I can't seem to PM the OP or DarkMoon17, but someone on here has pm'ed me to say that my fish looks like it had this:
I put him in water in the freezer yesterday, to preserve him until I can take him back to Petco (I'd bury him, but it's 10 degrees out, and if he has this, I guess he shouldn't be buried). Anyway, there was no other way to preserve his body--we had a major snowstorm yesterday--but I'm wondering if I should/can send him and how to get in touch with someone about it. If he does look like this is the cause of death from the pics in my thread, let me know. I'm so sad that I've failed him, but if his death can help in some way, that would be some positive from all of this.