Weird lump

[Abif]

Does anyone know why my guppy has suddenly developed a big lump in its tail? I have two others which look normal, but this is large!!

 

[GaryE]

There is a very strong chance it's cancer. If not, it could be a bacterial cyst, in which case you don't want him in the tank with the other fish. The bacteria bursts out of the cyst as the fish dies.

 

[Abif]

Thanks - I will remove it

 

[emeraldking]

This is something you just can not heal. How fast did this develop?