Schooling Fish In Small Numbers.

[ythao01]

What would happen if schooling fish were kept in small group like 2-3? I know that they should be kept in schools of 6+ but never really knew why? Does it stress the fish to death? More aggression?

 

[eaglesaquarium]

Yes, and yes.
 
It increases the stress - they feel (correctly) that they have been lost from the rest of the school and feel more vulnerable.
 
Also, smaller numbers means that the weakest in the group can be picked on much more by the strongest in the group.  In a larger group, the aggression is more spread out - and they won't pick on any individual as much.  In some cases, they will turn on other tank mates in smaller numbers, because they feel more threatened - fight or flight sets in.  Some will hide (the more peaceful species), whereas the more aggressive species will become more so.

 

[fluttermoth]

The actual stress of being alone, or in a very small number,doesn't usually kill the fish directly (that's why so many people have one neon, or one rummynose that lives on) but it does leave them open to catching stress related illnesses, like fungus, ich  and fin rot.
 
It really is bordering on cruel to keep shoaling fish in very small numbers. They have evolved to live in huge shoals, often consisting of hundreds, if not thousands of individuals, in the wild and so they do need plenty of company.
 
Being completely honest, even the oft quoted 'at least six' is barely acceptable; it would be great if we could aim for a dozen instead! Certainly, when I was building up my shoal of green neons (I don't know why they're called 'green', when they're quite obviously blue, but there you are!) I noticed a big difference in behaviour when I got past the dozen mark; they suddenly became much more confident.
 
And aggression within the group is also a big factor, as eagles points out; even neons can be surprisingly aggressive and nippy, if in small numbers.