Let me apologize in advance. I have been pacing around my place for about an hour now in indignation (probably a small town about 30 miles south of Chicago.) (The good news is the fish have been fed some treats and the water has been changed in a couple of places, so long as the adrenaline flows.) I'm not trying to flame you or be a grand inquisitor, but there are a couple of warning signs/ alarm bells in what you write above. I would be glad to be wrong in every instance. :)

What fungus would you be treating for? What specific disease(s) are you treating for? What were you treating with? When did this take place? What impact might that treatment have on your nitrogen cycle?

When you say the water conditions are optimal, what ammonia, nitrite and nitrate readings would it test for? If you don't have a test kit, that is ok. But do you have a shop which can test for those?

Thanks! I look forward to your answers. They may help you unravel what has been going on.