Why are black skirt tetra so mean to each other

[Steven Williams]

I’ve got a 60 gal stocked with a variety of fish but my black skirts are definetly what people see the most. Few things I’ve observed however, starting with they are huge *******s to one another.
Got them all at the same time but a few have outgrown the rest. I’ve got a school of seven 3 are an inch, one is an inch and a bit more and very plump, one is below an inch, one is almost two inches, and I’ve got a monster 2.5 inch one that is clearly the boss. But the plump and big one always fight and I see nipped fins, they heal, then they nipped, the other small ones don’t grow but still eat a lot, and the plump one and another isolate from the school unless they feel really unsafe like during water changes and they seem to have boundaries. I’ve got other tetras in the tank and they don’t mess with em just each other. Thoughts? Can’t really increase the school cause I’ve got other fish, two are really plump also and those are also the two that fight
The largest one compared to the 6 inch parrot cichlid

 

[Mighty Wizard]

Tetras belong to the family Characidae, of the order Characiformes, which also include such species as piranha, wolf fish and tiger fish. The piranhas, or Serrasalmids, are by far the closest to tetras in terms of relation, and where up until recently classified as a species of tetra, and indeed the only real thing that sets Tetras and Serrasalmids apart is size.

Now, a defining characteristic of Characins is their extremely well developed teeth. Some examples would be certain Alestids and Serrasalmids, but also tetras:


I couldn't find any pictures of the black tetra, but those are the jaws of Moenkhausia megalops, a closely related species with comparable a jaw structure. Impressive to say the least.

There are several different species of tetra, and as with serrasalmids, there are docile species and there are aggressive ones. Some have evolved to eat plant matter, and some have evolved to be aggressive little ****s. The latter holds true for the black tetra, and they bear more than a passing resemblance to piranhas.

So there you have it, your tetras are essentially small piranhas. Only way that I know to relieve aggression would be to get a larger school. Here are some more pictures, to show just how ****ed up tetras really are: