You (dormfish) have realized the issue and are taking corrective action, so that is to your credit. I can't pass up this opportunity to comment on the tetra behaviour, since many other members will likely read this and might benefit; so please don't take my comments are direct criticism of you.
I simply want to take advantage of a prime example.
Shoaling fish need to be kept in groups. Most species require at least six--more when space permits is always better and safer--and some species need more than that. They also need space. The behaviour of two white skirt tetra noted by dormfish is nothing out of the ordinary; in fact, it is exactly what one should expect. Text book.
Reasons for shoaling are several, and can include security (safety in numbers), social interaction, including a pecking order in some. This facet is "programmed" into the species by nature, through evolution. It cannot be changed.
Kept in too small a group, or in insufficient space, often causes increased aggressive behaviour. In a species that is not normally aggressive at all, it can cause aggression itself. The reason is that the fish are using they only means they have to "lash out" at their frustration. Kept in a group of their own species and in sufficient space for their needs, they are fine. Less stress, which means better health. We simply cannot fly in the face of nature without consequences.
Point made (I hope). Enough of that.
Byron.
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