The characiformes family is very large; primarily in South America (and a bit further north for a very few species) with fewer species in Africa, showing that they evolved after the continents separated. All characiforms are shoaling fish, living in large (very large) groups, and this is an essential aspect of their care.
There is now scientific evidence that groups of ten (or more) are essential for the well-being of all species in this family. The "shoaling" aspect can manifest itself in differing ways, but one or two factors is without question: aggression is increased with smaller shoals, and there is a latency to feed. These aspects show the extreme importance of maintaining the species in decent-sized shoals.
Second observation, I agree with members who have said "just wait and see." Fish in a new environment (involves water parameters, tank size, aquascaping, etc) often require time to settle in, but when they do, the natural behaviours become more obvious. Numbers in the group are key, as is tank size. Fish within the confines of even the largest aquarium may behave differently from those in the wild. With most such species, fish in the wild are less likely to be bullied or killed by aggressive traits in the other fish, but in the aquarium they cannot usually escape.
Third, this is programmed into the genetics of the species. It is not learned, it is a vital aspect of the species. That means we cannot change their behaviours, we can only provide as close to what the fish "expect" and hope it is sufficient to satisfy.
Lastly, some species have a stronger "aggressive" nature than some others. Numbers are vital, as is space, but the individual species have clear distinctions in how they may "act out" their aggression. Even the most peaceful tetras will exhibit aggressive behaviour of some sort if the group is less than ten+. It may or may not be easily observable, but to the fish who communicatee not only by sight but by chemical pheromones and allomones, this is present.
