You asked near the same question in another thread, to which I replied at length, so I will copy that post over for the benefit of this thread. And BTW, try not to raise the same issue in more than one thread; it helps us and you if all posts on the topic are together in one place.
When we speak of shoaling fish (some sources will call this schooling, though that is technically a bit different) we are referring to fish species that live in large groups. Often hundreds and even thousands of fish live in the group, though in some species it might be fewer. The species has evolved to require this, and it may have several impacts on the fish. Frequently there is "safety in numbers" and when the fish is on its own or in too small a group (for that species, as I said, it can vary) this causes stress and stress is the cause of just about all fish disease. Stress weakens the immune system, but even more than that it can cause damage to the fish's physiology. You can read more in my article on stress in the Freshwater Articles section.
Aside from the above, many species have a social structure within the group. This can vary and may include a "pecking order" with dominant fish and submissive fish, and this is natural and necessary if that species is to be healthy. When we see tetra "playing" or what I call "sparring," we are seeing this social structure at work. It may seem like play to us, but this is a very serious social development that is absolutely essential to the fish.
Many of us have long understood the need for groups, but only last year the first scientific study on the effects of insufficient numbers within a species group came out with some factual evidence. Fish that were kept in groups under six developed increased aggression; species that one would consider peaceful actually became aggressive. Aggression took the form of fin nipping up to actual physical biting and death. And all because the fish just didn't have enough of its own species around it. The same thing happened when the tank was too small, and this shows that aggression is probably the fish's only available weapon to deal with frustration. In some cases, the exact opposite will occur; the fish will become so withdrawn because of these factors that it wastes away.
So, tetra being shoaling fish, they should never be in groups less than 6. This is a number many sources use, and generally it is the minimum, though it must be said that when space is available, increasing the group above 6 will always
be better for the fish. Some species are inherently aggressive to begin with, like Serpae Tetra
, and the group must be much larger. Group sizes are mentioned in our profiles where applicable.