Originally Posted by aussieJJDude
please expand, i want to learn something new!
You asked for it, dude.
"Shoaling" and "schooling" are different. Schooling applies to marine fish. Freshwater fish that "shoal" live in large groups. They have evolved to need these groups for several reasons that can vary depending upon the species. Everyone agrees that security is common to all; the more fish there are, the more secure they will feel. On their own, they are severely stressed. I'll come back to numbers in a moment.
Some species have a well-defined hierarchy within their groups. Cichlids are notable for this; the several species of Apistogramma wherein submissive males will appear externally the same as females (shorter fins, less bright colouration, etc) so long as the dominant male is present illustrate this; remove the dominant male and one of the other males will suddenly develop the extended fins and colouration.
Some fish are very social; loaches for instance. They have a pecking order of sorts, but they also simply have the need to interact continually. We might call it play, and sometimes it probably is, but it can also be much more serious to the fish. It is essential to their life, and again stress will result if they are denied this.
Anyone who has maintained a largish group (8 or more) of many of the tetra will have seen the continual interaction among males, forms of display, sometimes to entice a female but more often just doing it. I ave 10 Congo Tetra in my River Habitat tank, five males and five females. The males remain close, the females slightly above and to one side. The males will pair up and race down the length of the tank several times; a third male sometimes joins in, or another male will approach and race with one of the others. Fins flared, colours at their brightest. This is simply the fish behaving as nature made them. But put a male/female pair in a small tank, and this is gone--along with the fish's health.
Coming back to the numbers. The more the better, undoubtedly. But the first scientific studies carried out a year or so ago in this area have shown that behaviours alter due to stress if there are not more than 5 of the species together. Studies were done with angelfish and a number of characins and I believe rasbora. Fish in groups less than five showed increased aggressive behaviour to other fish in the tank; even normally peaceful tetra became nasty. Fish that are naturally feisty, like Tiger Barb, will almost always be less likely to fin nip other species if they have 12 or more in their group. The aggression is then confined to within the group, as nature intended. What we can learn from this is that keeping any fish healthy means providing it with the environment that it has been designed to fit.
I relate this to stress in my article on that subject: http://www.tropicalfishkeeping.com/f...um-fish-98852/
Hope this was instructive.