Glo tetra "normallt" are the gebetic altered leucistic(white widows : there is one in between) type of Black.Widows.
Looks as if one of those where "crossbred" whit the common / normal type resulting in this one colored type.
They don't look of great quality (looks like a hobby breeder result)
These black skirts look dyed the not-genetic way. They could be injected.
The difference in coloring made.me think differently as did the word "permanently" but you could bw right for sure.
Only realised now OP is in Pakistan