From:
http://badmanstropicalfish.com/articles/article27.html
"Schooling and shoaling fish
Sometimes, one fish truly is not enough. These are my favorite fish, and my down-falling.
I will loosely define schooling as fish who stick pretty closely together most of the time. Schoaling is the fish who need some friends, but its more a social thing. Its a little easier to define by sorting fish into the groups. (You will also find that everyone has their own definitions of schooling and schoaling)
I would consider tetras, cories, and barbs, rainbows as schooling, and many loaches, cories, and some cichlids are schoalers. Schooling fish MUST be kept in large groups. Minimum is 6, better is 10+, best is 24+.
Schoalers need at least one or 2 friends of the same kind.
I'm going to use my good friends, the clown loaches and cardinal tetras for examples. By my definition, Clown Loaches are schoalers. I have 9, which makes for a decent school of anything. However, these guys will split apart into groups. The only time I see them all in the same place is at dinner time. Cardinal tetras are schoolers. When relaxed, they will spread out throughout the tank, but every now and then, they have to do a head count, and they all amass and circle the tank, leaving out no one. They will lurk in the plants together, rarely splitting up. I only noticed splits when I had a group of 23 green neons and 11 cardinals in a 125 gallon tank. Even then, a group of merely 3 was unthinkable. When one fish split off from the pack and started exploring, the entire group started streaming after them.
This will also bring us to another issue within this area. With shoalers, it is easier to get a larger variety of similar species. But when it comes down to schooling fish, I highly recommend minimizing the different kinds, unless you can find out if those tetras are accustomed to seeing each other in the wild.. a true biotope.
While its desirable to have 2 each of tetras A through Z, it really isn't fair to them. And it isn't fair to you either. Some will school together, (more so those with similar body structures), but you will never see anything so impressive as a true school. There also is increased stress with the numerous kinds. You will find that keeping a single kind will allow for more fish than keeping mixed. For this reason, I do not advise more than one type of tetra in a tank less than 4 feet. Perhaps in a 3ft, you might get away with 2 kinds (smaller species).
Shoalers... Don't plan on being able to place as many in a tank as the schoolers. While you could have 24 small tetras in a 20L, you don't want to stuff a ton of botias or schisturas, they might get a spine in their eye and decide to take out their tankmates. Not usually the case, but shoaling fish can be slightly territorial as well as requiring friends to spend the night with. I placed cories in both sections, because I'm still at a bit of a loss with these guys. I've only kept pigmy cories. Those guys are definitely schoolers, and anything less than 10 isn't proper. The regular cories, I have not kept, but everyone seems to find a group of 3 enough, which means they don't fit in my schooling definition. Personally, I'd want to keep them all in groups of 6.. but that's just me. "