Fin missing, body turning black

[solmartin]

I was just informed my sister's comet is sick. Upon looking at it, the top fin is totally rotted off, and a big portion of the body around the fin has turned black. What could this be and how would you treat it?

It's a 2.5 G Tank with 2 comets in it in case that helps.

 

[smb]

Sounds like fin rot.

I would make sure you have good water quality and treat with an antibiotic for fin rot.

Also, make sure you watch for secondary infections.