Help with black skirt aggression

[El_diablo]

So iv go 4 black skirt tetras that iv had from day 1! My tank has been up n running 2years and my black skirts hav been in since the beginning. Now I'm aware they show aggression and tend to nip between themselves but apart from the odd scuffle here and there they have lived peacefully together. Now, all of a sudden they seem to be bullying the smallest 1 and now he has chunks out of his top sail fin, how can I prevent this and why all of a sudden have they started to do this? My tank is a 30G bow front so it would b awkward trying to separate them and my spare/QT tank is out on loan to a friend of mine

 

[rich311k]

He is probably sick or getting weak in some way. More of them could help solve the problem. The more there are the more the aggression gets spread out.