

An important recommendation is that Black Tetra larvae should be fed live food depending on their mouth size. Once the yolk sac is consumed, they should first be fed infusoria and then Artemia nauplii (baby brine shrimp).

Sarma et al. (2003) studied the feeding behavior of Black Widow Tetra larvae, from hatching up to 8 weeks, and concluded the following:

• The rotifer Brachionus calyciflorus was attacked, captured, and ingested by the Black Tetra larvae from the first to the eighth week.
• The Black Widow Tetra larvae did not capture or ingest water fleas (Daphnia pulex and Moina macrocopa) until the fifth week.
• The Black Widow Tetra larvae showed an increased consumption of the rotifer Brachionus patulus as the age of the larvae increased.

Furthermore, Lipscomb et al. (2020) recommends weaning Black Widow Tetras from consuming Artemia onto microparticle diets at 12 days post-hatch because they exhibit higher survival.

Reproduction

Black Widow Tetras are oviparous, highly prolific, and relatively easy to breed. An important characteristic is that juveniles of Gymnocorymbus ternetzi are hermaphrodites, and then a sexual reversal process occurs. If you want to learn more about this aspect, I recommend that you review the study by Mazzoni et al. (2015).

Breeding Setup

To breed Gymnocorymbus ternetzi, you can use a 50-liter aquarium with abundant vegetation, low light, and a ratio of 2 male Black Widow Tetras for every female fish. The water temperature should be in the range of 27 to 29 °C, and you should include floating plants such as “foxtail” (Ceratophyllum sp.) to provide suitable conditions for the breeders.

During courtship, the male Black Widow Tetra swims with its fins spread in circles or zig-zags around the female until she lays between 300 and 400 eggs. Lyngdoh et al. (2024) found that the fish spawned during dawn, and that courtship lasted 3 hours and resulted in the release of eggs. The fertilized eggs were slightly adhesive, demersal (sinking/bottom-dwelling), and spherical in shape.

Hatching and Fry Development

According to Çelik et al. (2012), hatching occurs 20-21 hours after spawning at an average temperature of 24 °C. The mouth of the Gymnocorymbus ternetzi fry opens 3 days after hatching, when the yolk sac is completely absorbed and they begin to swim actively. The researchers also found that the Black Tetra fry completed metamorphosis and transformed into juveniles 32 days after hatching.

Egg Care

An important aspect during reproduction is the care of the fertilized eggs. In this regard, Chambel et al. (2014) determined that the optimal doses of the chemicals used in the disinfection of the eggs of the G. ternetzi, to increase hatching, were 1 mg/L for the iodine solution, 25 mg/L for hydrogen peroxide, and 3 mg/L for methylene blue.

Triploidization

If you are interested in producing Black Widow Tetras for sale, the use of triploidization techniques will allow you to obtain fish that grow faster. Uma and Chandran (2008) successfully induced triploidy by heat shock in the Black Tetra, Gymnocorymbus ternetzi.

Black Widow Tetra Care and Breeding Conditions

To ensure the well-being of your Black Skirt Tetras, consider the following parameters:

• Aquarium Size: A minimum of 75 liters (20 gallons) is recommended for a school of 6-8 individuals. Longer tanks (like 80-100 cm) are preferable to give them more horizontal swimming space. Priestley et al. (2006) described that group size does not affect the growth rate of black tetras, even when raised in groups of up to 16 individuals in a 50-liter tank.
• Filtration: They need clean and well-oxygenated water. A filter suitable for the aquarium’s capacity is essential, but avoid excessively strong currents.
• Lighting: They prefer moderate or subdued lighting. Lights that are too intense can stress them. Floating plants can help diffuse the light.
• Substrate and Decoration: A dark substrate will enhance their colors. They appreciate a well-planted aquarium (with natural or artificial plants) that offers them hiding places and shaded areas, combined with open areas for swimming. Roots and driftwood are also welcome.

