Hot Water Fish

Nun Fish or Monjita Fish

  • Scientific name:  Gymnocorymbus ternetzi
  • Common name:  Nun fish, nun fish, black tetra, black widow
  • Aquarium size:  40 to 80 liters
  • Temperament:  Peaceful, quick and awake
  • Temperature:  Between 20º and 26ºC
  • pH: Neutral, 7
  • Diet: Omnivores
  • Length:  Males up to 7.5 cms. and females only 5 cms.


The fish black tetra , better known as nun fish is a specimen from the Characidae (family characins ). This animal is highly sought after by those interested in aquariums, as it has an attractive coloration, it adapts well to different environments and does not require a very specific pH.
Another point in its favor is its behavior in the aquarium. It is a very suitable fish to live in a community aquarium of the Amazonian type , where ideally it is necessary to introduce a shoal, which will move with speed and agility, generating great visual appeal.


This type of nun fish has a very particular shape, it has a raised body that thickens in half, but at the same time, it is compressed on the sides. Its eyes are very large (considering the size of its body), the mouth is positioned downwards and it is small.
On the other hand, you have what is called one caudal fin cut and lobed , and the anal fin, so named for being in the back of the fish is long and very large, totally different from the rest of the body.
This fin is of the adipose type, however, through different crosses and selection of specimens, they have evolved to varieties with other types of fins that are even longer and darker in color.

Nun fish coloring

In the case of external physical appearance, it could also be said that, unlike others of its kind, the Gymnocorymbus ternetzi has a special color , since it is not unicolor, but diffuses into various shades.
As a background it has a gray color that is maintained until the middle, you can see two black stripes that are vertical, one just behind the eyes and have another even wider stripe in the center of the body, which sometimes also appears in the dorsal fin.
Due to its great popularity in the market, the color of the fantasy nun fish has been altered thanks to artificial genetic selection that is done in many laboratories.
There are groups of blue, gold, albino and semi-albino nun fish.The problem with this class of animals is that genetic modification can alter their life expectancy, since they can cause different types of diseases. So there is no point in buying them like that, either because you will not be able to enjoy them for long and because you do not have to support this type of practice, it is always better to resort to what comes from nature.

Sexual Dimorphism

Sexual dimorphism in male nun fish and female nun fish is seen in that the male’s fin is wider at the bottom, very different from that of the female, which is more pointed.
In addition to this, the female sex can also be larger and thicker, when compared to the male. In this way, they can be differentiated very easily.

Distribution and habitat

Are you interested in knowing where you can get a fantasy nun fish? Surely you did not know, but if you live in the South American continent (like Paraguay and Argentina) you are very close to them.
This animal exists in areas that have really abundant and jungle vegetation, so it is normal to see them in places such as basins, rivers and sides.
The most important thing for them is that it is a dark place, where the lighting does not penetrate so much and they can be hidden efficiently, that is why their color plays such an indispensable role for their survival.

Aquarium conditions

Do you want to have a nun fish inside your house? To do this you must be informed, since these animals need a good aquarium with a capacity of up to 80 liters , where they can feel comfortable, as in their natural habitat. What you should consider when buying the aquarium is the following: do not put many aquatic plants , choose dark colors for the substrate and, if you want them to adapt faster to the environment, buy an aquarium filterwith moderate power so that it resembles a stream of water. In addition to all this, you must also make sure that the aquarium lighting is not too intense. Among other technical aspects that you must remember to create the perfect aquarium for nun fish, the quality of the water has to move within the following parameters:

  • The hardness of the water must be between 3 ° and 10 ° gH.
  • Aquarium pH , between 6 and 8.
  • Water temperature between 20 ° and 26 ° C.

Aquarium decoration

  • Best aquarium sand
  • Best gravel for aquarium
  • Best akadama for aquarium
  • Best Aquarium Ornaments 
  • Best logs for aquariums
  • Best rocks for aquariums
  • Best backgrounds for aquariums
  • Best artificial plants


This is a very important and interesting topic, since this type of fish may not eat the same freely, as in its life in the aquarium.
However, it easily adapts to
all kinds of feeding options . Thanks to its omnivorous condition, it eats from live worms, crustaceans or insects and at the same time, you can feed it at home with fish food that is frozen, granular or freeze-dried . You see that it is very comfortable to keep them alive.

Everything you need to maintain your aquarium

  • Best digital pH meter
  • Best Aquarium Test
  • Best anti-algae
  • Best bacteria for aquarium
  • Best siphoner for aquarium
  • Best Aquarium Water Clarifiers
  • Best reverse osmosis filters for aquarium
  • Best UV lamps for aquariums

Behavior and compatibility

Many people consider the nuns as friendly and pretty, and this fish is really fast and awake. However, create an environment where they do not feel threatened as otherwise they may behave nervously, but that is achieved with a dim level of darkness and an aquarium filter that can create drafts.
When a nun fish does not feel well in an aquarium, it shows it by staying still, mixing with the environment and hiding in the vegetation.
If you want to keep him company, you can also do it by creating a school of between 8 or 10 specimens of his kind. They like to be in a group, they play chase and they don’t hurt each other, so you won’t have to worry about anything.
With age, they tend to be calmer and less active than in their younger stage.
They are usually not aggressive, except with long-finned fish … which they tend to chase around the aquarium. It is not advisable to mix with Guppies or Scalars , it is best to mix them with other Tetra fish.

Fish compatible with nuns

Compatible fish without restrictions

  • Goldfish
  • Corydoras
  • Barbel Cherry
  • Tiger Barbel
  • Ax fish
  • Mollys
  • Guppy endler
  • Kribensis
  • Snail Apple

Compatible fish without restrictions

  • Betta fish
  • Telescope Fish
  • Chinese Neon
  • Ramirezi
  • Harlequin fish
  • Otocinclus
  • Botia Clown

Compatible fish without restrictions

  • Rasbora Galaxy
  • Angel fish
  • Platy Fish
  • Ancistrus
  • Tetra Borrachito
  • Neon Tetra
  • Boraras brittae
  • Guppy
  • Silver dollar
  • Zebra fish

Fish compatible with restrictions (territorial problems)

  • Gourami Pearl
  • Eat Siamese seaweed
  • Killi


In relation to the reproduction and pregnancy of the nun fish, we can say that they usually reproduce very regularly.
They have great sunsets. The courtship of the male begins by chasing  the females tirelessly, until at a certain moment he gives it «a little bump» and at that moment the female expels a certain amount of eggs that are simultaneously fertilized by the male. They can lay between 300 and 400 eggs , with the first hours of the day.
It is for this reason that the breeding tanks should be about 50 liters . The eggs can take between 18 to 26 hours to hatch and that, once they hatch, you must feed the fry for the first time with infusoriaAnd, when the time is up, you can try brine shrimp nauplii.
It is always important to follow them up and see how they develop.

Photos of the nun fish

Publicaciones relacionadas

Deja una respuesta

Tu dirección de correo electrónico no será publicada. Los campos obligatorios están marcados con *

Mira también
Botón volver arriba