I've noticed this with my Espei, it's been too recurrent to ignore now.

Ever so often, one of my Espei will get a white streak on it's body. This white vertical streak will usually develop into one or two more, then the fish starts to be a loner, and eventually dies. So far, about 6 of my Espei has succumbed to this.

Another similar case will be here.

Anyone has a similar problem/knows what disease this is? It's unlike anything I can find in books.