Water Quality

Table 1. Black Widow Tetra Aquarium Breeding Parameters

Parameter	Ideal Value
Fish tank volume	75 to 120 liters.
Temperature	20-28 °C (79-82 °F)
pH	6.0-8.0
Hardness	3-10 dGH
Nitrates	< 50 mg/L
Lighting	Moderate
Filtration	Good
Water Changes	25% – 30% weekly

Black Widow Tetra Behavior and Compatibility

Behavior and Temperament

• Gregarious Nature: They are schooling fish and should be kept in groups of at least 6 individuals, preferably more. Alone or in small groups, they become shy, stressed, and may develop aggressive behaviors.
• Peaceful but ‘Nippers’: They are generally peaceful and good community tank mates. However, they have a known tendency to nip at the long, flowing fins of slower fish like bettas, guppies, or angelfish. It is recommended to avoid this combination.
• Swimming Level: They mainly occupy the middle level of the aquarium, although they will explore all levels.

Compatibility: What Fish Can Live with the Black Skirt Tetra?

When choosing tank mates, consider their fin-nipping tendency:

• Ideal Tank Mates: Other robust and fast tetras (e.g., Neon Tetra, Cardinal Tetra, Serpae Tetra – although the latter can also be nippy), Corydoras, Ancistrus, Rasboras, Danios.
• Tank Mates to Avoid: Fish with long fins and slow swimming speeds like Guppies, Bettas, Angelfish. Excessively small or timid fish that might be intimidated. Large and aggressive fish.

Diseases that Affect Gymnocorymbus ternetzi

White Spot Disease “Ich”

Ichthyophthirius multifiliis is a ciliate protozoan that causes “White Spot Disease” or “Ich”. Aydogan et al. (2010) reported natural infection of the Black Widow Tetra (Gymnocorymbus ternetzi) with Ich.

You can reduce the risk of Ich problems by regularly testing water quality and making necessary changes (Yang, 2022).

Sodium chloride (salt) and formalin are frequently used as therapeutics in fish; however, Rothen et al. (2002) recommends not using these inputs for Black widow Tetra larvae as their tolerance has not been demonstrated.

Parasites

Acchile et al. (2017) reported that monogenean parasites like Diaphorocleidus kabatai attach to the gills of the black tetra. Likewise, Malek et al. (2021) reported infestations of Tetrahymena sp. and Dactylogyrus sp. in the GloFish tetra (Gymnocorymbus ternetzi), and recommends treatment with salt baths and 37% formalin.

Conclusion

Black Widow Tetras (Gymnocorymbus ternetzi) are ideal for beginners in the aquarium hobby. Their beauty and hardiness make them easy to breed and care for. They are a fascinating and easy-to-care-for aquarium fish, making them an ideal pet for beginners and experienced hobbyists alike. By following the tips in this article, you can provide your Black Tetras with a long and healthy life.

Fluorescent Black Tetras are gaining popularity due to their vibrant colors. However, there are concerns about the environmental impacts of their release into the wild.

References

Acchile Santos Monyele, Gabriela Tomas Jerônimo, Lucas Cardoso, Karen Roberta Tancredo, Paula Brando Medeiros, José Victor Ferrarezi, Eduardo Luiz Tavares Gonçalves, Guilherme da Costa Assis, Maurício Laterça Martins. Parasitic fauna and histopathology of farmed freshwater ornamental fish in Brazil. Aquaculture, Volume 470, 1 March 2017, Pages 103–109. http://dx.doi.org/10.1016/j.aquaculture.2016.12.032

Aydogan A., H. Avci, S. Birincioglu. 2010. Ichthyophthirius multifiliis Infection in A Black Tetra (Gymnocorymbus ternetzi). Kafkas Univ Vet Fak Derg 16 (1): 135-137, 2010 DOI:10.9775/kvfd.2009.320

Çelik, I., Çelik, P., Cirik, S., Gürkan, M. and Hayretdag, S. (2012), Embryonic and larval development of black skirt tetra (Gymnocorymbus ternetzi, Boulenger, 1895) under laboratory conditions. Aquac. Res., 43: 1260-1275. https://doi.org/10.1111/j.1365-2109.2011.02930.x

Chambel, J., Costa, R., Gomes, M. et al. Hydrogen peroxide, iodine solution and methylene solution highly enhance the hatching rate of freshwater ornamental fish species. Aquacult Int 22, 1743–1751 (2014). https://doi.org/10.1007/s10499-014-9779-1

Dhewantara, Y. L. D., Firsty Rahmatia, & Armen Nainggolan. (2023). EFEKTIVITAS VITAMIN C DALAM PAKAN BUATAN TERHADAP KINERJA PERTUMBUHAN DAN KELANGSUNGAN HIDUP IKAN BLACK TETRA (Gymnocorymbus ternetzi). JURNAL MINA SAINS, 9(1). https://doi.org/10.30997/jmss.v9i1.7326

