I moved this mini-thread out of the other which was not directly related.
Tetra are characins and all characins are shoaling fish, meaning they live in large groups and must be kept in a group. Six is often considered the minimum, though more than this will always be better. There are reasons for this, from social interaction, to security, to inherent pecking orders, etc. When kept in smaller groups like 2, 3 or even 4, the fish will be under stress and this may exhibit itself in several ways including aggression. Species which are not normally prone to nip may do so solely because of the stress. Black Phantom Tetra
is the species named, although the other thread concerned Black Skirt or Black Widow Tetra
. The latter have a basic desire to fin nip, and small groups or confined quarters intensifies this significantly. The Black Phantom generally does not nip, unless stressed.
Water parameters and conditions also impact, as Blackfeet said. The Phantom is much more sensitive to parameters and quality.
You can read more on both species in the profiles, click the shaded names.