I agree with King Fisher, the problem described sounds like Columnaris, a very bad bacteria.
Columnaris seldom has a happy ending. If you have a seperate hospital tank, remove the obviously infected fish and medicate with an antibiotic. Don't get your hopes up. You would be hard pressed to find anyone who has successfully cured a fish with a true case of Columnaris.
, your goal should be to save the apparently unaffected tankmates. I'd do a large 50% WC
, asap. Keep a close watch on the survivors and be prepared to remove a sick fish at first sign.
One of the big reasons Columnaris is almost impossible to heal is that it starts inside the fish. By the time the exterior symptoms are seen the lethal internal damage has already occured.