Frankel J. S., Inheritance of Trunk Banding in the Tetra (Gymnocorymbus ternetzi Characidae), Journal of Heredity, Volume 95, Issue 3, May 2004, Pages 262–264, https://doi.org/10.1093/jhered/esh036

Kuhn, J., Azari, S., & Volkoff, H. (2023). Effects of temperature on food intake and the expression of appetite regulators in three Characidae fish: The black-skirted tetra (Gymnocorymbus ternetzi), neon tetra (Paracheirodon innesi) and Mexican cavefish (Astyanax mexicanus). Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology, 275, 111333.

Lipscomb Taylor N., Roy P. Yanong, Shane W. Ramee, Matthew A. DiMaggio. 2020. Histological, histochemical and biochemical characterization of larval digestive system ontogeny in black tetra Gymnocorymbus ternetzi to inform aquaculture weaning protocols. Aquaculture, Volume 520, 2020, 734957, ISSN 0044-8486,
https://doi.org/10.1016/j.aquaculture.2020.734957

Lyngdoh, A. S., Khongngain, O., Vyas, V., & Minare, A. (2024). Breeding of an ornamental fish – black skirt tetra (gymnocorymbus ternetzi, boulenger, 1895) and study of its embryonic development. International Journal of Fisheries and Aquatic Studies, 12(4), 88-92. https://doi.org/10.22271/fish.2024.v12.i4a.2948

Malek Ahmadi, B., Rahmati-Holasoo, H., & Momeninejad, A. 2021. Report of Tetrahymena sp. and Dactylogyrus sp. infestation in glofish tetra (Gymnocorymbus ternetzi): Diagnosis and treatment.

Mazzoni, T.S., Grier, H.J. and Quagio-Grassiotto, I. (2015), The Basement Membrane and the Sex Establishment in the Juvenile Hermaphroditism During Gonadal Differentiation of the Gymnocorymbus ternetzi (Teleostei: Characiformes: Characidae). Anat. Rec., 298: 1984-2010. https://doi.org/10.1002/ar.23270

Pan, X., Zhan, H. & Gong, Z. Ornamental Expression of Red Fluorescent Protein in Transgenic Founders of White Skirt Tetra (Gymnocorymbus ternetzi). Mar Biotechnol 10, 497–501 (2008). https://doi.org/10.1007/s10126-008-9094-9

Rothen Daniel E., Eric W. Curtis & Roy P. E. Yanong (2002) Tolerance of Yolk Sac and Free-Swimming Fry of the Zebra Danio Brachydanio rerio, Black Tetra Gymnocorymbus ternetzi, Buenos Aires Tetra Hemigrammus caudovittatus, and Blue Gourami Trichogaster trichopterus to Therapeutic Doses of Formalin and Sodium Chloride, Journal of Aquatic Animal Health, 14:3, 204-208, DOI: 10.1577/1548-8667(2002)014<0204:TOYSAF>2.0.CO;2

Sarma, S., Amador López-Rómulo, J. & Nandini, S. Larval feeding behaviour of blind fish Astyanax fasciatus (Characidae), black tetra Gymnocorymbus ternetzi (Characidae) and angel fish Pterophyllum scalare (Cichlidae) fed zooplankton. Hydrobiologia 510, 207–216 (2003). https://doi.org/10.1023/B:HYDR.0000008646.82042.7c

Sharpe Shirlie. 2020. Black Widow Tetra Fish Species Profile: Characteristics, Origin, and Helpful Information for Hobbyists. The Spruce Pets.

Steven M. Priestley, Abigail E. Stevenson, Lucille G. Alexander, Growth Rate and Body Condition in Relation to Group Size in Black Widow Tetras (Gymnocorymbus ternetzi) and Common Goldfish (Carassius auratus), The Journal of Nutrition, Volume 136, Issue 7, 1 July 2006, Pages 2078S–2080S, https://doi.org/10.1093/jn/136.7.2078S

Uma, B., & Chandran, M. R. (2008). Induction of triploidy in Gymnocorymbus ternetzi (Boulenger). Research journal of fisheries and hydrobiology, 3(2), 41-47.

Yang Alison. 2022. Black Skirt Tetra 101: Care, Tank Mates, Breeding & More. Aquarium Source.