The Black Widow Tetra, Gymnocorymbus ternetzi a medium sized tetra, growing to about two inches (6 cm). Other names for this fish are: Black Tetra, Black Skirt Tetra and Petticoat Fish. Some people find Black Widow tetras aggressive, but this is not my own observation. However, the possibility needs to be borne in mind when choosing this fish. It grows to about two inches (five and a half cm). The Black Widow Tetra is an easily kept fish. It lives about five years
Suitable companions include nearly all the tetras. There is a doubt about putting a big Black Widow Tetra with small Neon Tetras, but all the larger tetras should be OK. Other suitable companions include nearly all other small and medium size fish like platies, swordtails, all the small and medium size barbs, all the common Rasboras, all the Danios and White Cloud Mountain Minnows. It can also be kept with small and medium size goldfish and other small coldwater fish inside. This fish, like most of the tetras is a schooling fish and is best kept in groups of at least four. It is much more likely to remain its normal peaceful self in a school than by itself.
The Black Widow Tetra comes from South American rivers including the Rio Guapore, in Bolivia and the Rio Paraguay in Paraguay. It is a tropical fish, but its range extends outside the tropics. This fish can be acclimatized (slowly!) to slightly colder water than most tetras, but it will not take very coldwater. In South Australia it is not suitable for ponds in the winter, but can be kept in indoor aquariums without a heater as long as the room they are in does not get very cold. This is more likely to be successful in a larger aquarium because the water temperature will not change so rapidly.
The water in the Black Widow Tetra’s natural habitat is soft and acid and this is the ideal water for the fish. However, they will adapt to harder water and more alkaline conditions. I suggest that for a mixed community tank including this tetra, a pH of 7(Neutral) should be aimed for. They can be kept in the tap water of Adelaide and the Hills as long as the Chlorine or Chloramine is removed and the pH adjusted to about 7.
The Black Widow Tetra can survive temperatures as high as 34 degrees C ( 93 degrees F), and if its aquarium gets infected with White Spot, one way of eradicating the disease is to heat the tank to 34degrees for two days.
This fish is omnivorous and will happily eat any normal fish food, either flakes or granules. They relish live food like daphnia and wrigglers, and benefit from a change in diet like frozen blood worms.
It is not very easy to distinguish the sexes of this fish, but the males tend to be smaller than the females. The female will normally have a rounder body than the males. The males often have a broader anal fin and a narrower, more pointed dorsal fin.
They are an egg scatterer. They tend to spawn among fine leaved plants. They are fairly easy to breed. The babies can be fed on dried or liquid fry food. I have known instances where a few babies have survived in a large, well planted community aquarium which is not too crowded. As with any fish they benefit enormously from suitable sized live food.