I am sorry for your loss. Swim in peace, Alpha.
It's hard to say what caused it. I'm pretty sure Kanaplex treats it, though, although I know that you didn't even have time to treat it. (Kanaplex also treats dropsy. I will NEVER not have it on hand again in my life, it is so incredibly useful.) culomnaris has two types. One that develops over a couple days and one that kills quickly. I know you can probably guess which one he had. It is usually caused by a lowered immune system or cross-contamination. It is very contagious. I would thoroughly clean any equipment you plan to reuse.
Some fish are just hardier than others. One of my females passed away from Dropsy three days after a one hour power outage that made her tank temp fluctuate, although none of my other four fish got sick. Unfortunately, sometimes really small things like that can make them sick. :(