02-20-2011, 12:49 PM
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I agree with aunt kymmie.
Tetra are shoaling fish, meaning they must be in a group. Six is usually considered minimum for most species, some need more. When not maintained in groups, it is common for them to display even more aggression that normal. And most characins (tetra, pencilfish, hatchetfish) have some degree of aggressive behaviour within the group, which is why they must be in groups, to be normal.
Blackskirt tetra [also commonly known as Black Widow Tetra] are also notorious for being fin nippers, so with them this will be heightened in too small a group and too small a space. Your 25g will help partly, but if you really like this species, please get 4 more of them.
You can read about many fish in our profiles, second tab from the left in the blue bar across the top of the page, or if the name is the same in posts it will be shaded and you can click on that to see the species profile, example Black Widow Tetra. The profile will explain the above further.
And, welcome to Tropical Fish Keeping forum.