Hi iversotl. Silverfang asked me to take a look at this thread. First, did the loach and rasbora show any of the same gray as your betta?
From the pics you've posted here and in the mystery disease thread, it looks like there's a good chance your betta has that disease. If the loach and rasbora didn't have any of the same symptoms, I would say their deaths were from other causes and not related to this disease. However, if they DID have the same gray patch of tissue necrosis, then there's a possiblity we've just discovered the disease either jumps species OR we're one step closer to figuring out whether or not this disease is an acute form of columnaris.
Either way, the med you want to get is API Furan-2.