Get all fish showing symptoms out of there fast an eythanise, as this is highly contagious. Acting slowly here will potentialy wipe your tank...Been there and done that

As Colin says, you need to eutanise, but I Dont agree with his mwthod, as it most likely will be slow and very painful for the fish concerend in the event that they are able to feel pain, which IMO fish can...
Clove oil can be used to euthinise, if you add suffice. Keep adding a cap at a time every 5 mins untill the fish stops breeting. At this point leve it for 1/2 an hour to ensure that it doesn't come back, and then dispose of the body

If clove oil is not available now, take it out of the tank and smack it with something very heavy over the head, to cruch the head. This method will be almost instant, though few people have the guts to use this method.
Treat the main tank with the Interpet anti-internal bacteria med, to ensure that it doesn't spread.
All the best of luck fixing this
Rabbut