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Black Widow Tetra: health and care

Common name: “Black Widow Tetra”, “Black Tetra” (the last two are shared with other characins, only the “Black Widow Tetra” being useful). Gymnocorymbus ternetzi.

Characteristics and care.


• Order: Characiformes.

• Family: Characidae (Caracids).


In areas covered with vegetation and calm waters, including lakes, basins and lagoons.


South America, (Bolivia, Peru, Brazil, Paraguay, Argentina) in the area of ​​Mato Grosso, Guapore River, Matos River, Negro River and Paraguay River.


Tetra with a very high body, slightly thickened in the middle of its length, and quite compressed laterally. Other varieties have been developed, including one with longer fins that maintains the black coloring, the result of selective breeding.


The background color is gray in the front half of the body, furrowed by black bands, two on each side very visible, one behind the eyes and a wider one under the beginning of the dorsal fin.

The back of the body is a deeper black. The beginning of the dorsal and anal fins is a more pronounced black that fades into the rest of the fin. There is a golden variety, genetically selected through selective breeding, as well as albino (white with red eyes) and semi-albino (white with black eyes) varieties. Unfortunately, these fish – especially the albino and semi-albino forms – are sometimes artificially tinted, either pink or blue. These tinted fish are very prone to disease and are very short-lived, so we should refrain from buying them.


4 to 6 cm.

Sexual differences

Females are larger and have a more rounded body, something that is easily seen from above. The caudal fin of the male may have white dots. Males are usually smaller, staying at 4 to 5 cm, while females reach 5 or 6 cm. In males, the anal fin is wider at the front and the dorsal is more pointed than in females.


Between 23ºC and 26ºC.


Acid and soft or neutral, pH: 6.0 – 7.5 GH: 3ºd to 10ºd.


Aquarium from 40 liters, although on this point many differ. It is advisable to start from 60 or 80 liters. Not very populated, with plenty of space to swim. It looks much better if the background chosen as the substrate is dark in color and the aquarium is well planted.


Omnivorous, but like all tetras, essentially carnivorous: it will accept worms, small crustaceans, insects and dried food (scales, flakes, freeze-dried, etc.).


It is an active but passive fish, of group association, so it must live in a school of 8 to 10 specimens. When it is not comfortable, it usually remains static in the water, hidden among the vegetation. It is common for them to chase each other, although we should not be alarmed, since they do not usually cause damage.Amoxicillin for fish

Life expectancy

With an average of 7 years of longevity.


The Gymnocorimbus ternetzi (Black Widow Tetra) is an oviparous free-laying egg that is easy to reproduce. To reproduce the Black Widow Tetra, an aquarium of about 50 liters is recommended, since the clutches can be very numerous. It is necessary to place numerous plants with fine foliage at the ends (cabombas, riccia or ambulias are ideal) leaving a lot of free space in the center. The bottom covered with a plastic mesh and a good layer of java moss. The crystals must be covered as the eggs and fry are sensitive to light. The water is slightly softer than the maintenance water, the temperature is slightly higher, 26-27ºC, and the ph is slightly acidic.

A very chubby female, between one and two years old, is chosen and puts her in the aquarium alone for a couple of days. In the afternoon of that same day, the male is introduced and the laying usually takes place the next morning. , are stimulated if the aquarium receives the rays of the morning sun. The male dances around the female before she lays her eggs for her to fertilize. They spawn next to the surface of the water where the males then fertilize the eggs, which when fertilized will turn cream-colored.

The number of eggs is highly variable depending on the age of the breeders, the quality of their diet and genetic factors. From 200 to 400 is very normal, but they can be more than a thousand, for this reason it is important that the breeding tank is not too small, they do not tolerate transfers and there can be many fry. The parents must be removed as soon as they are finished or they will devour the eggs.

At 26ºC hatching takes about 18-26 hours. The fry take 3 or 4 days to absorb the yolk sac, which is very large. You have to start feeding them with infusoria and then with brine shrimp. The fry are very sensitive to light during the first week, so they have to live in semi-darkness, for which we will gradually uncover the crystals (see the Neon Tetra file, P. innesi).

If you do not want to remove the parents from the tank, because there is no other, there is the option of placing marbles at the bottom or a net that prevents the passage of adults to the bottom, where they would eat eggs and fry.