Strange lump on cardinal tetra

[Catfish586]

Following my poorly ember tetra (see thread below) now a cardinal tetra has developed a strange white lump on its tail, which makes me fear I'm on the verge of an outbreak. Could it be some sort of parasite? Ideas welcome

 

[GaryE]

It looks to me like you may have added new fish unquarantined, and they may have introduced a flavibacter/"columnaris" infection. Check it against online photos.

What you can use to treat it will vary from country to country. Success rates are not good.