Sudden so far as I can tell. What else do you think it might be? I was thinking maybe septicemia but it doesn't really look like it ... The red spots aren't blotchy, they're usually one large spot (so far) and under the skin, no lesions or sores as of yet. I wish I had taken some pictures before I got rid of the fish...
Sorry for gunking up this thread! D:
If there are red areas under the skin, it sounds bacterial rather than parasitic. How are the survivors? Do they have a grey film? Are they breathing hard? Lethargic?
And don't be concerned about gunking up the thread. We're all here to help each other.