The Monja Fish or also known as monjita fish is a species with a rounded body and compressed on its sides, it will reach a length of 7.5 cm in total diameter, however, in the aquarium it does not exceed five centimeters in diameter.
It is black or grayish in color, on the body it has two vertical black stripes, its dorsal and anal fins are highly developed and these resemble the fish having a skirt with most of its mass in the lower half of the body. , the fine black color of the juvenile fades to a gray as it matures.
It is a freshwater fish belonging to the Characidae family, its order is the Characiformes.This fish is also known by the common name of black tetra. There is another fish named with the same name, but it is important not to confuse them since this species is not the same, the Gymnocorymbus thayeri is only a cousin species, this fish has the common name Tetra Gymnocorymbus.