Sorry to hear that :-( ...don't feel too bad, sometimes the stock we get are already on their way out and we can't tell. It's possible that it had nothing to do with your tank or husbandry.
Good luck moving forward!
Is this one brown too?
Only thing I can think of is the beginning of HLLE.
I was thinking, Lymphocytis but that is normally white-grey clumps, typically at the base of fins, and is a viral infection.
Hope this helps, sorry I can't tell you more.
I looked at HLLE and I don't think that's it. This looks crusty, almost like a booger. This one is by the fish's nose, right in front of the eye. He doesn't seem lethargic, but the one that died always was. I'm not sure if it's contagious so I moved him to a bag in the tank